Equatorial African climate teleconnections

Teleconnections between equatorial African climate and the surrounding circulation are examined using a convective index over the Congo River Basin in March to May (MAM) and July to September (JAS) seasons. Its influence on the wider region is determined through lag correlation and cross-wavelet analysis. During seasons of deeper convection, easterly winds weaken over the tropical Atlantic (anomalous flow toward Africa), whilst upper westerly winds weaken over southern Africa (in JAS). We view this as zonal overturning with ascent over the equatorial African lowlands and Congo River Basin that spreads moisture to the North African Sahel, with influence from the Pacific El Niño. Another facet of our study is the relationship between East African highlands rainfall and the Indian Ocean circulation. We find coupling between the Indian Ocean Rossby wave, a thermocline oscillation and Walker cell over the Indian Ocean that induces shifts in rainfall, particularly in the October to December season.     

The self-organizing map,a new approach to apprehend the Madden–Julian Oscillation influence on the intraseasonal variability of rainfall in the southern African region

The Madden–Julian Oscillation (MJO) is the major mode of intraseasonal variability (30–60 days) in the tropics, having large rainfall impacts globally, and possibly on southern Africa. However, the latter impact is not well understood and needs to be further explored. The life cycle of the MJO, known to be asymmetric, has been nevertheless analyzed usually through methods constrained by both linearity and orthogonality, such as empirical orthogonal function analysis. Here we explore a non-linear classification method, the self-organizing map (SOM), a type of artificial neural network used to produce a low-dimensional representation of high-dimensional datasets, to capture more accurately the life cycle of the MJO and its global impacts. The classification is applied on intraseasonal anomalies of outgoing longwave radiation within the tropical region over the 1980–2009 period. Using the SOM to describe the MJO is a new approach, complimentary to the usual real-time multivariate MJO index. It efficiently captures this propagative phenomenon and its seasonality, and is shown to provide additional temporal and spatial information on MJO activity. For each node, the subtropical convection is analyzed, with a particular focus on the southern Africa region. Results show that the convection activity over the central tropical Indian Ocean is a key factor influencing the intraseasonal convective activity over the southern African region. Enhanced (suppressed) convection over the central Indian Ocean tends to suppress (enhance) convection over the southern African region with a 10-day lag by modulating the moisture transport.     

SOI and QBO signals in the African region

Summary Climatic determinants of summer (Nov-Mar) rainfall over southern Africa are investigated through analysis of sea surface temperatures (SST), outgoing longwage radiation (OLR) and tropospheric wind with respect to the Southern Oscillation Index (SOI) and the stratospheric quasi-biennial oscillation (QBO). Index-to-field correlation maps are presented at various lags for the austral spring and summer seasons to establish the spatial dependence and evolution of coherent, statistically significant features. The SOI signal is reflected in upper-level zonal wind anomalies over the equatorial Atlantic Ocean during spring. SSTs in the central Indian Ocean are significantly negatively correlated with the SOI in summer. On the other hand, OLR correlations are weak over southern Africa in the summer, implying that the SOI signal may not dominate interannual convective variability.QBO correlations with SST are relatively weak, but with 200 hPa zonal winds over the western equatorial Ocean, positive correlations are noted. A standing wave pattern is described in the sub-tropics. The OLR correlation pattern represents a dipole with increased convection over eastern and southern Africa in contrast to reduced convection over Madagascar when the QBO is in west phase.Contingency analyses indicate that the global indices are unreliable predictors in isolation. However the characteristics and domain of influence of SOI and QBO signals are identified and may offer useful inputs to objective multivariate models for different modes of southern African rainfall variability.With 12 Figures     

Modification of the southern African rainfall variability/ENSO relationship since the late 1960s

Analysis of 149 raingauge series (1946–1988) shows a weak positive correlation between late summer rainfalls (January–March) in tropical southern Africa and the Southern Oscillation Index (SOI). The correlation coefficients have been unstable since World War II. They were close to zero before 1970 and significant thereafter. Before 1970, southern African late summer rainfalls were more specifically correlated with regional patterns of sea surface temperature (SST), mainly over the southwestern Indian Ocean. After 1970, teleconnections with near global SST anomaly patterns, i.e. over the central Pacific and Indian oceans, dominate the regional connections. The increase in the sensitivity of the southern African rainfall to the global SO-related circulation anomalies is simultaneous with the correlation between SOI and more extensive SST anomalies, particularly over the southern Indian Ocean. This feature is part of longer term (decadal), global SST variability, as inferred from statistical analyses. Numerical experiments, using the Météo-France general circulation model ARPEGE-Climat, are performed to test the impact of the observed SST warming in the southern Indian and extratropical oceans during El Niño Southern Oscillation (ENSO) events on southern African rainfall. Simulated results show that ENSO events, which occurred in the relatively cold background of the pre-1970 period in the southern oceans, had a little effect on southern Africa climatic conditions and atmospheric circulation. By contrast, more recent ENSO events, with warmer SST over the southern oceans, lead to a climatic bipolar pattern between continental southern African and the western Indian Ocean, which is characterized by reduced (enhanced) deep convection and rainfall over the subcontinent (the western Indian Ocean). A weaker subtropical high-pressure belt in the southwestern Indian Ocean is also simulated, along with a reduced penetration of the moist southern Indian Ocean trade winds over the southern African plateau. These results are consistent with the strong droughts observed over all southern Africa during ENSO events since 1970.     

African climate variability in the satellite era

Interannual fluctuations in rainfall and ocean-atmosphere fields over and around Africa were studied in the satellite era of 1979–2007 using singular value decomposition. The leading modes of rainfall variability in GPCP satellite-gauge merged fields include a leading mode over central Africa, two modes of marine origin in the Gulf of Guinea and Eastern Africa, and two sub-tropical modes over the Sahel and Southern Africa. This differs from earlier gauge-based studies that tend to isolate three leading modes over western, eastern, and southern Africa. In the sea-surface temperature, sea-level pressure and upper wind fields, ENSO signals dominate the leading modes. However, for the low-level wind field, a trough circulation over the southeast Atlantic – Kalahari is the leading mode. It demonstrates predictive potential when cross-correlated with rainfall at 6- to 12-month lead time. Based on continuous filtered data, the value of various indices and the predictability of different zones are examined. The Sahel achieves the highest rank followed by the Congo and southern zones in the next tier. The Guinea and East African rains, which peak in the March-to-May season, appear least predictable. The seasonal rainfall is shown to modulate economic growth rate, and multi-variate predictive algorithms are tested at 6-month lead time.     

Seasonal variation of the ITCZ and its characteristics over central Africa

We investigated the seasonal march of the Intertropical Convergence Zone (ITCZ) shown by the 22 coupled general circulation models of the 20th Century Climate in Coupled Models experiment in seven regions (Africa, Indian Ocean, western Pacific, central Pacific, eastern Pacific, South America, and Atlantic Ocean). Inter-model differences in the seasonal march of the ITCZ over Africa (10?C40°E) were significantly smaller than those over other regions. This finding indicates that the seasonal march of the ITCZ over Africa is insensitive to differences in model physics and resolution and suggests that the seasonal march of the African ITCZ is controlled by robust and simple mechanisms. Motivated by this result, we tried to understand the process of the seasonal march of the ITCZ over central Africa (15?C30°E) based on an analysis of ERA-40 data. The analysis results revealed the following features of the ITCZ in this region: (1) The ITCZ itself produces large convective available potential energy that generates deep convection. (2) The abundant water vapor within the ITCZ is maintained by horizontal moisture flux. (3) Outside but near the ITCZ, shallow convection exists and may act to pre-moisten deep convection in spring and autumn. (4) The seasonal change of the ITCZ is preceded by that of the vertical moist instability in the lower free atmosphere caused by the seasonal change in insolation.     

Impacts of convection schemes on simulating tropical-temperate troughs over southern Africa

This study examines southern African summer rainfall and tropical temperate troughs (TTTs) simulated with three versions of an atmospheric general circulation model differing only in the convection scheme. All three versions provide realistic simulations of key aspects of the summer (November–February) rainfall, such as the spatial distribution of total rainfall and the percentage of rainfall associated with TTTs. However, one version has a large bias in the onset of the rainy season. Results from self-organizing map (SOM) analysis on simulated daily precipitation data reveals that this is because the occurrence of TTTs is underestimated in November. This model bias is not related to westerly wind shear that provides favorable conditions for the development of TTTs. Rather, it is related to excessive upper level convergence and associated subsidence over southern Africa. Furthermore, the model versions are shown to be successful in capturing the observed drier (wetter) conditions over the southern African region during El Niño (La Niña) years. The SOM analysis reveals that nodes associated with TTTs in the southern (northern) part of the domain are observed less (more) often during El Niño years, while nodes associated with TTTs occur more frequently during La Niña years. Also, nodes associated with dry conditions over southern Africa are more (less) frequently observed during El Niño (La Niña) years. The models tend to perform better for La Niña events, because they are more successful in representing the observed frequency of different synoptic patterns.     

Divergent and non-divergent water vapour transport over southern Africa during wet and dry conditions

Summary Inter-seasonal and inter-annual variations of low-level tropospheric water vapour transport over southern Africa and adjacent oceans have been examined using the ECMWF III-B global analysis data set. Velocity potential and stream function analyses have been used to investigate differences between the large-scale circulation fields and the sources and sinks of water vapour over subtropical southern Africa in wet and dry early (October) and late (January) summer months. Water vapour transported over the region in early summer has been found to originate from both the Atlantic and Indian oceans. Transport takes place in a southerly direction over the subcontinent with eddy transport playing an important role only in early summer. During late summer transport is effected mainly by the mean circulation fields. At this time the tropical Indian Ocean becomes the most important source for water vapour. Variations in the intensity and position of the Walker and Hadley cells are important controls of moisture availability over southern Africa. Wet late summers are characterized by an anomalous Hadley cell over tropical and subtropical Africa such that excess vapour transport occurs across southern Africa from the north in the second half of the summer rainfall season. During dry late summers the vapour source region locates over the south-western Indian Ocean and diminished vapour transport takes place across southern Africa from the south east.With 8 Figures     

On convection and static stability during the AMMA SOP3 campaign

Using radiosonde dataset from 15 weather stations over West Africa, this paper investigates the contribution of the couple convection-static stability in the framework of the African monsoon multidisciplinary analyses Special Observing Period 3 (AMMA SOP3) experiment. Within this 31-day period, the boundary layer-winds depictions have revealed the West African monsoon’s (WAM) depth (around 1500 m) is not thick enough to trigger intense convection. However, the midlevel winds distribution (700–600 hPa) has shown the average African easterly jet core strength (15 m s^?1) is sufficient to allow the development of African easterly waves (AEWs) necessary for squall lines activities. In return, in the upper levels (200–100 hPa), the speed (below 18 m s^?1) of the mean Tropical easterly jet (TEJ) core cannot favor midlevel updrafts. The free tropospheric humidity (FTH) depiction has indicated convective events are more likely in the western Sahel where the highest FTH (FTH >50 %) are recorded. The static stability analysis has testified that convection is stronger in the semi-arid (SA) area during night time (0000 GMT). However, convective activities are inhibited in the wet equatorial (WE) region due to mean low-level stability. We used METEOSAT Second Generation (MSG) infrared (IR10.8) imagery of the 8th September 2006 to confirm that result. Furthermore, a maximum midtropospheric static stability combined with maximum relative humidity (RH) was found on the fringe of the Saharan air layer’s (SAL) top (altitude around 5.3 km) in the WE region.     

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.     

The bi-decadal rainfall cycle,Southern Annular Mode and tropical cyclones over the Limpopo River Basin,southern Africa

The association between bi-decadal rainfall variability over southern Africa and the rainfall contributed by tropical cyclonic systems from the Southwest Indian Ocean (SWIO) provides a potential means towards understanding decadal-scale variability over parts of the region. A multi-decadal period is considered, focusing on the anomalous tropospheric patterns that induced a particularly wet 8-year long sub-period over the Limpopo River Basin. The wet sub-period was also characterized by a larger contribution to rainfall by tropical cyclones and depressions. The findings suggest that a broadening of the Hadley circulation underpinned by an anomalous anticyclonic pattern to the east of southern Africa altered tropospheric steering flow, relative vorticity and moisture contents spatially during the sub-period of 8 years. These circulation modulations induced enhanced potential for tropical systems from the SWIO to cause precipitation over the Limpopo River Basin. The same patterns are also conducive to increasing rainfall over the larger subcontinent, therefore explaining the positive association in the bi-decadal rainfall cycle and rainfall contributed by tropical cyclonic systems from the SWIO. An overview of regional circulation anomlies during alternating near-decadal wet and dry epochs is given. The regional circulation anomalies are also explained in hemispheric context, specifically in relation to the Southern Annular Mode, towards understanding variation over other parts of the Southern Hemisphere at this time scale.     

Instability of squall-inducing waves, global sea-surface temperature anomalies and climate change in tropical north Africa

Summary An inviscid form of the hydrodynamical equations is solved with enhanced horizontal shear, which is a synoptic feature consistent with stronger African Easterly Jet (AEJ) in Sahelian dry years, for unstable waves generated along the boundary between the two tropospheric air masses in tropical north Africa (i.e. the moist south-westerlies and the dry north-easterlies). Using a two-layer model of the atmosphere in order to correctly simulate the tropospheric synoptic situation in the sub-region, results show that the mode of the waves which is known to be fundamental to the development of West African squall lines is more unstable in dry years. This instability is found to be most-pronounced when the surface of discontinuity between the south-westerlies and the north-easterlies is at 700 mb level. Further, it is shown that in Sahelian dry years, the zone of these unstable waves shifts slightly southwards. This shift causes a deficit in rainfall in West African isohyet bands north of latitude 12°. The persistence of this deficit is linked with the continuous warming, in July, August and September of the 18-year period 1969–1986, of the three oceans (Indian, Pacific and South Atlantic) whose sea-surface temperature (SST) anomalies influence rainfall in tropical north Africa. It is shown that anytime these oceans warm up anomalously, the strength of the AEJ is enhanced leading to the climate-change process of: SST anomaly, increased AEJ strength, southward shift of the zone of squall-inducing waves and consequent reduction in total annual rainfall north of latitude 12° in tropical north Africa.With 5 Figures     

中国北方秋雨与热带中太平洋海表冷却的关系

本文利用1951～2011年中国160站降水、NCEP/NCAR再分析资料和NOAA延长重建海表温度 (NOAA extended reconstructed SST) 资料,研究了中国9月北方秋雨的年际变化特征及其成因,并用ECHAM5大气环流模式开展了数值试验,最后对2011年9月历史罕见秋雨进行了分析。研究发现,中国北方秋雨有明显的年际和年代际变化,19世纪60年代到1980年代中期,北方秋雨偏多,1950年代、1980年代后期和1990年代秋雨偏少。北方秋雨与西太平洋副热带高压的西伸有密切联系,北方秋雨偏多时,副热带高压偏西偏强,有利于偏南风向北输送水汽并在中国北方辐合。西太平洋副热带高压的加强西伸与热带中太平洋的海表冷却密切有关,偏低的热带中太平洋海表温度(CTSSTI)使其上的对流活动受到抑制,热带西太平洋对流异常旺盛,在西北太平洋出现异常反气旋,加强东亚—西北太平洋的EAP波列,引起西太平洋副热带高压明显西伸,导致秋雨偏多。反之,热带中太平洋海表偏暖,副热带高压偏弱偏南,秋雨偏少。2011年9月北方秋雨的环流异常及成因与统计分析和数值模拟结果基本一致。     

Composite climatic patterns associated with extreme modes of summer rainfall over southern Africa: 1975–1984

Summary Climatic patterns associated with extreme modes of summer rainfall over southern Africa are investigated using composite techniques. Differences between the wet summers of the mid-1970s and the dry summers of the early 1980s are highlighted. In dry summers both the Southern Oscillation Index (SOI) and Quasi-Biennial Oscillation (QBO) are negatively biased. Composite difference fields of outgoing longwave radiation (OLR), sea surface temperature (SST), and upper and lower tropospheric wind are analysed. The OLR difference field indicates the widespread nature of convective variations with a consistent sign in the domain 15–33° S, 0–40° E. An area of opposing sign is conspicuous over the southwest Indian Ocean and represents a dipole, whereby wet summers over southern Africa coincide with dry summers over the adjacent ocean. This dipole behaviour is an expression of the primary mode of interannual climatic variability in the region. SST composite differences are negative over a wide portion of the central equatorial Indian Ocean and SE Atlantic, and positive to the south of Africa where the Agulhas Current flows. Wind composites reveal distinctive circulation differences in the extreme summers considered. In the tropical zone off the east coast of Africa difference vectors indicate upper westerly and lower easterly circulation anomalies, and distinguish a pathway for moist Indian Ocean air. A deep anticyclonic gyre is located over the region of positive SST differences in the sub-tropics to the SE of Africa. The identification of climatic patterns in extreme summers offers some useful guidelines in seasonal forecasts.With 6 Figures     

The application of a present-day climatic model to the late quaternary in southern Africa

A present-day climatic model is presented in which extended wet spells of near-decadal duration and dry spells of similar length are explained on the basis of surface and upper tropospheric circulation variations. Wet spells are shown to be the result of increased tropical atmospheric disturbances and tropical-temperate interaction, and to be linked to variations in the Walker Circulation. Conversely, dry spells are shown to result from diminished tropical activity over southern Africa, equatorward movement of westerly storm tracks and temperate perturbations in the westerlies.The present-day analogue is compared to preliminary spatial reconstructions of the climate of southern Africa over the last twenty-five millennia and is shown to have wide applicability in the explanation of the late-Quaternary palaeoclimates of the subcontinent. In particular, it is argued that the Last Glacial Maximum was associated with northward-displaced circulation conditions similar to those of present-day dry spells over the summer rainfall region, whereas the extensive moist conditions that prevailed for several thousand years after 9000 BP were analogous to present-day wet spell conditions with little apparent displacement of major circulation features.     

A model investigation of recent ENSO impacts over southern Africa

Summary This study investigates the impacts of five recent ENSO events on southern Africa, the associated circulation anomalies and the ability of an atmospheric general circulation model (UKMO HadAM3) to represent these impacts when forced by observed sea-surface temperature (SST). It is found that the model is most successful for the 1997/8 El Niño but does less well for the 1991/2 and 2002/3 El Niños and the 1995/6 and 1999/00 La Niña events. Diagnostics from the model and NCEP re-analyses suggest that modulations to the Angola low, an important centre of tropical convection over southern Africa during austral summer, are often important for influencing the rainfall impacts of ENSO over subtropical southern Africa. Since the model has difficulty in adequately representing this regional circulation feature and its variability, it has problems in capturing ENSO rainfall impacts over southern Africa. During 1997/8, modulations to the Angola low were weak and Indian Ocean SST forcing strong and the model is relatively successful. The implications of these results for dynamical model based seasonal forecasting of the region are discussed.Current affiliation: CSIR Centre for Mathematical Modelling and Computer Simulation, Bangalore, India.     

Circulation anomalies associated with tropical-temperate troughs in southern Africa and the south west Indian Ocean

Daily rainfall variability over southern Africa (SA) and the southwest Indian Ocean (SWIO) during the austral summer months has recently been described objectively for the first time, using newly derived satellite products. The principle mode of variability in all months is a dipole structure with bands of rainfall orientated northwest to southeast across the region. These represent the location of cloud bands associated with tropical temperate troughs (TTT). This study objectively identifies major TTT events during November to February, and on the basis of composites off NCEP reanalysis data describes the associated atmospheric structure. The two phases of the rainfall dipole are associated with markedly contrasting circulation patterns. There are also pronounced intra-seasonal variations. In early summer the position of the temperate trough and TTT cloud band alternates between the SWIO and southwest Atlantic. In late summer the major TTT axis lies preferentially over the SWIO, associated with an eastward displacement in the Indian Ocean high. In all months, positive events, in which the TTT cloud band lies primarily over the SWIO, are associated with large-scale moisture flux anomalies, in which convergent fluxes form a pronounced poleward flux along the cloud band. This suggests that TTT events are a major mechanism of poleward transfer of energy and momentum. Moisture transport occurs along three principle paths: (1) the northern or central Indian Ocean (where anomalous fluxes extend eastward to the Maritime Continent), (2) south equatorial Africa and the equatorial Atlantic, (3) from the south within a cyclonic flow around the tropical-temperate trough. The relative importance of (2) is greatest in late summer. Thus, synoptic scale TTT events over SA/SWIO often result from large-scale planetary circulation patterns. Hovmoeller plots show that TTT development coincides with enhanced tropical convection between 10°–30°E (itself exhibiting periodicity of around 5 days), and often with convergence of eastward and westward propagating convection around 40°E. Harmonic analysis of 200 hPa geopotential anomalies show that TTT features are forced by a specific zonally asymmetric wave pattern, with wave 5 dominant or significant in all months except February when quasi-stationary waves 1, 2 and 3 dominate. These findings illustrate the importance of tropical and extratropical dynamics in understanding TTT events. Finally, it is suggested that in November–Januar TTT rainfall over SA/SWIO may be in phase with similar rainfall dipole structures observed in the South Pacific and South Atlantic convergence zones. Received: 11 August 1998 / Accepted: 28 May 1999     

A revised picture of the structure of the “monsoon” and land ITCZ over West Africa

This article presents an overview of the land ITCZ (Intertropical Convergence Zone) over West Africa, based on analysis of NCAR–NCEP Reanalysis data. The picture that emerges is much different than the classic one. The most important feature is that the ITCZ is effectively independent of the system that produces most of the rainfall. Rainfall linked directly to this zone of surface convergence generally affects only the southern Sahara and the northern-most Sahel, and only in abnormally wet years in the region. A second feature is that the rainbelt normally assumed to represent the ITCZ is instead produced by a large core of ascent lying between the African Easterly Jet and the Tropical Easterly Jet. This region corresponds to the southern track of African Easterly Waves, which distribute the rainfall. This finding underscores the need to distinguish between the ITCZ and the feature better termed the “tropical rainbelt”. The latter is conventionally but improperly used in remote sensing studies to denote the surface ITCZ over West Africa. The new picture also suggests that the moisture available for convection is strongly coupled to the strength of the uplift, which in turn is controlled by the characteristics of the African Easterly Jet and Tropical Easterly Jet, rather than by moisture convergence. This new picture also includes a circulation feature not generally considered in most analyses of the region. This feature, a low-level westerly jet termed the African Westerly Jet, plays a significant role in interannual and multidecadal variability in the Sahel region of West Africa. Included are discussions of the how this new view relates to other aspects of West Africa meteorology, such as moisture sources, rainfall production and forecasting, desertification, climate monitoring, hurricanes and interannual variability. The West African monsoon is also related to a new paradigm for examining the interannual variability of rainfall over West Africa, one that relates changes in annual rainfall to changes in either the intensity of the rainbelt or north–south displacements of this feature. The new view presented here is consistent with a plethora of research on the synoptic and dynamic aspects of the African Easterly Waves, the disturbances that are linked to rainfall over West Africa and spawn hurricanes over the Atlantic, and with our knowledge of the prevailing synoptic and dynamic features. This article demonstrate a new aspect of the West Africa monsoon, a bimodal state, with one mode linked to dry conditions in the Sahel and the other linked to wet conditions. The switch between modes appears to be linked to an inertial instability mechanism, with the cross-equatorial pressure gradient being a critical factor. The biomodal state has been shown for the month of August only, but this month contributes most of the interannual variability. This new picture of the monsoon and interannual variability shown here appears to be relevant not only to interannual variability, but also to the multidecadal variability evidenced in the region between the 1950s and 1980s.     

Warm and cold events in the southeast Atlantic/southwest Indian Ocean region and potential impacts on circulation and rainfall over southern Africa

Summary An important pattern of interannual variability in the southern African region is one where sea surface temperature (SST) in neighbouring waters, particularly in the Agulhas Current, its retroflection region and outflow across the southern midlatitudes of the Indian Ocean, is anomalously warm or cool. Evidence exists of significant rainfall anomalies over large parts of southern Africa during these warm or cool SST events. Here, a general circulation model is used to study the response of the atmosphere in the region to an idealised representation of these SST anomalies. The induced atmospheric circulation and precipitation anomalies over the adjacent southern African landmass on intraseasonal through to interannual time scales are investigated.A nonlinear response to the SST anomalies is found in that the changes to the model atmosphere when warm SST forcing is used are not the reverse (in either pattern or magnitude) to that when cold SST forcing is imposed. For the warm SST anomaly, it is found that the atmospheric response is favourable for enhancement of the original SST anomaly on scales up to, and including, annual. However, as the scale becomes interannual (i.e., 15–21 months after imposition of the anomaly), the model response suggests that damping of the original SST anomaly becomes likely. However, no such coherent timescale dependent response is found when the cold SST anomaly is impose. It is suggested that the relationship of the SST anomaly to the background seasonal climatology may help explain this fundamental difference in the response.Examination of the circulation and rainfall patterns under warm SST forcing indicates that there are significant anomalies over large parts of southern Africa on all scales from intraseasonal through to interannual. On the south coast, rainfall anomalies result from enhanced evaporation of moisture off the SST anomaly. Over the interior, changer in the convergence of moist air streams together with suggestions of a shift in the Walker circulations between southern Africa and the bordering tropical South Atlantic and Indian Oceans appear to be associated with the rainfall anomalies. Similar mechanisms of rainfall perturbation are found when the cold SST anomaly is imposed; however, there is a significant response only on intra-annual to interannual scales. In all cases, the magnitude of the rainfall anomalies accumulated over a 90 day season were of the order of 90–180 mm, and therefore represent a significant fraction of the annual total of many areas. These model results re-inforce previous observational work suggesting that SST anomalies south of Africa, particularly in the retroflection region of the Agulhas Current, are linked with significant rainfall anomalies over the adjacent subcontinent.With 12 Figures     

Indian summer monsoon rainfall predictability and variability associated with Northwest Pacific circulation in a suit of coupled model hindcasts

The Northwest Pacific (NWP) circulation (subtropical high) is an important component of the East Asian summer monsoon system. During summer (June–August), anomalous lower tropospheric anticyclonic (cyclonic) circulation appears over NWP in some years, which is an indicative of stronger (weaker) than normal subtropical high. The anomalous NWP cyclonic (anticyclonic) circulation years are associated with negative (positive) precipitation anomalies over most of Indian summer monsoon rainfall (ISMR) region. This indicates concurrent relationship between NWP circulation and convection over the ISMR region. Dry wind advection from subtropical land regions and moisture divergence over the southern peninsular India during the NWP cyclonic circulation years are mainly responsible for the negative rainfall anomalies over the ISMR region. In contrast, during anticyclonic years, warm north Indian Ocean and moisture divergence over the head Bay of Bengal-Gangetic Plain region support moisture instability and convergence in the southern flank of ridge region, which favors positive rainfall over most of the ISMR region. The interaction between NWP circulation (anticyclonic or cyclonic) and ISMR and their predictability during these anomalous years are examined in the present study. Seven coupled ocean–atmosphere general circulation models from the Asia-Pacific Economic Cooperation Climate Center and their multimodel ensemble mean skills in predicting the seasonal rainfall and circulation anomalies over the ISMR region and NWP for the period 1982–2004 are assessed. Analysis reveals that three (two) out of seven models are unable to predict negative (positive) precipitation anomalies over the Indian subcontinent during the NWP cyclonic (anticyclonic) circulation years at 1-month lead (model is initialized on 1 May). The limited westward extension of the NWP circulation and misrepresentation of SST anomalies over the north Indian Ocean are found to be the main reasons for the poor skill (of some models) in rainfall prediction over the Indian subcontinent. This study demonstrates the importance of the NWP circulation variability in predicting summer monsoon precipitation over South Asia. Considering the predictability of the NWP circulation, the current study provides an insight into the predictability of ISMR. Long lead prediction of the ISMR associated with anomalous NWP circulation is also discussed.