Professor Adrian Matthews School of Environmental Sciences and School of Mathematics, University of East Anglia, Norwich, UK
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Centre for Ocean and Atmospheric Sciences

Abstracts

Xu G, Osborn TJ, Matthews AJ, 2017: Moisture transport by Atlantic tropical cyclones onto the North American continent. Climate Dyn., 48, 3161-3182.

Tropical Cyclones (TCs) are an important source of freshwater for the North American continent. Many studies have tried to estimate this contribution by identifying TC-induced precipitation events, but few have explicitly diagnosed the moisture fluxes across continental boundaries. We design a set of attribution schemes to isolate the column-integrated moisture fluxes that are directly associated with TCs and to quantify the flux onto the North American Continent due to TCs. Averaged over the 2004-2012 hurricane seasons and integrated over the western, southern and eastern coasts of North America, the seven schemes attribute 7-18% (mean 14%) of total net onshore flux to Atlantic TCs. A reduced contribution of 10% (range 9-11%) was found for the 1980-2003 period, though only two schemes could be applied to this earlier period. Over the whole 1980-2012 period, a further 8% (range 6-9% from two schemes) was attributed to East Pacific TCs, resulting in a total TC contribution of 19 (range 17-22%) to the ocean-to-land moisture transport onto the North American continent between May and November. Analysis of the attribution uncertainties suggests that incorporating details of individual TC size and shape adds limited value to a fixed radius approach and TC positional errors in the ERA-Interim reanalysis do not affect the results significantly, but biases in peak wind speeds and TC sizes may lead to underestimates of moisture transport. The interannual variability does not appear to be strongly related to the El Nino-Southern Oscillation phenomenon.

Baranowski DB, Flatau MK, Flatau PJ, Matthews AJ, 2016: Phase locking between atmospheric convectively coupled equatorial Kelvin waves and the diurnal cycle of precipitation over the Maritime Continent. Geophys. Res. Lett., 43, 8269-8276.

Convectively coupled Kelvin waves (CCKWs) are a major component of the tropical atmospheric circulation, propagating eastward around the equatorial belt. Here we show there are scale interactions between CCKWs and the diurnal cycle over the Maritime Continent. In particular, CCKW packets that pass a basepoint in the eastern Indian Ocean at 90E between 0600-0900 UTC subsequently arrive over Sumatra in phase with the diurnal cycle of convection. As the distance between Sumatra and Borneo is equal to the distance travelled by a CCKW in one day, these waves are then also in phase with the diurnal cycle over Borneo. Consequently, this subset of CCKWs has a precipitation signal up to a factor of 3 larger than CCKWs that arrive at other times of the day, and a 40% greater chance of successfully traversing the Maritime Continent.

Baranowski DB, Flatau MK, Flatau PJ, Matthews AJ, 2016: Impact of atmospheric convectively-coupled Kelvin waves on upper ocean variability. J. Geophys. Res., 121, 2045-2059.

Convectively coupled Kelvin waves (CCKWs) are atmospheric weather systems that propagate eastward along the equatorial wave guide with phase speeds between 11 and 14 m s-1. They are an important constituent of the convective envelope of the Madden-Julian Oscillation (MJO), for which ocean-atmosphere interactions play a vital role. Hence, ocean-atmosphere interactions within CCKWs may be important for MJO development and prediction, and for tropical climate in general. Although the atmospheric structure of CCKWs has been well studied, their impact on the underlying ocean is unknown. In this paper, the ocean-atmosphere interactions in CCKWs are investigated by a case study from November 2011 during the CINDY/DYNAMO field experiment, using in situ oceanographic measurements from an ocean glider. The analysis is then extended to a 15-year period using precipitation data from the Tropical Rainfall Measuring Mission (TRMM) and surface fluxes from the TropFlux analysis. A methodology is developed to calculate trajectories of CCKWs. CCKW events are strongly controlled by the MJO, with twice as many CCKWs observed during the convectively active phase of the MJO compared to the suppressed phase. Coherent ocean-atmosphere interaction is observed during the passage of a CCKW, which lasts approximately 4 days at any given longitude. Surface wind speed and latent heat flux are enhanced, leading to a transient suppression of the diurnal cycle of sea surface temperature (SST), and a sustained decrease in bulk SST of 0.1 degC. Given that a typical composite mean MJO SST anomaly is of the order of 0.3 degC, and more than one CCKW can occur during the active phase of a single MJO event, the oceanographic impact of CCKWs is of major importance to the MJO cycle.

Birch CE, Webster S, Peatman SC, Parker DJ, Matthews AJ, Li Y, Hassim ME, 2016: Scale interactions between the MJO and the western Maritime Continent. J. Climate, 29, 2471-2492.

State-of-the-art regional climate model simulations that are able to resolve key mesoscale circulations are used, for the first time, to understand the interaction between the large-scale convective environment of the MJO and processes governing the strong diurnal cycle over the islands of the Maritime Continent (MC). Convection is sustained in the late afternoon just inland of the coasts due to sea breeze convergence. Previous work has shown that the variability in MC rainfall associated with the MJO is manifested in changes to this diurnal cycle; land-based rainfall peaks before the active convective envelope of the MJO reaches the MC, whereas oceanic rainfall rates peak whilst the active envelope resides over the region. The model simulations show that the main controls on oceanic MC rainfall in the early active MJO phases are the large-scale environment and atmospheric stability, followed by high oceanic latent heat flux forced by high near-surface winds in the later active MJO phases. Over land, rainfall peaks before the main convective envelope arrives (in agreement with observations), even though the large-scale convective environment is only moderately favourable for convection. The causes of this early rainfall peak are convective triggers from land-sea breeze circulations that are strong due to high surface insolation and surface heating. During the peak MJO phases cloud cover increases and surface insolation decreases, which weakens the strength of the mesoscale circulations and reduces land-based rainfall, even though the large-scale environment remains favourable for convection at this time. Hence, scale interactions are an essential part of the MJO transition across the MC.

van der Wiel K, Matthews AJ, Joshi M, Stevens DP, 2016: The influence of diabatic heating in the South Pacific Convergence Zone on Rossby wave propagation and the mean flow. Quart. J. Roy. Meteorol. Soc., 142, 901-910.

The South Pacific Convergence Zone (SPCZ) is a northwest-southeast oriented precipitation band over the South Pacific Ocean. Latent heat release from condensation leads to substantial diabatic heating, which has potentially large impacts on local and global climate. The influence of this diabatic heating within the SPCZ is investigated using the Intermediate General Circulation Model (IGCM4). Precipitation in the SPCZ has been shown to be triggered by transient Rossby waves that originate in the Australian subtropical jet and are refracted towards the equatorial eastern Pacific. A Rossby wave triggers a SPCZ 'convective event', with associated diabatic heat release and vortex stretching. Consequently, the Rossby wave is dissipated in the SPCZ region. These features are simulated well in a control integration of IGCM4.

In an experiment, convective heating is prescribed to its 'climatological' value in the SPCZ region during the Rossby wave 'events' and dynamic forcing from Rossby waves is decoupled from the usual thermodynamic response. In this experiment Rossby waves over the SPCZ region are not dissipated, confirming the vortex stretching mechanism from previous studies. Furthermore, the change in Rossby wave propagation has an impact on momentum transport. Overall, the effect of the Rossby wave-induced convection in the SPCZ is to decrease the strength of the Pacific subtropical jet and the equatorial eastern Pacific upper-tropospheric westerlies, by about 2-6 m s-1.

Following these changes to the basic state, two potential feedbacks in the SPCZ and larger Pacific climate system are suggested: increased SPCZ convection due to the enhancement of negative zonal stretching deformation in the SPCZ region and decreased equatorward refraction of Rossby waves into the westerly duct leading to less SPCZ 'events'. As the convective events in the SPCZ have a significant impact on Pacific mean climate, it is crucial that the SPCZ is represented correctly in climate models.

Xu G, Osborn TJ, Matthews AJ, Joshi MM, 2016: Different atmospheric moisture divergence responses to extreme and moderate El Ninos. Climate Dyn., 47, 393-410.

On seasonal and interannual time scales, vertically integrated moisture divergence provides a useful measure of the tropical atmospheric hydrological cycle. It reflects the combined dynamical and thermodynamica effects, and is not subject to the limitations tat afflict observations of evaporation minus precipitation. An Empirical Orthogonal Function (EOF) analysis of the tropical Pacific moisture divergence fields reveals he dominant effects of the El Nino-Southern Oscillation (ENSO) on interannual time scales. Two EOFs are necessary to capture the ENSO signature, and regression relationships between their Principal Components and indices of equatorial Pacific sea surface temperature (SST) demonstrate that the transition from strong La Nina through to extreme El Nino events is not a linear one. The largest deviation from linearity is for the strongest El Ninos, and we interpret that this arises at least partly because the EOF analysis cannot easily separate different patterns of responses that are not orthogonal to each other.

To overcome the orthogonality constraints, a Self Organizing Map (SOM) analysis of the same moisture divergence fields was performed. The SOM analysis captures the range of responses to ENSO, including the distinction between the moderate and strong El Ninos identified by the EOF analysis. The work demonstrates the potential for the application of SOM to large scale climatic analysis, by virtue of its easier interpretation, relaxation of orthogonality constraints and its versatility for serving as an alternative classification method. Both the EOF and SOM analyses suggest a classification of "moderate" and "extreme" El Ninos by their differences in the magnitudes of the hydrological cycle responses, spatial patterns and evolutionary paths. Classification from the moisture divergence point of view shows consistency with results based on other physical variables such as SST.

Poulidis AP, Renfrew IA, Matthews AJ, 2016: Thermally induced convective circulation and precipitation over an isolated volcano. J. Atmos. Sci., 73, 1667-1686.

Intense rainfall over active volcanoes is known to trigger dangerous volcanic hazards, from remobilising loose volcanic surface material into lahars or mudflows, to initiating explosive activity including pyroclastic flows at certain dome-forming volcanoes. However, the effect of the heated volcanic surface on the atmospheric circulation, including any feedback with precipitation, is unknown. This is investigated here, using the Weather Research and Forecasting (WRF) model. The recent activity at the Soufriere Hills Volcano (SHV), Montserrat, is a well-documented case of such rainfall-volcano interaction, and is used as a template for our experiments. The volcano is represented in the model by an idealised Gaussian mountain, with an imposed realistic surface temperature anomaly on the volcano summit. A robust increase in precipitation over the volcano is simulated for surface temperature anomalies above approximately 40 degC, an area-average value that is exceeded at the SHV. For wind speeds less that 4 m s-1 and a range of realistic atmospheric conditions, the precipitation increase is well above the threshold required to trigger volcanic hazards (5-10 mm hr-1). Hence, the thermal atmospheric forcing due to an active, but non-erupting, volcano appears to be an important factor in rainfall-volcano interactions, and should be taken account of in future hazard studies.

van der Wiel K, Matthews AJ, Joshi M, Stevens DP, 2016: Why the South Pacific Convergence Zone is diagonal. Climate Dyn., 46, 1683-1698.

During austral summer, the majority of precipitation over the Pacific Ocean is concentrated in the South Pacific Convergence Zone (SPCZ). The surface boundary conditions required to support the diagonally (northwest-southeast) oriented SPCZ are determined through a series of experiments with an atmospheric general circulation model. Continental configuration and orography do not have a significant influence on SPCZ orientation and strength. The key necessary boundary condition is the zonally asymmetric component of the sea surface temperature (SST) distribution. This leads to a strong subtropical anticyclone over the southeast Pacific that, on its western flank, transports warm moist air from the equator into the SPCZ region. This moisture then intensifies (diagonal) bands of convection that are initiated by regions of ascent and reduced static stability ahead of the cyclonic vorticity in Rossby waves that are refracted toward the westerly duct over the equatorial Pacific. The climatological SPCZ is comprised of the superposition of these diagonal bands of convection. When the zonally asymmetric SST component is reduced or removed, the subtropical anticyclone and its associated moisture source is weakened. Despite the presence of Rossby waves, significant moist convection is no longer triggered; the SPCZ disappears. The diagonal SPCZ is robust to large changes (up to +/-6 degC) in absolute SST (i.e. where the SST asymmetry is preserved). Extreme cooling (change less than -6 degC) results in a weaker and more zonal SPCZ, due to decreasing atmospheric temperature, moisture content and convective available potential energy.

Peatman SC, Matthews AJ, Stevens DP, 2015: Propagation of the Madden-Julian Oscillation and scale interaction with the diurnal cycle in a high-resolution GCM. Climate Dyn., 45, 2901-2918.

The Madden-Julian Oscillation (MJO) is the chief source of tropical intra-seasonal variability, but is simulated poorly by most state-of-the-art GCMs. Common errors include a lack of eastward propagation at the correct frequency and zonal extent, and too small a ratio of eastward- to westward-propagating variability. Here it is shown that HiGEM, a high-resolution GCM, simulates a very realistic MJO with approximately the correct spatial and temporal scale. Many MJO studies in GCMs are limited to diagnostics which average over a latitude band around the equator, allowing an analysis of the MJO's structure in time and longitude only. In this study a wider range of diagnostics is applied. It is argued that such an approach is necessary for a comprehensive analysis of a model's MJO. The standard analysis of Wheeler and Hendon (Mon Wea Rev 132(8):1917-1932, 2004; WH04) is applied to produce composites, which show a realistic spatial structure in the MJO envelopes but for the timing of the peak precipitation in the inter-tropical convergence zone, which bifurcates the MJO signal. Further diagnostics are developed to analyse the MJO's episodic nature and the "MJO inertia" (the tendency to remain in the same WH04 phase from one day to the next). HiGEM favours phases 2, 3, 6 and 7; has too much MJO inertia; and dies out too frequently in phase 3. Recent research has shown that a key feature of the MJO is its interaction with the diurnal cycle over the Maritime Continent. This interaction is present in HiGEM but is unrealistically weak.

van der Wiel K, Matthews AJ, Stevens DP, Joshi M, 2015: A dynamical framework for the origin of the diagonal South Pacific and South Atlantic convergence zones. Quart. J. Roy. Meteorol. Soc., 141, 1997-2010.

The South Pacific Convergence Zone (SPCZ) and South Atlantic Convergence Zone (SACZ) are diagonal bands of precipitation that extend from the equator southeastward into the Southern Hemisphere over the western Pacific and Atlantic Oceans, respectively. With mean precipitation rates over 5 mm day-1, they are a major component of the tropical and global climate in austral summer. However, their basic formation mechanism is not fully understood. Here, a conceptual framework for the diagonal convergence zones is developed, based on calculations of the vorticity budget from reanalysis and Rossby wave theory.

Wave trains propagate eastward along the Southern Hemisphere subtropical jet, with initially quasi-circular vorticity centres. In the zonally sheared environment on the equatorward flank of the jet, these vorticity centres become elongated and develop a northwest-southeast tilt. Ray tracing diagnostics in a non-divergent, barotropic Rossby wave framework then explain the observed equatorward propagation of these diagonal vorticity structures toward the westerly ducts over the equatorial Pacific and Atlantic. The baroclinic component of these circulations leads to destabilisation and ascent ahead of the cyclonic vorticity anomaly in the wave, triggering deep convection because of the high sea surface temperatures in this region. Latent heat release then forces additional ascent and strong upper-tropospheric divergence, with an associated anticyclonic vorticity tendency. A vorticity budget shows that this cancels out the advective cyclonic vorticity tendency in the wave train over the SPCZ, and dissipates the wave within a day. The mean SPCZ is consequently comprised of the sum of these pulses of diagonal bands of precipitation.

Similar mechanisms also operate in the SACZ. However, the vorticity anomalies in the wave trains are stronger, and the precipitation and negative feedback from the divergence and anticyclonic vorticity tendency are weaker, resulting in continued propagation of the wave and a more diffuse diagonal convergence zone.

Niznik MJ, Lintner BR, Matthews AJ, Widlansky MJ, 2015: The role of tropical-extratropical interaction and synoptic variability in maintaining the South Pacific Convergence Zone in CMIP5 models. J. Climate, 28, 3353-3374.

The South Pacific Convergence Zone (SPCZ) is simulated as too zonal a feature in current generation climate models, including those in Phase 5 of the Coupled Model Intercomparison Project (CMIP5). This zonal bias induces errors in tropical convective heating, with subsequent effects on global circulation. The SPCZ structure, particularly in the subtropics, is governed by the tropical-extratropical interaction between transient synoptic systems and the mean background state. However, the fidelity of synoptic-scale interactions as simulated by CMIP5 models has not yet been evaluated. In this study, analysis of synoptic variability in the simulated subtropical SPCZ reveals that the basic mechanism of tropical-extratropical interaction is generally well simulated, with storms approaching the SPCZ along comparable trajectories to observations. However, there is a broad spread in mean precipitation and its variability across the CMIP5 ensemble. Inter-model spread appears to relate to a biased background state in which the synoptic waves propagate. In particular, the region of mean negative zonal stretching deformation or "storm graveyard" in the upper troposphere (a feature previously determined to play a key role in SPCZ-storm interactions) is typically displaced in CMIP5 models to the northeast of its position in reanalysis data, albeit with individual model graveyards displaying a pronounced (25 degree) longitudinal spread. From these findings, we suggest that SPCZs simulated by CMIP5 models are not simply too zonal; rather, in models the subtropical SPCZ manifests a diagonal tilt similar to observations while SST biases force an overly zonal tropical SPCZ, resulting in a more disjointed SPCZ than observed.

Matthews AJ, Baranowski DB, Heywood KJ, Flatau PJ, Schmidtko S, 2014: The surface diurnal warm layer in the Indian Ocean during CINDY/DYNAMO. J. Climate, 27, 9101-9122.

A surface diurnal warm layer is diagnosed from Seaglider observations, and develops on half the days in the CINDY/DYNAMO Indian Ocean experiment. The diurnal warm layer occurs on days of high solar radiation flux (>80 W m-2) and low wind speed (<6 m s-1), and preferentially in the inactive stage of the Madden-Julian Oscillation. Its diurnal harmonic has an exponential vertical structure with a depth scale of 4-5 m (dependent on chlorophyll concentration), consistent with forcing by absorption of solar radiation. The effective sea surface temperature (SST) anomaly due to the diurnal warm layer often reaches 0.8°C in the afternoon, with a daily mean of 0.2°C, rectifying the diurnal cycle onto longer time scales. This SST anomaly drives an anomalous flux of 4 W m-2 that cools the ocean. Alternatively, in a climate model where this process is unresolved, this represents an erroneous flux that warms the ocean. A simple model predicts a diurnal warm layer to occur on 30-50% of days across the tropical warm pool. On the remaining days, with low solar radiation and high wind speeds, a residual diurnal cycle is observed by the Seaglider, with a diurnal harmonic of temperature that decreases linearly with depth. As wind speed increases, this already weak temperature gradient decreases further, tending towards isothermal conditions.

Webber BGM, Matthews AJ, Heywood KJ, Kaiser J, Schmidtko S, 2014: Seaglider observations of equatorial Indian Ocean Rossby waves associated with the Madden-Julian Oscillation. J. Geophys. Res., 119, 3714-3731.

During the CINDY-DYNAMO field campaign of September 2011 - January 2012, a Seaglider was deployed at 80 degE and completed 10 north-south sections between 3 and 4 degS, measuring temperature, salinity, dissolved oxygen concentration and chlorophyll fluorescence. These high-resolution subsurface observations provide insight into equatorial ocean Rossby wave activity forced by three Madden-Julian Oscillation (MJO) events during this time period. These Rossby waves generate variability in temperature O(1 degC), salinity O(0.2 g kg-1), density O(0.2 kg m-3) and oxygen concentration O(10 micromol kg-1), associated with 10 m vertical displacements of the thermocline. The variability extends down to 1000 m, the greatest depth of the Seaglider observations, highlighting the importance of surface forcing for the deep equatorial ocean. The temperature variability observed by the Seaglider is greater than that simulated in the ECCO-JPL reanalysis, especially at depth. There is also marked variability in chlorophyll fluorescence at the surface and at the depth of the chlorophyll maximum. Upwelling from Rossby waves and local wind stress curl leads to an enhanced shoaling of the chlorophyll maximum by 10-25 m in response to the increased availability of nutrients and light. This influence of the MJO on primary production via equatorial ocean Rossby waves has not previously been recognised.

Peatman SC, Matthews AJ, Stevens DP, 2014: Propagation of the Madden-Julian Oscillation through the Maritime Continent and scale interaction with the diurnal cycle of precipitation. Quart. J. Roy. Meteorol. Soc., 140, 814-825.

The convectively active part of the Madden-Julian Oscillation (MJO) propagates eastward through the warm pool, from the Indian Ocean through the Maritime Continent (the Indonesian archipelago) to the western Pacific. The Maritime Continent's complex topography means the exact nature of the MJO propagation through this region is unclear. Model simulations of the MJO are often poor over the region, leading to local errors in latent heat release and global errors in medium-range weather prediction and climate simulation.

Using 14 northern winters of TRMM satellite data it is shown that, where the mean diurnal cycle of precipitation is strong, 80% of the MJO precipitation signal in the Maritime Continent is accounted for by changes in the amplitude of the diurnal cycle. Additionally, the relationship between outgoing long-wave radiation (OLR) and precipitation is weakened here, such that OLR is no longer a reliable proxy for precipitation. The canonical view of the MJO as the smooth eastward propagation of a large-scale precipitation envelope also breaks down over the islands of the Maritime Continent. Instead, a vanguard of precipitation (anomalies of 2.5 mm day^-1 over 10^6 km^2) jumps ahead of the main body by approximately 6 days or 2000 km. Hence, there can be enhanced precipitation over Sumatra, Borneo or New Guinea when the large-scale MJO envelope over the surrounding ocean is one of suppressed precipitation.

This behaviour can be accommodated into existing MJO theories. Frictional and topographic moisture convergence and relatively clear skies ahead of the main convective envelope combine with the low thermal inertia of the islands, to allow a rapid response in the diurnal cycle which rectifies onto the lower-frequency MJO. Hence, accurate representations of the diurnal cycle and its scale interaction appear to be necessary for models to simulate the MJO successfully.

Hicks PD, Cooker MJ, Matthews AJ, 2014: Saturation front evolution for liquid infiltration into a gas-filled porous medium with counter-current flow. Europ. J. Mech. - B/Fluids, 43, 202-215.

The infiltration of liquid into a gas saturated porous network is investigated. Particular attention is paid to the situation in which a pressure gradient in the porous medium drives a gas flow upwards, while a more dense liquid infiltrates down into the reservoir due to gravity. There are two flows in opposite directions. A model is proposed, based upon a compressible gas phase and an incompressible liquid phase. The volume fluxes in each phase are assumed to be governed by Darcy type flow laws, modified to include the permeability caused by both the solid matrix and the impeding of the gas flow by the liquid phase. Isothermal flows are examined in the absence of phase changes. The proposed model is an extension of the traditional Buckley-Leverett model, and is used to consider a variety of flows, including carbon sequestration in a porous medium below the seabed and rainfall infiltration into a lava dome.

Matthews AJ, Pickup G, Peatman SC, Clews P, Martin J, 2013: The effect of the Madden-Julian Oscillation on station rainfall and river level in the Fly River system, Papua New Guinea. J. Geophys. Res., 118, 10926-10935.

The Madden-Julian oscillation (MJO) is the dominant mode of intraseasonal variability in tropical rainfall on the large scale, but its signal is often obscured in individual station data, where effects are most directly felt at the local level. The Fly River system, Papua New Guinea, is one of the wettest regions on Earth and is at the heart of the MJO envelope. A 16 year time series of daily precipitation at 15 stations along the river system exhibits strong MJO modulation in rainfall. At each station, the difference in rainfall rate between active and suppressed MJO conditions is typically 40% of the station mean. The spread of rainfall between individual MJO events was small enough such that the rainfall distributions between wet and dry phases of the MJO were clearly separated at the catchment level. This implies that successful prediction of the large-scale MJO envelope will have a practical use for forecasting local rainfall. In the steep topography of the New Guinea Highlands, the mean and MJO signal in station precipitation is twice that in the satellite Tropical Rainfall Measuring Mission 3B42HQ product, emphasizing the need for ground-truthing satellite-based precipitation measurements. A clear MJO signal is also present in the river level, which peaks simultaneously with MJO precipitation input in its upper reaches but lags the precipitation by approximately 18 days on the flood plains.

Dawson A, Matthews AJ, Stevens DP, Roberts MJ, Vidale PL, 2013: Importance of oceanic resolution and mean state on the extra-tropical response to El Nino in a matrix of coupled models. Climate Dyn., 41, 1439-1452.

The extra-tropical response to El Nino in configurations of a coupled model with increased horizontal resolution in the oceanic component is shown to be more realistic than in configurations with a low resolution oceanic component. This general conclusion is independent of the atmospheric resolution. Resolving small-scale processes in the ocean produces a more realistic oceanic mean state, with a reduced cold tongue bias, which in turn allows the atmospheric model component to be forced more realistically. A realistic atmospheric basic state is critical in order to represent Rossby wave propagation in response to El Nino, and hence the extra-tropical response to El Nino. Through the use of high and low resolution configurations of the forced atmospheric-only model component we show that, in isolation, atmospheric resolution does not significantly affect the simulation of the extra-tropical response to El Nino. It is demonstrated, through perturbations to the SST forcing of the atmospheric model component, that biases in the climatological SST field typical of coupled model configurations with low oceanic resolution can account for the erroneous atmospheric basic state seen in these coupled model configurations. These results highlight the importance of resolving small-scale oceanic processes in producing a realistic large-scale mean climate in coupled models, and suggest that it might may be possible to 'squeeze out' valuable extra performance from coupled models through increases to oceanic resolution alone.

Oppel S, Hilton GM, Allcorn R, Fenton C, Matthews AJ, Gibbons DW, 2013: The effects of rainfall on different components of seasonal fecundity in a tropical forest passerine. Ibis, 155, 464-475.

Seasonal fecundity is a composite metric that is determined by component parameters such as clutch size, nest survival and re-nesting probability. Many of these component parameters are known to vary with environmental conditions, in particular rainfall prior to or during the breeding season. In some species, seasonal fecundity is positively related to rainfall, but little is known about which component parameters of seasonal fecundity respond most strongly to rainfall. We used intensive nest monitoring of a multi-brooded tropical forest passerine, the Montserrat Oriole Icterus oberi, to examine the effects of rainfall during the pre-breeding season on component parameters of annual fecundity. We monitored all nests of a total of 42 pairs over 5 years in which rainfall varied substantially. We then related clutch size, nest survival, onset and length of the breeding season, re-nesting probability and re-nesting interval to pre-breeding season rainfall using generalized linear mixed models that accounted for random variation across sites and individual pairs, and incorporated other variables known to affect the response. Higher pre-breeding season rainfall led to an increase in clutch size and a decrease in re-nesting interval, but nest survival, re-nesting probability and length of the breeding season were not affected by variation in rainfall. The onset of the breeding season was delayed in very dry years. We conclude that higher rainfall is likely to increase food availability and thus body condition of female Montserrat Orioles, leading to an increase in fecundity due to larger clutch sizes.

Matthews AJ, 2012: A multiscale framework for the origin and variability of the South Pacific Convergence Zone. Quart. J. Roy. Meteorol. Soc., 138, 1165-1178.

The diagonal South Pacific Convergence Zone (SPCZ) is the major climatological precipitation feature over the Pacific region during the Northern Hemisphere winter. However, the basic mechanisms that control its structure and variability are only partly understood. Here, an analysis of the SPCZ is carried out in a multiscale framework. This identifies two modes that dominate: a (westward) shifted SPCZ and an enhanced SPCZ, which occur independently of each other. Within both modes, the primary mechanism for the initiation of precipitation is a transient synoptic wave propagating along the subtropical jet, which is then refracted by the basic state toward the westerly duct over the central equatorial Pacific. Individual vorticity centres in the wave become elongated, with a diagonal (northwest-southeast) tilt. Convection then occurs in a diagonal band in the poleward flow ahead of the cyclonic vorticity anomaly in the wave. However, latent heat release in the convection leads to upper-tropospheric divergence and anticyclonic vorticity forcing, which dissipates the wave, shutting off the convective forcing and stopping the precipitation. Hence, each individual wave or event only lasts a few days and contributes a discrete pulse of diagonally oriented precipitation to the region. The sum of these events leads to the diagonal climatological SPCZ. Event occurrence is a stochastic process, the probability of which is modified by lower-frequency variability of the basic state, including the Madden-Julian Oscillation (MJO) and El Nino-Southern Oscillation (ENSO). For example, during periods of enhanced convection over the eastern Indian Ocean to western Pacific (MJO phases 3-6 and La Nina) the westerly duct expands westwards, allowing synoptic waves to refract equatorwards earlier and increasing the probability of westward-shifted SPCZ events. Hence, both the existence and variability of the SPCZ depend fundamentally on scale interactions between dynamical processes on time-scales ranging from daily to interannual.

Webber BGM, Stevens DP, Matthews AJ, Heywood KJ, 2012: Dynamical ocean forcing of the Madden-Julian Oscillation at lead times of up to five months. J. Climate, 25, 2824-2842.

The authors show that a simple three-dimensional ocean model linearized about a resting basic state can accurately simulate the dynamical ocean response to wind forcing by the Madden-Julian oscillation (MJO). This includes the propagation of equatorial waves in the Indian Ocean, from the generation of oceanic equatorial Kelvin waves to the arrival of downwelling oceanic equatorial Rossby waves in the western Indian Ocean, where they have been shown to trigger MJO convective activity. Simulations with idealized wind forcing suggest that the latitudinal width of this forcing plays a crucial role in determining the potential for such feedbacks. Forcing the model with composite MJO winds accurately captures the global ocean response, demonstrating that the observed ocean dynamical response to the MJO can be interpreted as a linear response to surface wind forcing. The model is then applied to study "primary" Madden-Julian events, which are not immediately preceded by any MJO activity or by any apparent atmospheric triggers, but have been shown to coincide with the arrival of downwelling oceanic equatorial Rossby waves. Case study simulations show how this oceanic equatorial Rossby wave activity is partly forced by reflection of an oceanic equatorial Kelvin wave triggered by a westerly wind burst 140 days previously, and partly directly forced by easterly wind stress anomalies around 40 days prior to the event. This suggests predictability for primary MaddenJulian events on times scales of up to five months, following the reemergence of oceanic anomalies forced by winds almost half a year earlier.

Webber BGM, Matthews AJ, Heywood KJ, Stevens DP, 2012: Ocean Rossby waves as a triggering mechanism for primary Madden-Julian events. Quart. J. Roy. Meteorol. Soc., 138, 514-527.

The Madden-Julian Oscillation (MJO) is sporadic, with episodes of cyclical activity interspersed with inactive periods. However, it remains unclear what may trigger a Madden-Julian (MJ) event which is not immediately preceded by any MJO activity: a 'primary' MJ event. A combination of case-studies and composite analysis is used to examine the extent to which the triggering of primary MJ events might occur in response to ocean dynamics. The case-studies show that such events can be triggered by the arrival of a downwelling oceanic equatorial Rossby wave, which is shown to be associated with a deepening of the mixed layer and positive sea-surface temperature (SST) anomalies of the order of 0.5-1 degC. These SST anomalies are not attributable to forcing by surface fluxes which are weak for the case-studies analysed. Furthermore, composite analysis suggests that such forcing is consistently important for triggering primary events. The relationship is much weaker for successive events, due to the many other triggering mechanisms which operate during periods of cyclical MJO activity. This oceanic feedback mechanism is a viable explanation for the sporadic and broadband nature of the MJO. Additionally, it provides hope for forecasting MJ events during periods of inactivity, when MJO forecasts generally exhibit low skill.

Dawson A, Matthews AJ, Stevens DP, 2011: Rossby wave dynamics of the North Pacific extra-tropical response to El Nino: Importance of the basic state in coupled GCMs. Climate Dyn., 37, 391-405.

The extra-tropical response to El Nino in a "low" horizontal resolution coupled climate model, typical of the Intergovernmental Panel on Climate Change fourth assessment report simulations, is shown to have serious systematic errors. A high resolution configuration of the same model has a much improved response that is similar to observations. The errors in the low resolution model are traced to an incorrect representation of the atmospheric teleconnection mechanism that controls the extra-tropical sea surface temperatures (SSTs) during El Nino. This is due to an unrealistic atmospheric mean state, which changes the propagation characteristics of Rossby waves. These erroneous upper tropospheric circulation anomalies then induce erroneous surface circulation features over the North Pacific. The associated surface wind speed and direction errors create erroneous surface flux and upwelling anomalies which finally lead to the incorrect extra-tropical SST response to El Nino in the low resolution model. This highlights the sensitivity of the climate response to a single link in a chain of complex climatic processes. The correct representation of these processes in the high resolution model indicates the importance of horizontal resolution in resolving such processes.

Love BS, Matthews AJ, Lister GMS, 2011: The diurnal cycle of precipitation over the Maritime Continent in a high-resolution atmospheric model. Quart. J. Roy. Meteorol. Soc., 137, 934-947.

Climate models can exhibit systematic errors in their mean precipitation over the maritime continent of the Indonesian archipelago at the heart of the tropical warm pool. These can often be traced back to an erroneous simulation of the diurnal cycle, and can lead to errors in global climate, through planetary wave propagation. Here, we examine the simulation of the diurnal cycle over the maritime continent in a series of high-resolution integrations of the UK Met Office atmospheric model, with horizontal resolutions of 40 and 12 km (where the convection is parameterised) and 4 km (where the convection is explicitly resolved), as part of the Cascade project. In these models, the vertical heating profile over the islands changes from a convective profile with a mid-tropospheric maximum in the early afternoon to a more stratiform profile with upper tropospheric heating and mid-tropospheric cooling later. The convective heating profile forces a first internal mode gravity wave that propagates rapidly offshore; the deep warm anomalies behind its downwelling wavefront suppress convection offshore during early afternoon. The stratiform heating profile forces a gravity wave with a higher order vertical mode, that propagates slowly offshore later in the afternoon. This mode has a negative, destabilising temperature anomaly in the mid-troposphere. Together with the convergence zone between the wave fronts of the two modes, favourable conditions are created for offshore convection. In the 4 km explicit convection model, the offshore convection responds strongly to this gravity wave forcing, in agreement with observations, supporting a gravity wave--convection paradigm for the diurnal cycle over the maritime continent. However, the convective response in the lower resolution models is much less coherent, leading to errors in the diurnal cycle and mean precipitation. Hence, to improve climate model simulations, sensitivity to gravity wave forcing should be a factor in future convective parameterisation schemes.

Webber BGM, Matthews AJ, Heywood KJ, 2010: A dynamical ocean feedback mechanism for the Madden-Julian Oscillation. Quart. J. Roy. Meteorol. Soc., 136, 740-754.

Composite analysis is applied to study the dynamical ocean response to Madden-Julian (MJ) events, measured by anomalies in sea surface height from the merged TOPEX/Poseidon-European Remote Sensing satellite altimetry dataset. In each of the tropical ocean basins, significant equatorial waves are forced, which are shown to modulate the sea surface temperature (SST) by 0.2-0.3 degC in the absence of strong surface heat fluxes. In the Indian Ocean there is a clear dynamical response which may play a significant role in generating later MJ events. Surface westerly winds, associated with the active phase of the Madden-Julian Oscillation (MJO), force an eastward-propagating oceanic downwelling equatorial Kelvin wave, which, on reaching the eastern boundary at Sumatra, forces reflected downwelling equatorial Rossby waves and coastal Kelvin waves. The coastal Kelvin waves propagate southwards towards northern Australia and northwards into the Bay of Bengal, and will be important for local physical, chemical and biological processes. The equatorial Rossby waves propagate westward across the Indian Ocean, arriving in the western Indian Ocean approximately 80-100 days after the initial Kelvin wave was generated. The arrival of these waves generates positive SST anomalies which leads to convection and may trigger the next-but-one MJ event, or amplify the low-frequency tail of the MJO. This constitutes a coupled feedback mechanism from the ocean dynamics onto the MJO, somewhat similar to the delayed oscillator mechanism for the El Nino Southern Oscillation.

Matthews AJ, Singhruck P, Heywood KJ, 2010: Ocean temperature and salinity components of the Madden-Julian Oscillation observed by Argo floats. Climate Dyn., 35, 1149-1168.

New diagnostics of the Madden-Julian Oscillation (MJO) cycle in ocean temperature and, for the first time, salinity are presented. The MJO composites are based on 4 years of gridded Argo float data from 2003 to 2006, and extend from the surface to 1,400 m depth in the tropical Indian and Pacific Oceans. The MJO surface salinity anomalies are consistent with precipitation minus evaporation fluxes in the Indian Ocean, and with anomalous zonal advection in the Pacific. The Argo sea surface temperature and thermocline depth anomalies are consistent with previous studies using other data sets. The near-surface density changes due to salinity are comparable to, and partially offset, those due to temperature, emphasising the importance of including salinity as well as temperature changes in mixed-layer modelling of tropical intraseasonal processes. The MJO-forced equatorial Kelvin wave that propagates along the thermocline in the Pacific extends down into the deep ocean, to at least 1,400 m. Coherent, statistically significant, MJO temperature and salinity anomalies are also present in the deep Indian Ocean.

Lavender SL, Taylor CM, Matthews AJ, 2010: Coupled land-atmosphere intraseasonal variability of the West African monsoon in a GCM. J. Climate, 23, 5557-5571.

Recent observational studies have suggested a role for soil moisture and land--atmosphere coupling in the 15-day westward-propagating mode of intraseasonal variability in the West African monsoon (WAM). This hypothesis is investigated with a set of three atmospheric general circulation model (AGCM) experiments. 1) When soil moisture is fully coupled with the atmospheric model, the 15-day mode of land--atmosphere variability is clearly identified. Precipitation anomalies lead soil moisture anomalies by 1--2 days, similar to the results from satellite observations. 2) In a sensitivity experiment, soil moisture is externally prescribed with all intraseasonal fluctuations suppressed. The 15-day precipitation signal is still present, as a purely internal atmospheric free mode, though its propagation is less coherent. 3) In a final experiment, the atmospheric model is forced with a 15-day westward-propagating cycle of regional soil moisture anomalies based on the observed mode. Through a reduced surface sensible heat flux, the imposed wet soil anomalies induce negative low-level temperature anomalies and increased pressure (a cool high). An anticyclonic circulation then develops around the region of wet soil which enhances northward moisture advection and convection to the west. This favours westward propagation, such that the atmospheric 15-day free mode becomes weakly phase locked to the imposed soil moisture forcing. Hence, although the 15-day wave can exist as a purely internal atmospheric mode, soil moisture and land--atmosphere coupling act to further organise the mode and increase its coherence.

Hicks PD, Matthews AJ, Cooker MJ, 2010: Triggering of a volcanic dome collapse by rainwater infiltration. J. Geophys. Res., 115, B09212, doi: 10.1029/2009JB006831.

The thermodynamic processes in a one-dimensional model of a porous lava dome are considered in the presence of a rising magmatic gas flux through the void spaces and rainfall interacting with the dome surface. The steady state surface temperature of the dome depends on both magmatic gas mass flux and rainfall rate. A critical rainfall rate is determined, that cools the dome surface to 100 degC. Rainfall rates above this critical value allow liquid infiltration into the void spaces of the dome, thus restricting the escape of magmatic gas. A model which restricts the gas flow through the surface predicts internal gas pressures much higher than the overburden pressure in the top few meters, approximately one hour after the onset of rainfall. For a marginally stable dome, this could cause small Vulcanian explosions, which (depending on their location) could trigger a dome collapse, on a timescale consistent with observations.

Goldblatt C, Claire MW, Lenton TM, Matthews AJ, Watson AJ, Zahnle KJ, 2009: Nitrogen enhanced greenhouse warming on the early Earth. Nature Geoscience, 2, 891-896.

Early in Earth's history, the Sun provided less energy to the Earth than it does today. However, the Earth was not permanently glaciated, an apparent contradiction known as the faint young Sun paradox. By implication, the Earth must have been warmed by a stronger greenhouse effect or a lower planetary albedo. Here we use a radiative?convective climate model to show that more N2 in the atmosphere would have increased the warming effect of existing greenhouse gases by broadening their absorption lines. With the atmospheric CO2 and CH4 levels estimated for 2.5 billion years ago, a doubling of the present atmospheric nitrogen (PAN) level would cause a warming of 4.4 degrees C. Our new budget of Earth's geological nitrogen reservoirs indicates that there is a sufficient quantity of nitrogen in the crust (0.5 PAN) and mantle (greater than 1.4 PAN) to have supported this, and that this nitrogen was previously in the atmosphere. In the mantle, N correlates with 40Ar, the daughter product of 40K, indicating that the source of mantle N is subducted crustal rocks in which NH4+ has been substituted for K+. We thus conclude that a higher nitrogen level probably helped warm the early Earth, and suggest that the effects of N2 should be considered in assessing the habitable zone for terrestrial planets.

Love BS, Matthews AJ, 2009: Real-time localised forecasting of the Madden-Julian Oscillation using neural network models. Quart. J. Roy. Meteorol. Soc., 135, 1471-1483.

Existing statistical forecast models of the Madden-Julian Oscillation (MJO) are generally of very low order and predict the evolution of a small number (typically two) of principal components (PCs). While such models are skilful up to 25 days lead time, by design they only predict the very largest-scale features of the MJO. Here we present a higher-order MJO statistical forecast model that is able to predict MJO variability on smaller, more localised scales, that will be of more direct benefit to national weather agencies and regional government planning. The model is based on daily outgoing long-wave radiation (OLR) data that are intraseasonally filtered using a recently developed technique of empirical mode decomposition that can be used in real time. A standard truncated PC analysis is then used to isolate the maximum amount of variance in a finite number of modes. The evolution of these modes is then forecast using a neural network model, which does not suffer from the parametrisation problems of other statistical forecast techniques when applied to a higher number of modes. Compared to a standard 2-PC model, the higher-order PC model showed improved skill over the whole MJO domain, with substantial improvements over the western Pacific, Arabian Sea, Bay of Bengal, South China Sea and Phillipine Sea.

Lavender SL, Matthews AJ, 2009: Response of the West African monsoon to the Madden-Julian Oscillation. J. Climate, 22, 4097-4116.

Observations show that rainfall over West Africa is influenced by the Madden-Julian Oscillation (MJO). A number of mechanisms have been suggested: 1) forcing by equatorial waves; 2) enhanced monsoon moisture supply; and 3) increased African easterly wave (AEW) activity. However, previous observational studies are not able to unambiguously distinguish between cause and effect. Carefully designed model experiments are used to assess these mechanisms. Intraseasonal convective anomalies over West Africa during the summer monsoon season are simulated in an atmosphere-only global circulation model as a response to imposed sea surface temperature (SST) anomalies associated with the MJO over the equatorial warm pool region. 1) Negative SST anomalies stabilize the atmosphere leading to locally reduced convection. The reduced convection leads to negative midtropospheric latent heating anomalies that force dry equatorial waves. These waves propagate eastward (Kelvin wave) and westward (Rossby wave), reaching Africa approximately 10 days later. The associated negative temperature anomalies act to destabilize the atmosphere, resulting in enhanced monsoon convection over West and central Africa. The Rossby waves are found to be the most important component, with associated westward-propagating convective anomalies over West Africa. The eastward-propagating equatorial Kelvin wave also efficiently triggers convection over the eastern Pacific and Central America, consistent with observations. 2) An increase in boundary layer moisture is found to occur as a result of the forced convective anomalies over West Africa rather than a cause. 3) Increased shear on the African easterly jet, leading to increased AEW activity, is also found to occur as a result of the forced convective anomalies in the model.

Matthews AJ, Barclay J, Johnstone JE, 2009: The fast response of volcano-seismic activity to intense precipitation: Triggering of primary volcanic activity by rainfall at Soufriere Hills Volcano, Montserrat. J. Volcanol. Geotherm. Res., 184, 405-415.

One-minute resolution time series of rainfall and seismic data from the Soufriere Hills Volcano, Montserrat are analysed to explore the mechanism of external forcing of volcanic eruptions by rainfall over three years of activity. The real-time seismic amplitude (RSAM) shows a narrow, statistically significant, peak within 30 min after the start of intense rainfall events, and a much broader peak with a lag of 6?40 h. The classified seismic events indicate that the volcanic response to rainfall begins at the surface and gradually penetrates deeper into the dome, as there is an increase in the pseudo-magnitude of: surface rockfall events (including pyroclastic flows) with lags from the first 30 min to 40 h, long-period rockfalls (from shallow degassing) at lags of 4 and 14 h, and long-period and hybrid events (source depth approximately 1 km) with lags at 14 and 24 h after the start of rainfall events. There was no rainfall-related change in deeper, volcano-tectonic activity. There was no change in the frequency of any type of classified event, indicating that the rainfall acts to modulate existing, internal processes, rather than generating new events itself. These robust results are due to many (229) different rainfall events, and not just to a few, large magnitude cases. The rainfalltriggered volcanic activity examined here is consistent with a model of fast, shallow interactions with rainfall at the dome surface, after which, a deeper dome collapse follows.

Hicks PD, Matthews AJ, Cooker MJ, 2009: The thermal structure of a gas-permeable lava dome and time-scale separation in its response to perturbation. J. Geophys. Res., 114, B07201, doi: 10.1029/2008JB006198.

The thermal boundary layer at the surface of a volcanic lava dome is investigated through a continuum model of the thermodynamic advection diffusion processes resulting from magmatic gas flow through the dome matrix. The magmatic gas mass flux, porosity and permeability of the rock are identified as key parameters. New, theoretical, nonlinear steady-state thermal profiles are reported which give a realistic surface temperature of 210 degC for a region of lava dome surface through which a gas flux of 3.5 x 10-3 kg s-1 m-2 passes. This contrasts favourably with earlier purely diffusive thermal models, which cool too quickly. Results are presented for time-dependent perturbations of the steady states as a response to: changes in surface pressure, a sudden rockfall from the lava dome surface, and a change in the magmatic gas mass flux at depth. Together with a generalized analysis using the method of multiple scales, this identifies two characteristic time scales associated with the thermal evolution of a dome carapace: a short time scale of several minutes, over which the magmatic gas mass flux, density, and pressure change to a new quasi-steady-state, and a longer time scale of several days, over which the thermal profile changes to a new equilibrium distribution. Over the longer time scale the dynamic properties of the dome continue to evolve, but only in slavish response to the ongoing temperature evolution. In the light of this time scale separation, the use of surface temperature measurements to infer changes in the magmatic gas flux for use in volcanic hazard prediction is discussed.

Love BS, Matthews AJ, Janacek GJ, 2008: Real-time extraction of the Madden-Julian Oscillation using empirical mode decomposition and statistical forecasting with a VARMA model. J. Climate, 21, 5318-5335.

A simple guide to the new technique of empirical mode decomposition (EMD) in a meteorological-climate forecasting context is presented. A single application of EMD to a time series essentially acts as a local high-pass filter. Hence, successive applications can be used to produce a bandpass filter that is highly efficient at extracting a broadband signal such as the Madden-Julian Oscillation (MJO). The basic EMD method is adapted to minimize end effects, such that it is suitable for use in real time. The EMD process is then used to efficiently extract the MJO signal from gridded time series of outgoing longwave radiation (OLR) data.

A range of statistical models from the general class of vector autoregressive moving average (VARMA) models was then tested for their suitability in forecasting the MJO signal, as isolated by the EMD. A VARMA (5, 1) model was selected and its parameters determined by a maximum likelihood method using 17 yr of OLR data from 1980 to 1996. Forecasts were then made on the remaining independent data from 1998 to 2004. These were made in real time, as only data up to the date the forecast was made were used. The median skill of forecasts was accurate (defined as an anomaly correlation above 0.6) at lead times up to 25 days.

Matthews AJ, 2008: Primary and successive events in the Madden-Julian Oscillation. Quart. J. Roy. Meteorol. Soc., 134, 439-453.

Conventional analyses of the MJO tend to produce a repeating cycle, such that any particular feature cannot be unambiguously attributed to the current or previous event. We take advantage of the sporadic nature of the MJO and classify each observed Madden-Julian (MJ) event as either primary, with no immediately preceding MJ event, or successive, which does immediately follow a preceding event. 40% of MJ events are primary events. Precursor features of the primary events can be unambiguously attributed to that event. A suppressed convective anomaly grows and decays in situ over the Indian Ocean, prior to the start of most primary MJ events. An associated mid-tropospheric temperature anomaly destabilises the atmosphere, leading to the generation of the active MJ event. Hence, primary MJ events appear to be thermodynamically triggered by a previous dry period, although stochastic forcing may also be important. Other theories predict that boundary-layer convergence, humidity, propagation of dynamical structures around the Equator, sea surface temperatures, and lateral forcing by extratropical transients may all be important in triggering an event. Although precursor signals from these mechanisms are diagnosed from reanalysis and satellite observational data in the successive MJ events, they are all absent in the primary MJ events. Hence, it appears that these apparent precursor signals are part of the MJO once it is established, but do not play a role in the spontaneous generation of the MJO. The most frequent starting location of the primary events is the Indian Ocean, but over half of them start elsewhere, from the maritime continent to the western Pacific.

Matthews AJ, Singhruck P, Heywood KJ, 2007: Deep ocean impact of a Madden-Julian Oscillation observed by Argo floats. Science, 318, 1765-1769.

Using the new Argo array of profiling floats that gives unprecedented space-time coverage of the upper 2000 meters of the global ocean, we present definitive evidence of a deep tropical ocean component of the Madden-Julian Oscillation (MJO). The surface wind stress anomalies associated with the MJO force eastward-propagating oceanic equatorial Kelvin waves that extend downward to 1500 meters. The amplitude of the deep ocean anomalies is up to 6 times the amplitude of the observed annual cycle. This deep ocean sink of energy input from the wind is potentially important for understanding phenomena such as El Nino-Southern Oscillation and for interpreting deep ocean measurements made from ships.

Pohl B, Matthews AJ, 2007: Observed changes in the lifetime and amplitude of the Madden-Julian Oscillation associated with interannual ENSO sea surface temperature anomalies. J. Climate, 20, 2659-2674.

The Madden-Julian Oscillation (MJO) is analysed using the reanalysis zonal wind and satellite outgoing longwave radiation-based indices of Wheeler and Hendon for the 1974-2005 period. The average life time of MJO events varies with season, being 36 days for events whose central date occurs in December, and 48 days for events in September. The life time of the MJO in the equinoctial seasons (March-May and October-December) is also dependent on the state of the El Nino-Southern Oscillation (ENSO). During October-December it is only 32 days under El Nino conditions, increasing to 48 days under La Nina conditions, with similar values in northern spring. This difference is due to faster eastward propagation of the MJO convective anomalies through the Maritime Continent and western Pacific during El Nino, consistent with theoretical arguments concerning equatorial wave speeds.

The analysis is extended back to 1950 by using an alternative definition of the MJO based on just the zonal wind component of the Wheeler and Hendon indices. A rupture in the amplitude of the MJO is found in 1975, at the same time as the well known rupture in the ENSO time series, that has been associated with the Pacific Decadal Oscillation. The mean amplitude of the MJO is 16% larger in the post-rupture period (1976-2005) compared to the pre-rupture period (1950-1975). Before the 1975 rupture, the amplitude of the MJO is a maximum (minimum) under El Nino (La Nina) conditions during northern winter, and a minimum (maximum) under El Nino (La Nina) conditions during northern summer. After the rupture, this relationship disappears. When the MJO-ENSO relationship is analysed using all year round data, or a shorter data set, as in some previous studies, no relationship is found.

Barclay J, Johnstone JE, Matthews AJ, 2006: Meteorological monitoring of an active volcano: Implications for eruption prediction. J. Volcanol. Geotherm. Res., 150, 339-358.

Rainfall data collected on and around the Soufriere Hills Volcano, Montserrat, between 1998 and 2003 were analysed to assess the impact on primary volcanic activity, defined here as pyroclastic flows, dome collapses, and explosions. Fifteen such rainfall-triggered events were identified. If greater than 20 mm of rain fell on a particular day, the probability of a dome collapse occurring on that day increased by a factor of 6.3 to 9.2%, compared to a randomly chosen day. Similarly, the probability of observing pyroclastic flows and explosions on a day with >20 mm of rainfall increased by factors of 2.6 and 5.4, respectively. These statistically significant links increased as the rainfall threshold was increased. Seventy percent of these rainfall-induced dome collapse episodes occurred on the same calendar day (most within a few hours) as the onset of intense rainfall, but an extra 3 occurred one or two calendar days later. The state of the volcano was important, with the rainfall-volcanic activity link being strongest during periods of unstable dome growth and weakest during periods of no dome growth or after a recent major collapse. Over 50% of the heavy rain days were associated with large-scale weather systems that can potentially be forecast up to a few days ahead. However, the remaining heavy rain days wer associated with small-scale, essentially unpredictable weather systems. There was significant variability in the amount of rainfall recorded by different rain gauges, reflecting topographic variations around the volcano but also the inherent small-scale variability within an individual weather system. Hence, any monitoring/warning program is recommended to use a network, rather than just a single gauge. The seasonal cycle in rainfall was pronounced, with nearly all the heavy rain days occurring in the May-December wet season. Hence, the dome was at its most vulnerable at the beginning of the wet season after a period of uninterrupted growth. Interannual variability in the rainfall was related to tropical Pacific and Atlantic sea surface temperature anomalies, and holds out the prospect of some limited skill in volcanic hazard forecasts at even longer lead times.

Handoh IC, Matthews AJ, Bigg GR, Stevens DP, 2006: Interannual variability of the tropical Atlantic independent of and associated with ENSO: Part I. The North Tropical Atlantic. Int. J. Climatol, 26, 1937-1956.

The interannual variability of the tropical Atlantic ocean-atmosphere system is examined using 50 years of sea-surface temperature (SST) and re-analysis data, and satellite data when available. A singular value decomposition analysis of 12- to 72-month bandpass filtered SST and zonal wind stress reveals two dominant modes of interannual variability. The SST anomalies are confined to the North Tropical Atlantic (NTA) in the first mode and extend over the equatorial and South Tropical Atlantic in the second mode. No evidence is found for an Atlantic SST dipole. The structure of the first (NTA) mode is examined in detail here, while the second mode has been described in a companion paper. In particular, the relationship of the NTA mode with El Nino-Southern Oscillation (ENSO) is investigated. There are 12 NTA events (seven warm and five cold) that are associated with ENSO, and 18 NTA events (seven warm and 11 cold) that are independent of ENSO.

The ENSO-associated NTA events appear to be a passive response to remote ENSO forcing, mainly via a Pacific-North America (PNA)-like wave train that induces SST anomalies over the NTA through changes in the surface wind and latent heat flux. The NTA anomalies peak four months after ENSO. There does not appear to be an atmospheric response to the NTA SST anomalies as convection over the Atlantic is suppressed by the anomalous Walker circulation due to ENSO.

The ENSO-independent NTA events also appear to be induced by an extratropical wave train from the Pacific sector (but one that is independent of Pacific SST), and forcing by the North Atlantic Oscillation (NAO) also contributes. As the event matures, the atmosphere does respond to the NTA SST anomalies, with enhanced convection over the Caribbean and a wave train that propagates northeastward to Europe.

Handoh IC, Bigg GR, Matthews AJ, Stevens DP, 2006: Interannual variability of the tropical Atlantic independent of and associated with ENSO: Part II. The South Tropical Atlantic. Int. J. Climatol, 26, 1957-1976.

Two dominant ocean-atmosphere modes of variability on interannual timescales were defined in Part I of this work, namely, the North Tropical Atlantic (NTA) and South Tropical Atlantic (STA) modes. In this paper we focus on the STA mode that covers the equatorial and sub-tropical South Atlantic. We show that STA events occurring in conjunction with ENSO have a preference for the southern summer season and seem to be forced by an atmospheric wave train emanating from the central tropical Pacific and travelling via South America, in addition to the more direct ENSO-induced change in the Walker circulation. They are lagged by one season from the peak of ENSO. These events show little evidence for other-than-localised coupled ocean-atmosphere interaction.

In contrast, STA events occurring in the absence of ENSO favour the southern winter season. They appear to be triggered by a Southern Hemisphere wave train emanating from the Pacific sector, and then exhibit features of a self-sustaining climate mode in the tropical Atlantic. The southward shift of the inter tropical convergence zone that occurs during the warm phase of such an event triggers an extra tropical wave train that propagates downstream in the Southern Hemisphere. We present a unified view of the NTA and STA modes through our observational analysis of the interannnual tropical Atlantic variability.

Batstone CP, Matthews AJ, Stevens DP, 2005: Coupled ocean-atmosphere interactions between the Madden-Julian Oscillation and synoptic-scale variability over the warm pool. J. Climate, 18, 2004-2020.

A principal component analysis of the combined fields of sea surface temperature (SST) and surface zonal and meridional wind reveals the dominant mode of intraseasonal (30-70-day) co-variability during northern winter in the tropical Eastern Hemisphere is that of the Madden-Julian Oscillation (MJO). Regression calculations show that the submonthly (30-day high-pass filtered) surface wind variability is significantly modulated during the MJO. Regions of increased (decreased) submonthly surface wind variability propagate eastward, approximately in phase with the intraseasonal surface westerly (easterly) anomalies of the MJO. Due to the dependence of the surface latent heat flux on the magnitude of the total wind speed, this systematic modulation of the submonthly surface wind variability produces a significant component in the intraseasonal latent heat flux anomalies, which partially cancels the latent heat flux anomalies due to the slowly varying intraseasonal wind anomalies, particularly south of 10S. A method is derived that demodulates the submonthly surface wind variability from the slowly varying intraseasonal wind anomalies. This method is applied to the wind forcing fields of a one-dimensional ocean model. The model response to this modified forcing produces larger intraseasonal SST anomalies than when the model is forced with the observed forcing over large areas of the southwest Pacific Ocean and southeast Indian Ocean during both phases of the MJO. This result has implications for accurate coupled modeling of the MJO. A similar calculation is applied to the surface shortwave flux, but intraseasonal modulation of submonthly surface shortwave flux variability does not appear to be important to the dynamics of the MJO.

Matthews AJ, Li HYY, 2005: Modulation of station rainfall over the western Pacific by the Madden-Julian Oscillation. Geophys. Res. Lett., 32, L14827, doi: 10.1029/2005GL023595.

Rainfall data from 140 stations in the PACRAIN network in the tropical western Pacific are analysed to assess the signal due to the Madden-Julian Oscillation (MJO). During northern winter, the station rainfall difference between the wet and dry phases of the MJO is up to 6 mm day-1, compared to the climatological mean value of 12 mm day-1. The anomalies have a strong spatial coherence, with over 80% of the individual point station anomalies having the same sign as the large-scale rainfall anomaly, as determined by the mainly satellite-derived CMAP rainfall product.

Matthews AJ, 2004: Intraseasonal variability over tropical Africa during northern summer. J. Climate, 17, 2427-2440.

The intraseasonal variability over Africa during northern summer was analyzed, using 25 years of NCEP/NCAR reanalysis and satellite data. The dominant pattern of variability was one of enhanced deep convection over the whole African monsoon region. It appeared to arise at least partly as a remote response to the intraseasonal (Madden--Julian) oscillation over the warm pool sector. Twenty days prior to the maximum in convection over Africa, there was no signal over Africa but convection was reduced over the equatorial warm pool. An equatorial Kelvin wave response to this change in warm pool convection propagated eastward and an equatorial Rossby wave response propagated westward and between them they completed a circuit of the equator and met up twenty days later over Africa, where the negative mid-tropospheric temperature anomalies in the Kelvin and Rossby wave favoured deep convection. Over West Africa, the Kelvin wave component contained lower-tropospheric westerly anomalies which acted to increase the boundary layer monsoon flow and moisture supply. The westerly anomalies also increased the cyclonic shear on the equatorward flank of the African easterly jet, leading to enhanced African easterly wave and transient convective activity, which then contributed to the enhanced convection over Africa on the longer intraseasonal time scale. The implications of this intraseasonal mode for predictability over Africa are discussed.

Matthews AJ, 2004: The atmospheric response to observed intraseasonal tropical sea surface temperature anomalies. Geophys. Res. Lett., 31, L14107, doi: 10.1029/2004GL020474.

The major tropical convective and circulation features of the intraseasonal or Madden-Julian Oscillation (MJO) are simulated as a passive response to observed MJO sea surface temperature (SST) anomalies in an atmospheric general circulation model (AGCM), strengthening the case for ocean-atmosphere interactions being central to MJO dynamics. However, the magnitude of the surface fluxes diagnosed from the MJO cycle in the AGCM, that would feed back onto the ocean in a coupled system, are much weaker than in observations. The phasing of the convective-dynamical model response to the MJO SST anomalies and the associated surface flux anomalies is too fast compared to observations of the (potentially) coupled system, and would act to damp the SST anomalies.

Matthews AJ, Hoskins BJ, Masutani M, 2004: The global response to tropical heating in the Madden-Julian Oscillation during northern winter. Quart. J. Roy. Meteorol. Soc., 130, 1991-2011.

A life cycle of the Madden-Julian Oscillation (MJO) was constructed, based on 21 years of outgoing longwave radiation data. Regression maps of NCEP-NCAR reanalysis data for northern winter show statistically significant upper-tropospheric equatorial wave patterns linked to the tropical convection anomalies, and extratropical wave patterns over the North Pacific, North America, the Atlantic, the Southern Ocean and South America. To assess the cause of the circulation anomalies, a global primitive equation model was initialised with the observed three-dimensional winter climatological-mean flow and forced with a time-dependent heat source derived from the observed MJO anomalies. A model MJO cycle was constructed from the global response to the heating, and both the tropical and extratropical circulation anomalies generally matched the observations well. The equatorial wave patterns are established in a few days, while it takes approximately two weeks for the extratropical patterns to appear. The model response is robust and insensitive to realistic changes in damping and basic state. The model tropical anomalies are consistent with a forced equatorial Rossby--Kelvin wave response to the tropical MJO heating, although it is shifted westward by approximately 20 longitude relative to observations. This may be due to a lack of damping processes (cumulus friction) in the regions of convective heating. Once this shift is accounted for, the extratropical response is consistent with theories of Rossby wave forcing and dispersion on the climatological flow, and the pattern correlation between the observed and modelled extratropical flow is up to 0.85. The observed tropical and extratropical wave patterns accounts for a significant fraction of the intraseasonal circulation variance, and this reproducibility as a response to tropical MJO convection has implications for global medium-range weather prediction.

Matthews AJ, Barclay J, 2004: A thermodynamical model for rainfall-triggered volcanic dome collapse. Geophys. Res. Lett., 31, L05614, doi: 10.1029/2003GL019310.

Dome-forming volcanic eruptions typically involve the slow extrusion of viscous lava onto a steep-sided volcano punctuated by collapse and the generation of hazardous pyroclastic flows. We show an unequivocal link between the onset of intense rainfall and lava dome collapse on short time scales (within a few hours) and develop a simple thermodynamical model to explain this behavior. The model is forced with rainfall observations from the Soufriere Hills Volcano, Montserrat, and suggests that when the dome is in a critical state, a minimum rainfall rate of approximately 15 mm hr-1 for 2-3 hr could trigger a dome collapse.

Matthews AJ, Meredith MP, 2004: Variability of Antarctic circumpolar transport and the southern annular mode associated with the Madden-Julian Oscillation. Geophys. Res. Lett., 31, L24312, doi: 10.1029/2004GL021666.

The variability of oceanic Antarctic circumpolar transport and the atmospheric Southern Annular Mode (SAM) on intraseasonal (30-70-day) time scales is shown to be related to the tropical atmospheric Madden--Julian Oscillation (MJO) during southern winter. Approximately 7 days after anomalous MJO convection in the equatorial Indian Ocean peaks, an atmospheric extratropical response is set up with anomalous surface westerlies around almost the entire latitude circle at 60S. This pattern projects strongly onto the SAM and leads to an acceleration of the eastward circumpolar transport around Antarctica, as measured by tide gauges and bottom pressure recorders. This ocean response is confirmed by a global ocean model, which shows a maximum in the transport through Drake Passage 3 days after the atmospheric extratropical response.

Matthews AJ, Barclay J, Carn S, Thompson G, Alexander J, Herd R, Williams C, 2002: Rainfall-induced volcanic activity on Montserrat. Geophys. Res. Lett., 29, 1644, doi: 10.1029/2002GL014863.

Dome-forming volcanic eruptions cyclically extrude bodies of lava over several months, which then become gravitationally unstable and collapse, generating pyroclastic flows. On 29 July 2001 extreme rainfall over Montserrat coincided with a major collapse of the Soufriere Hills lava dome. We present rainfall and seismic records that demonstrate, for the first time, a relationship between intense rainfall and lava dome collapse, with associated pyroclastic flow generation. After seven months of little rain and a period of sustained dome growth, the onset of intense rain was followed within hours by dome collapse and pyroclastic flows. The large-scale weather system responsible for the rain was identifiable in satellite images and predicted by meteorological forecasts issued 60 hours prior to the volcanic activity. It is suggested that weather prediction of intense rainfall be incorporated with existing geophysical and geochemical measurements to improve warnings of these hazardous events.

Hall JD, Matthews AJ, Karoly DJ, 2001: The modulation of tropical cyclone activity in the Australian region by the Madden-Julian Oscillation. Mon. Wea. Rev., 129, 2970-2982.

The observed relationship between tropical cyclone activity in the Australian region and the Madden-Julian Oscillation (MJO) has been examined using 20 years of outgoing longwave radiation, NCEP-NCAR reanalysis and best track tropical cyclone data. The MJO strongly modulates the climatological pattern of cyclogenesis in the Australian region, where significantly more (fewer) cyclones form in the active (inactive) phase of the MJO. This modulation is more pronounced to the north-west of Australia. The relationship between tropical cyclone activity and the MJO was strengthened during El Nino periods. Variation of the large-scale dynamical conditions necessary for cyclogenesis were explored, and it was found that MJO-induced perturbations of these parameters correspond with the observed variation in cyclone activity. In particular, 850-hPa relative vorticity anomalies attributable to the MJO were found to be an excellent diagnostic of the changes in the large-scale cyclogenesis patterns.

Meehl GA, Lukas R , Kiladis GN, Weickmann KM, Matthews AJ, Wheeler M, 2001: A conceptual framework for time and space scale interactions in the climate system. Climate Dyn., 17, 753-775.

Interactions involving various space and time scales, both within the tropics and between the tropics and midlatitudes, are ubiquitous in the climate system. The concept of longer time scales and larger space scales setting the base state for processes on shorter time scales and smaller space scales is explored. Decadal time scale base states of the coupled climate system set the context for the manifestation of interannual time scales (El Nino/Southern Oscillation, ENSO and tropospheric biennial oscillation, TBO) which are influenced by and interact with the annual cycle and seasonal time scales. Those base states in turn influence the large-scale coupled processes involved with intraseasonal and submonthly time scales, tied to tropical-tropical and tropical-midlatitude teleconnections. All of these set the base state for processes on the synoptic and mesoscale and regional/local space scales. Events at those relatively short time scales and small space scales may then affect the longer time scale and larger space scale processes in turn, reaching back out to submonthly, intraseasonal, seasonal, annual, TBO, ENSO and decadal. Global coupled models can capture some elements of the decadal, ENSO, TBO, annual and seasonal time scales with the associated global space scales. However, coupled models are less successful at simulating phenomena at subseasonal and shorter time scales with hemispheric and smaller space scales. Due to the synergistic interactions of the time and space scales, a high priority must be placed on improved simulations of all the time and space scales in the climate system (particularly the subseasonal time scales and hemispheric and smaller space scales which are not well simulated at present) if we hope to successfully forecast phenomena beyond the synoptic scales.

Matthews AJ, 2000: Propagation mechanisms for the Madden-Julian Oscillation. Quart. J. Roy. Meteorol. Soc., 126, 2637-2652.

The Madden-Julian Oscillation (MJO) is examined using 20 years of outgoing longwave radiation and NCEP-NCAR reanalysis data. Two mechanisms for the eastward propagation and regeneration of the convective anomalies are suggested.

The first is a local mechanism operating over the warm pool region. At the phase of the MJO with a dipole structure to the convection anomalies, there is enhanced tropical convection over the eastern Indian Ocean and reduced convection over the western Pacific. Over the equatorial western Indian Ocean, the equatorial Rossby wave response to the west of the enhanced convection includes a region of anomalous surface divergence associated with the anomalous surface westerlies and pressure ridge. This tends to suppress ascent in the boundary layer and shuts off the deep convection, eventually leading to a convective anomaly of the opposite sign. Over the Indonesian sector, the equatorial Kelvin wave response to the east of the enhanced convection includes a region of anomalous surface convergence into the anomalous equatorial surface easterlies and pressure trough, which will tend to favour convection in this region. The Indonesian sector is also influenced by an equatorial Rossby wave response (of opposite sign) to the west of the reduced convection over the western Pacific, which also has a region of anomalous surface convergence associated with its anomalous equatorial surface easterlies and pressure trough. Hence, convective anomalies of either sign tend to erode themselves from the west and initiate a convective anomaly of opposite sign via their equatorial Rossby wave response, and expand to the east via their equatorial Kelvin wave response.

The second is a global mechanism involving an anomaly completing a circuit of the equator. Enhanced convection over the tropical western Pacific excites a negative sea level pressure (SLP) anomaly which radiates rapidly eastward as a dry equatorial Kelvin wave at approximately 35 m s-1 over the eastern Pacific. It is blocked by the orographic barrier of the Andes and Central America for several days before propagating through the gap at Panama. After rapidly propagating as a dry equatorial Kelvin wave over the Atlantic, the SLP anomaly is delayed further by the East African Highlands before it reaches the Indian Ocean and coincides with the development of enhanced convection at the start of the next MJO cycle.

Matthews AJ, Kiladis GN, 2000: A model of Rossby waves linked to submonthly convection over the eastern tropical Pacific. J. Atmos. Sci., 57, 3785-3798.

Equatorward-propagating wave trains in the upper troposphere are observed to be associated with deep convection over the eastern tropical Pacific on the submonthly time scale during northern winter (Fig. 1). The convection occurs in the regions of ascent and reduced static stability ahead of cyclonic anomalies in the wave train. In this study an atmospheric primitive-equation model is used to examine the roles of the dry wave dynamics and the diabatic heating associated with the convection.

Many features of a dry integration initialized with a localized wave train in the African--Asian jet on a three-dimensional climatological basic state quantitatively agree with the observations, including the zonal-wavenumber 6--7 scale of the waves, the time period of approximately 12 days and the cross-equatorial Rossby wave propagation over the eastern Pacific. There is ascent and reduced static stability ahead of the cyclonic anomalies, consistent with the interpretation of the waves forcing the convection. The spatial scale of the waves appears to be set by the basic state; baroclinic growth upstream in the Asian jet favors waves with zonal-wavenumber 6. On reaching the Pacific sector, lower-wavenumber components of the wave train are not refracted so strongly equatorward, while higher-wavenumber components are advected quickly along the Pacific jet before they can propagate equatorward. Once over the Pacific, the wave train approximately obeys barotropic Rossby wave dynamics.

The observed lower-tropospheric anomalies include an equatorial Rossby wave that propagates westward from the region of cross-equatorial wave propagation and tropical convection. However, this equatorial Rossby wave is not triggered directly by the equatorward-propagating wave train, but appears in a separate integration as a forced response to the observed diabatic heating associated with the tropical convection.

Matthews AJ, Madden RA, 2000: Observed propagation and structure of the 33-h atmospheric Kelvin wave. J. Atmos. Sci., 57, 3488-3497.

The structure of the 33-hour Kelvin wave, a normal mode of the atmosphere, is examined in 6-hourly station and NCEP--NCAR reanalysis data. Cross-spectral analysis of 6 years (1993-98) of tropical station pressure data shows a peak in coherence in a narrow frequency band centered near 0.74 cycles per day, corresponding to a period of approximately 33 hours. The phase angles are consistent with an eastward-propagating zonal-wavenumber-one structure, implying an equatorial phase speed of approximately 340 m s-1. The global structure of the mode is revealed by empirical orthogonal function and regression analysis of 31 years (1968--98) of reanalysis data. The horizontal structure shows a zonal-wavenumber-one equatorial Kelvin wave with an equatorial trapping scale of approximately 34 degrees latitude. The vertical structure has zero phase change. The amplitude of the wave is approximately constant in the troposphere with an equatorial geopotential height perturbation of 0.9 m, and then increases exponentially with height in the stratosphere. Cross-spectral analysis between the station and reanalysis data shows that the results from the two data sets are consistent. No evidence can be found for forcing of the wave by deep tropical convection, which is is examined using a twice-daily outgoing longwave radiation data set.

Matthews AJ, Kiladis GN, 1999: The tropical-extratropical interaction between high-frequency transients and the Madden-Julian Oscillation. Mon. Wea. Rev., 127, 661-677.

The interaction between high-frequency transient disturbances and convection, and the Madden--Julian Oscillation (MJO), is investigated using NCEP-NCAR reanalysis and satellite outgoing longwave radiation data for 15 northern winters. During the phase of the MJO with enhanced convection over the East Indian Ocean and Indonesia, and suppressed convection over the South Pacific convergence zone, both the Asian--Pacific jet and the region of upper-tropospheric tropical easterlies over the warm pool are displaced westward. These changes in the basic state lead to a weaker or ``leakier'' waveguide in the Asian--Pacific jet, with a westward-displaced ``forbidden'' region of tropical easterlies, such that high-frequency transient waves propagate equatorwards into the deep tropics over the central Pacific near the date line. As these waves induce convection in the region of ascent and reduced static stability ahead of the upper-level cyclonic disturbances, there is an enhancement of high-frequency convective variability over the central Pacific intertropical convergence zone during this phase of the MJO. This enhanced high-frequency convective variability appears to project back onto intraseasonal timescales, and forms an integral part of the slowly varying diabatic heating field of the MJO. In the opposite phase of the MJO, the Asian--Pacific jet is extended eastward and there is an almost continuous waveguide across the Pacific. Together with the expanded forbidden region of tropical easterlies over the warm pool, this leads to a more zonal propagation of high-frequency transients along the waveguide with less equatorward propagation, and hence reduced high-frequency convective variability over the tropical central Pacific. There is also evidence of high-frequency waves propagating into the Indian Ocean region at the beginning of the MJO cycle, which may be important in the initiation of intraseasonal convective anomalies there.

Matthews AJ, Kiladis GN, 1999: Interactions between ENSO, transient circulation and tropical convection over the Pacific. J. Climate, 12, 3062-3086.

The interannual variability of transient waves and convection over the central and eastern Pacific is examined using 30 northern winters of NCEP-NCAR reanalyses (1968/69 - 1997/98) and satellite outgoing longwave radiation data starting in 1974. There is a clear signal associated with the El Nino-Southern Oscillation, such that differences in the seasonal-mean basic state lead to statistically significant changes in the behavior of the transients and convection (with periods less than 30 days), which then feed back onto the basic state. During a warm event (El Nino phase), the Northern Hemisphere subtropical jet is strengthened over the central Pacific; the region of upper-tropospheric mean easterlies over the tropical western Pacific expands eastwards past the dateline, and the upper-tropospheric mean ``westerly duct'' over the tropical eastern Pacific is weakened. The transients tend to propagate along the almost continuous waveguide of the subtropical jet; equatorward propagation into the westerly duct is reduced. The transient convective events over the ITCZ typically observed to be associated with these equatorward-propagating waves are subsequently reduced both in number and magnitude, leading to a seasonal-mean net negative diabatic heating anomaly over the central Pacific from 10N-20N, which then feeds back onto the basic state. During a cold event (La Nina phase), the situation is reversed. The different propagation characteristics of the transients in El Nino and La Nina basic states are well simulated in initial value experiments with a primitive equation model.

Matthews AJ, Slingo JM, Hoskins BJ, Inness PM, 1999: Fast and slow Kelvin waves in the Madden-Julian Oscillation of a GCM. Quart. J. Roy. Meteorol. Soc., 125, 1473-1498.

The structure of the Madden-Julian Oscillation (MJO) in an 1800-day integration of the Hadley Centre Unified Model was analysed, and interpreted within a Kelvin wave framework. The model was forced with constant equinoctial (March) boundary conditions so that a clean MJO signal could be separated from the effects of the seasonal cycle and forced interannual variability. The simulated MJO was fairly realistic in terms of its large-scale spatial structure and propagation characteristics, although its period of 30 days (corresponding to an average phase speed of 15 m s-1) was shorter than that observed. The signal in deep convection was less coherent than in observations, and appeared to move eastward as a sequence of discrete convective anomalies, rather than by a smooth eastward propagation. Both "fast" and "slow" equatorial Kelvin waves appeared to play an important role in the eastward propagation of the simulated MJO. Enhanced convection over the Indian Ocean was associated with a fast equatorial Kelvin wave that propagated eastward at 55 m s-1 over the Pacific. On reaching the west coast of South America, a component of this Kelvin wave propagated northward and southward as a trapped wave along the mountain ranges of Central America and the Andes, in agreement with observations. The anomalous surface easterlies over the tropical eastern Pacific associated with this fast Kelvin wave enhanced the climatological mean easterlies and led to positive convective anomalies over the eastern Pacific consistent with the WISHE mechanism. However, WISHE was not able to account for the eastward development of the convective anomalies over the Indian Ocean/western Pacific region. By splitting the equatorial divergence anomalies of the simulated MJO into their du/dx and dv/dy components, the role of Kelvin wave dynamics in the slow (15 m s-1) average eastward propagation of the simulated MJO was examined. Although the two components were of comparable magnitude, the du/dx component exhibited a pronounced eastward propagation which tended to be disrupted by the dv/dy component, thus supporting the paradigm of an underlying, but strongly modified, Kelvin wave mechanism.

Matthews AJ, Lander J, 1999: Physical and numerical contributions to the structure of Kelvin wave-CISK modes in a spectral transform model. J. Atmos. Sci., 56, 4050-4058.

Nonlinear Kelvin wave-CISK modes are critically assessed as a possible mechanism for the Madden-Julian Oscillation (MJO) with a global spectral transform model and a one-dimensional analog. Convection is parametrized using a simple "positive-only CISK" scheme, where tropospheric diabatic heating is proportional to the low-level convergence, and is set to zero in regions of low-level divergence. Although the modes have many properties that are consistent with the MJO, they also have a serious drawback. The growth rate of unstable modes depends crucially on the width of the heating region, which is resolution dependent. The "CISK catastrophe" has not been averted, and the heating region collapses to the smallest localized scale that the model can support. This scale is larger than the model resolution, as measured by both the gridpoint-scale and the inverse wavenumber or half-zonal-wavelength of the highest wavenumber basis function, and is associated with the appearance of negative Gibbs fringes which are then cut off by the positive-only CISK parametrization.

Matthews AJ, Hoskins BJ, Slingo JM, Blackburn M, 1996: Development of convection along the SPCZ within a Madden-Julian Oscillation. Quart. J. Roy. Meteorol. Soc., 122, 669-688.

A subtropical Rossby wave propagation mechanism is proposed to account for the poleward and eastward progression of intraseasonal convective anomalies along the South Pacific Convergence Zone (SPCZ) that is observed in a significant proportion of Madden-Julian Oscillations (MJOs). Large scale convection, associated with an MJO, is assumed to be already established over the Indonesian region. The latent heating associated with this convection forces an equatorial Rossby wave response with an upper tropospheric anticyclone, centred over or slightly to the west of the convection. Large potential vorticity (PV) gradients, associated with the subtropical jet and the tropopause, lie just poleward of the anticyclone and large magnitude PV air is advected equatorwards on the eastern side of the anticyclone. This ``high'' PV air, or upper tropospheric trough, is far enough off the equator that it has associated strong horizontal temperature gradients, and it induces deep ascent on its eastern side, at a latitude of about 15-30\degr. If this deep ascent is over a region susceptible to deep convection, such as the SPCZ region, then convection may be forced or triggered. Hence convection develops along the SPCZ as a forced response to convection over Indonesia. The response mechanism is essentially one of subtropical Rossby wave propagation. This hypothesis is based on a case study of a particularly strong MJO in early 1988, and is tested by idealised modelling studies. The mechanism may also be relevant to the existence of the mean SPCZ, as a forced response to mean Indonesian convection.

Slingo JM, Sperber KR, Boyle JS, Ceron JP, Dix M, Dugas B, Ebisuzaki W, Fyfe J, Gregory D, Gueremy JF, Hack J, Harzallah A, Inness P, Kitoh A, Lau WKM, McAvaney B, Madden R, Matthews AJ, Palmer TN, Park CK, Randall D, Renno N, 1996: Intraseasonal oscillations in 15 atmospheric general circulation models: results from an AMIP diagnostic subproject. Climate Dyn., 12, 325-358.

The ability of 15 atmospheric GCM models (AGCM) to simulate the tropical intraseasonal oscillation has been studied as part of AMIP. Time series of the daily upper tropospheric velocity potential and zonal wind, averaged over the equatorial belt, were provided from each AGCM simulation. These data were analyzed using a variety of techniques such as time filtering and space-time spectral analysis to identify eastward and westward propagating waves. The results have been compared with an identical assessment of ECMWF analyses for the period 1982-1991. The models display a wide range of skill in simulating the intraseasonal oscillation. Most models show evidence of an eastward propagating anomaly in the velocity potential field, although in some models there is a greater tendency for a standing oscillation, and in one or two the field is rather chaotic with no preferred direction of propagation. Where a model has a clear eastward propagating signal, typical periodicities seem quite reasonable although there is a tendency for the models to simulate shorter periods than in the ECMWF analyses, where it is near 50 days. The results of the space-time spectral analysis have shown that no model has captured the dominance of the intraseasonal oscillation found in the analyses. Several models have peaks at intraseasonal time scales, but nearly all have relatively more power at higher frequencies (<30 days) than the analyses. Most models underestimate the strength of the intraseasonal variability. The observed intraseasonal oscillation shows a marked seasonality in its occurrence with greatest activity during northern winter and spring. Most models failed to capture this seasonality. The interannual variability in the activity of the intraseasonal oscillation has also been assessed, although the AMIP decade is too short to provide any conclusive results. There is a suggestion that the observed oscillation was suppressed during the strong El Nino of 1982/83, and this relationship has also been reproduced by some models. The relationship between a model's intraseasonal activity, its seasonal cycle and characteristics of its basic climate has been examined. It is clear that those models with weak intraseasonal activity tend also to have a weak seasonal cycle. It is becoming increasingly evident that an accurate description of the basic climate may be a prerequisite for producing a realistic intraseasonal oscillation. In particular, models with the most realistic intraseasonal oscillations appear to have precipitation distributions which are better correlated with warm sea surface temperatures. These models predominantly employ convective parametrizations which are closed on buoyancy rather than moisture convergence.

Created: Mon Apr 24 02:02:20 2017