Convective Anomalies Research Articles

On the Connection between Continental-Scale Land Surface Processes and the Tropical Climate in a Coupled Ocean–Atmosphere–Land System

AbstractAn evaluation is presented of the impact on tropical climate of continental-scale perturbations given by different representations of land surface processes (LSPs) in a general circulation model that includes atmosphere–ocean interactions. One representation is a simple land scheme, which specifies climatological albedos and soil moisture availability. The other representation is the more comprehensive Simplified Simple Biosphere Model, which allows for interactive soil moisture and vegetation biophysical processes.The results demonstrate that such perturbations have strong impacts on the seasonal mean states and seasonal cycles of global precipitation, clouds, and surface air temperature. The impact is especially significant over the tropical Pacific Ocean. To explore the mechanisms for such impact, model experiments are performed with different LSP representations confined to selected continental-scale regions where strong interactions of climate–vegetation biophysical processes are present. The largest impact found over the tropical Pacific is mainly from perturbations in the tropical African continent where convective heating anomalies associated with perturbed surface heat fluxes trigger global teleconnections through equatorial wave dynamics. In the equatorial Pacific, the remote impacts of the convection anomalies are further enhanced by strong air–sea coupling between surface wind stress and upwelling, as well as by the effects of ocean memory. LSP perturbations over South America and Asia–Australia have much weaker global impacts. The results further suggest that correct representations of LSP, land use change, and associated changes in the deep convection over tropical Africa are crucial to reducing the uncertainty of future climate projections with global climate models under various climate change scenarios.

Comparison of the structure and evolution of intraseasonal oscillations before and after onset of the Asian summer monsoon

High-resolution satellite-derived data and NCEP-NCAR reanalysis data are used to investigate intraseasonal oscillations (ISO) over the tropical Indian Ocean. A composite evolution of the ISO life cycle is constructed, including the initiation, development, and propagation of rainfall anomalies over the tropical Indian Ocean. The characteristics of ISO over the tropical Indian Ocean are profoundly different before and after the onset of the Indian summer monsoon. Positive precipitation anomalies before monsoon onset appear one phase earlier than those after monsoon onset. Before monsoon onset, precipitation anomalies associated with ISO first initiate in the western tropical Indian Ocean and then propagate eastward along the equator. After monsoon onset, convective anomalies propagate northward over the Indian summer monsoon region after an initial eastward propagation over the equatorial Indian Ocean. Surface wind convergence and air-sea interaction play critical roles in initiating each new cycle of ISO convection.

Evaluation of mean and intraseasonal variability of Indian summer monsoon simulation in ECHAM5: identification of possible source of bias

The performance of ECHAM5 atmospheric general circulation model (AGCM) is evaluated to simulate the seasonal mean and intraseasonal variability of Indian summer monsoon (ISM). The model is simulated at two different vertical resolutions, with 19 and 31 levels (L19 and L31, respectively), using observed monthly mean sea surface temperature and compared with the observation. The analyses examine the biases present in the internal dynamics of the model in simulating the mean monsoon and the evolution of the boreal summer intraseasonal oscillation (BSISO) and attempts to unveil the reason behind them. The model reasonably simulates the seasonal mean-state of the atmosphere during ISM. However, some notable discrepancies are found in the simulated summer mean moisture and rainfall distribution. Both the vertical resolutions, overestimate the seasonal mean precipitation over the oceanic regions, but underestimate the precipitation over the Indian landmass. The performance of the model improves with the increment of the vertical resolution. The AGCM reasonably simulates some salient features of BSISO, but fails to show the eastward propagation of the convection across the Maritime Continent in L19 simulation. The propagation across the Maritime Continent and tilted rainband structure improve as one moves from L19 to L31. The model unlikely shows prominent westward propagation that originates over the tropical western Pacific region. L31 also produces some of the observed characteristics of the northward propagating BSISOs. However, the northward propagating convection becomes stationary in phase 5–7. The simulation of shallow diabatic heating structure and the heavy rainfall activity over the Bay of Bengal indicate the abundance of the premature convection-generated precipitation events in the model. It is found that the moist physics is responsible for the poor simulation of the northward propagating convection anomalies.

Examination of multi-perturbation methods for ensemble prediction of the MJO during boreal summer

The impact of initialization and perturbation methods on the ensemble prediction of the boreal summer intraseasonal oscillation was investigated using 20-year hindcast predictions of a coupled general circulation model. The three perturbation methods used in the present study are the lagged-averaged forecast (LAF) method, the breeding method, and the empirical singular vector (ESV) method. Hindcast experiments were performed with a prediction interval of 10 days for extended boreal summer (May–October) seasons over a 20 year period. The empirical orthogonal function (EOF) eigenvectors of the initial perturbations depend on the individual perturbation method used. The leading EOF eigenvectors of the LAF perturbations exhibit large variances in the extratropics. Bred vectors with a breeding interval of 3 days represent the local unstable mode moving northward and eastward over the Indian and western Pacific region, and the leading EOF modes of the ESV perturbations represent planetary-scale eastward moving perturbations over the tropics. By combining the three perturbation methods, a multi-perturbation (MP) ensemble prediction system for the intraseasonal time scale was constructed, and the effectiveness of the MP prediction system for the Madden and Julian oscillation (MJO) prediction was examined in the present study. The MJO prediction skills of the individual perturbation methods are all similar; however, the MP‐based prediction has a higher level of correlation skill for predicting the real-time multivariate MJO indices compared to those of the other individual perturbation methods. The predictability of the intraseasonal oscillation is sensitive to the MJO amplitude and to the location of the dominant convective anomaly in the initial state. The improvement in the skill of the MP prediction system is more effective during periods of weak MJO activity.

Atmospheric aerosol properties over the equatorial Indian Ocean and the impact of the Madden‐Julian Oscillation

The chemical, physical, and optical properties of sub‐ and supermicrometer aerosols over the equatorial Indian Ocean were measured on board the R/V Revelle during the fall 2011 Dynamics of the Madden‐Julian Oscillation field campaign. During this time, both the retreating of the Asian monsoon and two Madden‐Julian Oscillation (MJO) events were observed. The R/V Revelle was on station (0.1°N and 80.5°E) to measure atmospheric and oceanic conditions between 4 October and 30 October 2011 (Leg 2) and 11 November and 4 December 2011 (Leg 3). Throughout the campaign, background marine atmospheric conditions were usually observed. As the Asian monsoon season retreated over the boreal fall and the general wind direction changed from southerly to northerly transporting, respectively, clean marine and polluted continental air masses, the average submicrometer aerosol mass nearly doubled from Leg 2 to Leg 3 and the aerosol appeared to be more influenced by continental sources. The effect of MJO‐associated convection anomalies on aerosols in the remote marine boundary layer (MBL) was measured during November when a complete MJO convection wave moved over the equatorial Indian Ocean and during October when a partial MJO event was observed. MJO‐associated convection strongly affected the local aerosol as increased vertical mixing introduced new particles into the MBL, rainout cleared the atmosphere of submicrometer aerosol particles, and high winds enhanced the concentration of sea salt aerosol particles in the local atmosphere. Four stages of MJO‐affected aerosol population changes in the remote Indian Ocean are defined.

Simulation of boreal summer intraseasonal oscillations in the latest CMIP5 coupled GCMs

AbstractThe simulation of the boreal summer intraseasonal oscillation (BSISO) has been analyzed in the historical run of the 32 models, which participated in the Coupled Model Intercomparison Project phase5 (CMIP5), and it is shown that the current state‐of‐the‐art general circulation models (GCMs) still have difficulties to properly simulate the BSISO. Compared to CMIP3 models, more CMIP5 models simulated the northward propagation of BSISO. The majority of the models could not simulate the spatial pattern of BSISO variance over the Asian summer monsoon (ASM) region and many of them failed to capture all the three peak centers of BSISO variance over the Indian summer monsoon region. Many of the models underestimated the BSISO variance over the equatorial Indian Ocean, and it is associated with the seasonal mean dry biases over this region. A reasonable representation of the intraseasonal sea surface temperature and its coupling to the convection over the equatorial Indian Ocean and an equatorial eastward propagation of convective anomalies beyond 100°E assure realistic simulation of BSISO over the ASM domain. We found that the models MIROC5, IPSL‐CM5A‐LR, GFDL‐CM3, CMCC‐CM, and MPI‐ESM‐LR are able to represent reasonable BSISO characteristics and can be used to unravel the modulation of BSISO characteristics due to various projected climate changes.

Austral Summer Teleconnections of Indo-Pacific Variability: Their Nonlinearity and Impacts on Australian Climate

Abstract In austral summer, El Niño–Southern Oscillation (ENSO) covaries with the Indian Ocean Basin Mode (IOBM) and with the southern annular mode (SAM). The present study addresses how the IOBM and the SAM modulate the impact of ENSO on Australia. The authors show that the modulating effect of the SAM is limited; in particular, the SAM does not modify the ENSO teleconnection pattern. However, the IOBM extends ENSO-induced convection anomalies westward over northern Australia and over the eastern Indian Ocean, whereby extending the ENSO tropical teleconnection to the northwest of Australia. The IOBM also generates an equivalent-barotropic Rossby wave train through convection anomalies over northern Australia. The wave train shares an anomaly center over the Tasman Sea latitudes with the Pacific–South American (PSA) pattern, shifting the anomaly center of the PSA pattern to within a closer proximity to Australia. There is a strong asymmetry in the IOBM modulating effect. During an IOBM negative phase, which tends to coincide with La Niña events, the rainfall increase is far greater than the reduction during a positive IOBM phase, which tends to coincide with El Niño events. This modulation asymmetry is consistent with an asymmetry in the ENSO–rainfall teleconnection over Australia, in which the La Niña–rainfall teleconnection is stronger than the El Niño–rainfall teleconnection. This asymmetric ENSO–rainfall teleconnection ensures a higher coherence of northern Australia convective anomalies with La Niña or with a negative phase of the IOBM, hence a greater modification of the PSA pattern, underpinning the asymmetric modulating role of the IOBM.

Simulation of a persistent snow storm over southern China with a regional atmosphere-ocean coupled model

A regional atmosphere-ocean coupled model, RegCM3-POM, was developed by coupling the regional climate model (RegCM3) with the Princeton Ocean Model (POM). The performance of RegCM3-POM in simulating a persistent snow storm over southern China and the impact of the Madden-Julian oscillation (MJO) on this persistent snow storm were investigated. Compared with the stand-alone RegCM3, the coupled model performed better at reproducing the spatial-temporal evolution and intensity of the precipitation episodes. The power spectral analysis indicated that the coupled model successfully captured the dominant period between 30 and 60 days in the precipitation field, leading to a notable improvement in simulating the magnitude of intraseasonal precipitation variation, and further in enhancing the intensity of the simulated precipitation. These improvements were mainly due to the well-simulated low-frequency oscillation center and its eastward propagation characteristics in each MJO phase by RegCM3-POM, which improved the simulations of MJO-related low-frequency vertical motions, water vapor transport, and the deep inversion layer that can directly influence the precipitation event and that further improved the simulated MJO-precipitation relationship. Analysis of the phase relationship between convection and SST indicated that RegCM3-POM exhibits a near-quadrature relation between the simulated convection and SST anomalies, which was consistent with the observations. However, such a near-quadrature relation was not as significant when the stand-alone RegCM3 was used. This difference indicated that the inherent coupled feedback process between the ocean and atmosphere in RegCM3-POM played an important part in reproducing the features of the MJO that accompanied the snow storm.

Effects of the cold tongue in the South China Sea on the monsoon, diurnal cycle and rainfall in the Maritime Continent

AbstractWe investigate the effects of the cold tongue in the South China Sea (SCS) on the winter monsoon, rainfall and diurnal cycle in the maritime continent using a numerical model verified with satellite rainfall and reanalysis data. Composite analysis of the observation and reanalysis data based on cold tongue index indicates that the penetration of the monsoon to Java Sea is enhanced when the cold tongue is strong. A sensitivity experiment without the cold tongue shows that the winter monsoon is diminished over SCS and around coastal regions because of anomalous low‐level cyclonic circulation associated with enhanced convection over SCS due to the warmer sea surface temperature (SST). The diurnal cycle, in particular the night–morning rainfall, over the sea in coastal regions is modified. The effect on daytime rainfall over the land is relatively weaker. Along the northern coast of Java far from the SCS, the night–morning rainfall is much reduced over the Java Sea when the cold tongue is suppressed because of the weakened land‐breeze front as a result of the weakened northerly monsoon. In contrast, the afternoon–evening rainfall on Java Island is slightly enhanced, showing that the local impacts are not simply the result of large‐scale subsidence from the convective anomaly in the SCS. Along the northwestern coast of Borneo adjacent to the SCS, the weakened winter monsoon tends to reduce the rainfall at the land‐breeze front near the coastline. On the other hand, the warmer SST forces a stronger land breeze and the weakened monsoon encourages further and faster offshore propagation of the land‐breeze front. Consequently, the rainfall peak shifts further offshore in the sensitivity experiment. We conclude that the cold tongue has two effects, the sustenance of a strong monsoon (indirect effect) and the cooling of local SST (direct effect), which have opposite influences on the diurnal cycle in the Maritime Continent.

Interannual Variations of Wind Regimes off the Subtropical Western Australia Coast during Austral Winter and Spring

Abstract Off the Western Australia coast, interannual variations of wind regime during the austral winter and spring are significantly correlated with the Indian Ocean dipole (IOD) and the southern annular mode (SAM) variability. Atmospheric general circulation model experiments forced by an idealized IOD sea surface temperature anomaly field suggest that the IOD-generated deep atmospheric convection anomalies trigger a Rossby wave train in the upper troposphere that propagates into the southern extratropics and induces positive geopotential height anomalies over southern Australia, independent of the SAM. The positive geopotential height anomalies extended from the upper troposphere to the surface, south of the Australian continent, resulting in easterly wind anomalies off the Western Australia coast and a reduction of the high-frequency synoptic storm events that deliver the majority of southwest Australia rainfall during austral winter and spring. In the marine environment, the wind anomalies and reduction of storm events may hamper the western rock lobster recruitment process.

Intraseasonal temperature variability in the upper troposphere and lower stratosphere from the GPS radio occultation measurements

In this study, we examine the detailed spatiotemporal patterns and vertical structure of the intraseasonal temperature variability in the upper troposphere and lower stratosphere (UTLS) associated with the Madden‐Julian Oscillation (MJO) using the temperature profiles from the recent Global Positioning System radio occultation (GPS RO) measurements including the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission. The MJO‐related temperature anomalies in the UTLS are smaller near the equator (<0.6 K) than over the subtropics and extratropics (>1.2 K). Near the equator, the temperature anomalies exhibit an eastward tilt with height from the upper troposphere (UT) to the lower stratosphere (LS) and their magnitudes and signs are determined by the strength of convective anomalies and vertical pressure level. The subtropical temperature anomalies have similar magnitudes and patterns at a given location between the UT (250 hPa to 150 hPa) and the LS (150 hPa to 50 hPa) except for opposite signs that change around 150 hPa. The subtropical warm (cold) anomalies in the UT and cold (warm) anomalies in the LS are typically collocated with the subtropical positive (negative) tropopause height anomalies/cyclones (anticyclones) and flank or lie to the west of equatorial enhanced (suppressed) convection. We also compare the intraseasonal temperature variability in the UTLS related to the MJO between the GPS RO and Atmospheric Infrared Sounder (AIRS) measurements to highlight the new features of the GPS RO temperature anomalies and to evaluate the quality of the AIRS temperature in the UTLS considering the GPS RO temperature in the UTLS as the benchmark. Both AIRS and GPS RO have a very consistent vertical structure in the subtropical UTLS with a high correlation coefficient 0.92 and similar magnitudes. Both AIRS and GPS RO also show a generally consistent vertical structure of the intraseasonal temperature anomalies in the equatorial UTLS. However, GPS RO reveals many detailed fine‐scale vertical structures of the equatorial temperature anomalies between 150 and 50 hPa that are not well captured by AIRS. Furthermore, the equatorial temperature anomalies are about 40% underestimated in AIRS in comparison to GPS RO, over the equatorial Indian and western Pacific Oceans for 250 hPa and over all longitudes for 100 hPa. The low sampling within the optically thick clouds and low vertical resolution near the tropopause may both contribute to these deficiencies of AIRS.

Modulation of western North Pacific tropical cyclone genesis by intraseasonal oscillation of the ITCZ: A statistical analysis

The present study investigates modulation of western North Pacific (WNP) tropical cyclone (TC) genesis in relation to different phases of the intraseasonal oscillation (ISO) of ITCZ convection during May to October in the period 1979–2008. The phases of the ITCZ ISO were determined based on 30–80-day filtered OLR anomalies averaged over the region (5°–20°N, 120°–150°E). The number of TCs during the active phases was nearly three times more than during the inactive phases. The active (inactive) phases of ISO were characterized by low-level cyclonic (anticyclonic) circulation anomalies, higher (lower) midlevel relative humidity anomalies, and larger (smaller) vertical gradient anomalies of relative vorticity associated with enhanced (weakened) ITCZ convection anomalies. During the active phases, TCs tended to form in the center of the ITCZ region.

The Influence of the MJO on Upstream Precursors to African Easterly Waves

The Madden–Julian oscillation (MJO) produces alternating periods of increased and reduced precipitation and African easterly wave (AEW) activity in West Africa. This study documents the influence of the MJO on the West African monsoon system during boreal summer using reanalysis and brightness temperature fields. MJO-related West African convective anomalies are likely induced by equatorial Kelvin and Rossby waves generated in the Indian Ocean and West Pacific by the MJO, which is consistent with previous studies. The initial modulation of tropical African convection occurs upstream of West Africa, near the entrance of the African easterly jet (AEJ). Previous studies have hypothesized that an area to the east of Lake Chad is an initiation region for AEWs. Called the “trigger region” in this study, this area exhibits significant intraseasonal convection and wave activity anomalies prior to the wet and dry MJO phases in the West African monsoon region. In the trigger region, cold tropospheric temperature anomalies and high precipitable water, as well as an eastward extension of the African easterly jet, appear to precede and contribute to the wet MJO phase in West Africa. An anomalous stratiform heating profile is observed in advance of the wet MJO phase with anomalous PV generation maximized at the jet level. The opposite behavior occurs in advance of the dry MJO phase. The moisture budget is examined to provide further insight as to how the MJO modulates and initiates precipitation and AEW variability in this region. In particular, meridional moisture advection anomalies foster moistening in the trigger region in advance of the wet MJO phase across West Africa.

An Asymmetry in the IOD and ENSO Teleconnection Pathway and Its Impact on Australian Climate

Abstract Recent research has shown that the climatic impact from El Niño–Southern Oscillation (ENSO) on middle latitudes west of the western Pacific (e.g., southeast Australia) during austral spring (September–November) is conducted via the tropical Indian Ocean (TIO). However, it is not clear whether this impact pathway is symmetric about the positive and negative phases of ENSO and the Indian Ocean dipole (IOD). It is shown that a strong asymmetry does exist. For ENSO, only the impact from El Niño is conducted through the TIO pathway; the impact from La Niña is delivered through the Pacific–South America pattern. For the IOD, a greater convection anomaly and wave train response occurs during positive IOD (pIOD) events than during negative IOD (nIOD) events. This “impact asymmetry” is consistent with the positive skewness of the IOD, principally due to a negative skewness of sea surface temperature (SST) anomalies in the east IOD (IODE) pole. In the IODE region, convection anomalies are more sensitive to a per unit change of cold SST anomalies than to the same unit change of warm SST anomalies. This study shows that the IOD skewness occurs despite the greater damping, rather than due to a breakdown of this damping as suggested by previous studies. This IOD impact asymmetry provides an explanation for much of the reduction in spring rainfall over southeast Australia during the 2000s. Key to this rainfall reduction is the increased occurrences of pIOD events, more so than the lack of nIOD events.

Seasonal and interannual variability in the temperature structure around the tropical tropopause and its relationship with convective activities

Seasonal and interannual variability in the tropical tropopause temperatures and its relationship with convective activities are examined by using the ECMWF 40 year reanalysis data and NOAA/OLR data. Low temperatures generally occur over the equator in the eastern hemisphere and extend northwestward and southwestward in the subtropics to form a horseshoe‐shaped structure. Because this structure resembles a stationary wave response known as the Matsuno‐Gill pattern, which is a superposition of the Rossby and Kelvin responses, the two preliminary indices are defined to represent the two responses. The horseshoe‐shaped structure index is then calculated from the two indices. The seasonal cycle in the horseshoe‐shaped structure index is significantly related to that observed in convective activities adjacent to three monsoon regions: the South Asian monsoon (SoAM) and the North Pacific monsoon (NPM) areas during the northern summer and the Australian monsoon (AUM) area during the southern summer. The convective activities in the SoAM and NPM areas individually influence the horseshoe‐shaped structure. During the northern summer, interannual variation in the horseshoe‐shaped structure index in the NPM area is related to that observed in convective activities associated with the El Niño–Southern Oscillation (ENSO) cycle with about a half‐year time lag. In the SoAM area, the variation is mainly controlled by isolated high temperatures, which are surrounded by the horseshoe‐shaped temperature structures and are not related to convective activities. During the southern summer, the horseshoe‐shaped structure index is related to convective anomalies associated with the ENSO cycle, shifting eastward in El Niño years.

The effect of vertical shear orientation on tropical cyclogenesis

AbstractThe effect of the relative orientation of the vertical wind shear to the surface wind on tropical cyclogenesis is explored in environments of radiative‐convective equilibrium (RCE) through numerical simulation. This study serves as a companion paper to an earlier study on the thermodynamics of genesis in RCE.It is found, when the mean surface wind and shear are aligned, a negative surface wind anomaly arises from the superposition of the mean and vortex surface flows left of the shear vector. The resulting weak surface enthalpy fluxes and up‐shear quasi‐balanced subsidence leads to dry air being located cyclonically down‐wind of the down‐shear convective anomaly. Thus convection is inhibited from propagating cyclonically around the core leading to a large down‐shear vortex tilt. Conversely, in a counter‐aligned orientation, the negative surface wind anomaly and driest air is found right of the shear vector. Hence the driest air rotates into the down‐shear flank where it moistened by shear‐organized convection. Furthermore, the boundary layer is relatively moist left of shear due to the positive surface wind anomaly, therefore promoting the cyclonic propagation from down‐shear and constraining the magnitude of the vortex tilt.Genesis is intimately tied to the magnitude of the tilt and is found to occur once the mid‐level vortex has precessed into the up‐shear flank. For smaller values of maximum tilt, vortex precession is comparatively rapid, aided by “showerhead” moistening provided by the up‐shear advection of frozen condensate aloft. With the up‐shear flank pre‐moistened, rapid precession of the mid‐level vortex, at smaller radii, leads to near saturation on the mesoscale and the onset of rapid intensification. When the magnitude of the tilt is quite large, precession is much slower and the showerhead effect is significantly reduced until just prior to the emergence of the mid‐level vortex in the up‐shear flank. Copyright © 2011 Royal Meteorological Society

10–25-Day Intraseasonal Variability of Convection over the Sahel: A Role of the Saharan Heat Low and Midlatitudes

Abstract The understanding and forecasting of persistent dry or wet periods of the West African monsoon (WAM), especially those that occur at the intraseasonal time scale, are crucial to improve food management and disaster mitigation in the Sahel region. In the present study, the authors assess how the 10–25-day intraseasonal variability of convection over the Sahel is related to the recently documented intraseasonal variability of the Saharan heat low (SHL) and the associated extratropical circulation. Strongest and most frequent interactions occur when the SHL intraseasonal fluctuations lead those of convection over the Sahel with a 5-day lag. Using a nonlinear event-based approach, such a combination is shown to concern about one-third of Sahelian dry and wet spells and, in the case of dry spells, to yield convective anomalies that are stronger, last longer by at least 2 days, and reach a larger spatial scale. It is then argued that the 10–25-day intraseasonal variability of convection over the Sahel can be partly explained by the midlatitude intraseasonal variability, through a major role played by the SHL. The anomalous midlevel circulations observed during Sahelian wet and dry events can be shifted from the midlatitudes, which provide a complementary mechanism to that invoking equatorial Rossby wave dynamics. These two mechanisms are likely to interfere together in a constructive or destructive way, leading to high temporal and spatial variability of the Sahelian dry and wet spells. As a particular intraseasonal event, the WAM onset is shown to be clearly favored by phases of the SHL intraseasonal variability, when the Mediterranean ventilation is weakened and the SHL is able to strengthen. Conversely, the formation of a strong cold air surge over Libya and Egypt and its propagation toward the Sahel lead to the collapse of the SHL, which inhibits the WAM onset. From these extratropical–tropical interactions, more skillful forecasts of the Sahelian wet and dry spells and of the WAM onset can be expected. In particular, the monitoring of both the SHL intraseasonal activity and that of the equatorial Rossby wave should provide relevant information to forecast at least two-thirds of such high-impact events.

Impact of the Indian part of the summer MJO on West Africa using nudged climate simulations

Observational evidence suggests a link between the summer Madden Julian Oscillation (MJO) and anomalous convection over West Africa. This link is further studied with the help of the LMDZ atmospheric general circulation model. The approach is based on nudging the model towards the reanalysis in the Asian monsoon region. The simulation successfully captures the convection associated with the summer MJO in the nudging region. Outside this region the model is free to evolve. Over West Africa it simulates convection anomalies that are similar in magnitude, structure, and timing to the observed ones. In accordance with the observations, the simulation shows that 15–20 days after the maximum increase (decrease) of convection in the Indian Ocean there is a significant reduction (increase) in West African convection. The simulation strongly suggests that in addition to the eastward-moving MJO signal, the westward propagation of a convectively coupled equatorial Rossby wave is needed to explain the overall impact of the MJO on convection over West Africa. These results highlight the use of MJO events to potentially predict regional-scale anomalous convection and rainfall spells over West Africa with a time lag of approximately 15–20 days.

Tropical stratospheric cloud climatology from the PATMOS-x dataset: An assessment of convective contributions to stratospheric water

[1] The PATMOS-x level 2b climatology, generated using three decades of AVHRR measurements, contains valuable information about the past global cloud record. We extract climatologies of tropical deep convective clouds from the PATMOS-x data set, based on the 10.30–11.30 μm brightness temperature. A comparison of the cross tropopause convective cloud frequency between ISCCP and PATMOS-x shows that PATMOS-x has a greater frequency of occurrence than does the ISCCP, and this enhanced frequency is attributed to greater horizontal resolution (2 km) in the PATMOS-x data. The high resolution makes this dataset suitable for a search for cross tropopause convection, which happens on length scales down to 1 km. We find there have been several changes in deep convective activity over land during the period 1982 to 2009. We explore specifically the epoch of the HALOE satellite, and find a correlation between land deep convective activity and anomalies in the HALOE stratospheric water retrievals. A simple model is able to predict stratospheric water vapor concentrations highly correlated to that observed using only frequency of deep convection. From this we conclude that deep convection over land contributes to moistening of the lowest tropical stratosphere on seasonal, annual and decadal timescales.

The Influence of the Quasi-Biennial Oscillation on the Troposphere in Winter in a Hierarchy of Models. Part II: Perpetual Winter WACCM Runs

Abstract Experiments with the Whole Atmosphere Community Climate Model (WACCM) are used to understand the influence of the stratospheric tropical quasi-biennial oscillation (QBO) in the troposphere. The zonally symmetric circulation in thermal wind balance with the QBO affects high-frequency eddies throughout the extratropical troposphere. The influence of the QBO is strongest and most robust in the North Pacific near the jet exit region, in agreement with observations. Variability of the stratospheric polar vortex does not appear to explain the effect of the QBO in the troposphere in the model, although it does contribute to the response in the North Atlantic. Anomalies in tropical deep convection associated with the QBO appear to damp, rather than drive, the effect of the QBO in the extratropical troposphere. Rather, the crucial mechanism whereby the QBO modulates the extratropical troposphere appears to be the interaction of tropospheric transient waves with the axisymmetric circulation in thermal wind balance with the QBO. The response to QBO winds of realistic amplitude is stronger for perpetual February radiative conditions and sea surface temperatures than perpetual January conditions, consistent with the observed response in reanalysis data, in a coupled seasonal WACCM integration, and in dry model experiments described in Part I.