Diagnosing GCM errors over West Africa using relaxation experiments. Part II: intraseasonal variability and African easterly waves

A near-global grid-point nudging of the Arpege-Climat atmospheric General Circulation Model towards ECMWF reanalyses is used to diagnose the regional versus remote origin of the summer model biases and variability over West Africa. First part of this study revealed a limited impact on the monsoon climatology compared to a control experiment without nudging, but a significant improvement of interannual variability, although the amplitude of the seasonal anomalies remained underestimated. Focus is given here on intraseasonal variability of monsoon rainfall and dynamics. The reproducible part of these signals is investigated through 30-member ensemble experiments computed for the 1994 rainy season, a year abnormally wet over the Sahel but representative of the model systematic biases. In the control experiment, Arpege-Climat simulates too few rainy days that are associated with too low rainfall amounts over the central and western Sahel, in line with the seasonal dry biases. Nudging the model outside Africa tends to slightly increase the number of rainy days over the Sahel, but has little effect on associated rainfall amounts. However, results do indicate that a significant part of the monsoon intraseasonal variability simulated by Arpege-Climat is controlled by lateral boundary conditions. Parts of the wet/dry spells over the Sahel occur in phase in the 30 members of the nudging experiment, and are therefore embedded in larger-scale variability patterns. Inter-member spread is however not constant across the selected summer season. It is partly controlled by African Easterly Waves, which show dissimilar amplitude from one member to another, but a coherent phasing in all members. A lowpass filtering of the nudging fields suggests that low frequency variations in the lateral boundary conditions can lead to eastward extensions of the African Easterly Jet, creating a favorable environment for easterly waves, while high frequency perturbations seem to control their phasing.     

Diagnosing GCM errors over West Africa using relaxation experiments. Part I: summer monsoon climatology and interannual variability

The CNRM atmospheric general circulation model Arpege-Climat is relaxed towards atmospheric reanalyses outside the 10°S?C32°N 30°W?C50°E domain in order to disentangle the regional versus large-scale sources of climatological biases and interannual variability of the West African monsoon (WAM). On the one hand, the main climatological features of the monsoon, including the spatial distribution of summer precipitation, are only weakly improved by the nudging, thereby suggesting the regional origin of the Arpege-Climat biases. On the other hand, the nudging technique is relatively efficient to control the interannual variability of the WAM dynamics, though the impact on rainfall variability is less clear. Additional sensitivity experiments focusing on the strong 1994 summer monsoon suggest that the weak sensitivity of the model biases is not an artifact of the nudging design, but the evidence that regional physical processes are the main limiting factors for a realistic simulation of monsoon circulation and precipitation in the Arpege-Climat model. Sensitivity experiments to soil moisture boundary conditions are also conducted and highlight the relevance of land?Catmosphere coupling for the amplification of precipitation biases. Nevertheless, the land surface hydrology is not the main explanation for the model errors that are rather due to deficiencies in the atmospheric physics. The intraseasonal timescale and the model internal variability are discussed in a companion paper.     

The role of the Indian monsoon onset in the West African monsoon onset: observations and AGCM nudged simulations

In spring the inland penetration of the West African Monsoon (WAM) is weak and the associated rainband is located over the Guinean coast. Then within a few days deep convection weakens considerably and the rainband reappears about 20?days after over the Sahel, where it remains until late September signalling the summer rainy season. Over the period 1989–2008 a teleconnection induced by the Indian monsoon onset is shown to have a significant impact on the WAM onset, by performing composite analyses on both observational data sets and atmospheric general circulation model simulations ensembles where the model is nudged to observations over the Indian monsoon sector. The initiation of convective activity over the Indian subcontinent north of 15°N at the time of the Indian monsoon onset results in a westward propagating Rossby wave establishing over North Africa 7–15?days after. A back-trajectory analysis shows that during this period, dry air originating from the westerly subtropical jet entrance is driven to subside and move southward over West Africa inhibiting convection there. At the same time the low-level pressure field over West Africa reinforces the moisture transport inland. After the passage of the wave, the dry air intrusions weaken drastically. Hence 20?days after the Indian monsoon onset, convection is released over the Sahel where thermodynamic conditions are more favourable. This scenario is very similar in the observations and in the nudged simulations, meaning that the Indian monsoon onset is instrumental in the WAM onset and its predictability at intraseasonal scale.     

Summer Sahel-ENSO teleconnection and decadal time scale SST variations

The correlation between Sahel rainfall and El Niño–Southern Oscillation (ENSO) in the northern summer has been varying for the last fifty years. We propose that the existence of periods of weak or strong relationship could result from an interaction with the global decadal scale sea surface temperature (SST) background. The main modes of SST variability have been extracted through a principal component analysis with Varimax rotation. The correlations between a July-September Sahel rainfall index and these SST modes have been computed on a 20-year running window between 1945 and 1993. The correlations with the interannual ENSO-SST mode are negative, not significant in the 1960s during the transition period from the wet climate phasis to the long-running drought in the Sahel, but then were significant since 1976. During the former period, the correlations between the Sahel rainfall index and the other SST modes (expressing mostly on quasi and multi-decadal scales) are the highest, in particular correlations with the tropical Atlantic “dipole”. Correlations between Sahel and Guinea Coast rainfall are also significantly negative. After 1970, the Sahel-Guinea Coast rainfall correlations are no longer significant, and the ENSO-SST mode becomes the only one significantly correlated with Sahel rainfall, especially due to the impact of warm events. The partial correlations between the ENSO-SST mode and the Sahel rainfall index, when the influence of the other SST modes are eliminated, are significant over all the 20-year running periods between 1945 and 1993, suggesting that this summer teleconnection could be modulated by the decadal scale SST background. The NCEP/NCAR reanalyses reproduce accurately the interannual variability of the atmospheric circulation after 1968. In particular a regional West African Monsoon Index (WAMI), combining wind speed anomalies at 925 and 200?hPa, is highly correlated with the July-September Sahel rainfall index. A warm ENSO event is associated both with an eastward mean sea level pressure gradient between the eastern tropical Pacific and the tropical Atlantic and with a northward pressure gradient along the western coast of West Africa. This pattern leads to enhanced trade winds over the tropical Atlantic and to weaker moisture advection over West Africa, consistent with a weaker monsoon system strength and a weaker Southern Hemisphere Hadley circulation. The NCEP/NCAR reanalyses do not reproduce accurately the decadal variability of the atmospheric circulation over West Africa because of artifical biases. Therefore the impact of the decadal scale pattern of the atmospheric circulation has been investigated with atmospheric general circulation model (AGCM) sensitivity experiments, by forcing the ARPEGE-Climat model with different combinations of an El Niño-like SST pattern with the pattern of the main mode of decadal scale SST variability where the hightest weights are located in the Pacific and Indian basins. AGCM outputs show that the decadal scale SST variations weakly affect Sahel rainfall variability but that they do induce an indirect effect on Sahel rainfall by enhancing the impact of the warm ENSO phases after 1980, through an increase in the fill-in of the monsoon trough and a moisture advection deficit over West Africa.     

Multiyear simulation of the African climate using a regional climate model (RegCM3) with the high resolution ERA-interim reanalysis

This study examines the ability of the latest version of the International Centre for Theoretical Physics (ICTP) regional climate model (RegCM3) to reproduce seasonal mean climatologies, annual cycle and interannual variability over the entire African continent and different climate subregions. The new European Center for Medium Range Weather Forecast (ECMWF) ERA-interim reanalysis is used to provide initial and lateral boundary conditions for the RegCM3 simulation. Seasonal mean values of zonal wind profile, temperature, precipitation and associated low level circulations are shown to be realistically simulated, although the regional model still shows some deficiencies. The West Africa monsoon flow is somewhat overestimated and the Africa Easterly Jet (AEJ) core intensity is underestimated. Despite these biases, there is a marked improvement in these simulated model variables compared to previous applications of this model over Africa. The mean annual cycle of precipitation, including single and multiple rainy seasons, is well captured over most African subregions, in some cases even improving the quality of the ERA-interim reanalysis. Similarly, the observed precipitation interannual variability is well reproduced by the regional model over most regions, mostly following, and sometimes improving, the quality of the ERA-interim reanalysis. It is assessed that the performance of this model over the entire African domain is of sufficient quality for application to the study of climate change and climate variability over the African continent.     

Soil moisture memory and West African monsoon predictability: artefact or reality?

Besides sea surface temperature (SST), soil moisture (SM) exhibits a significant memory and is likely to contribute to atmospheric predictability at the seasonal timescale. In this respect, West Africa was recently highlighted as a “hot spot” where the land–atmosphere coupling could play an important role, through the recycling of precipitation and the modulation of the meridional gradient of moist static energy. Particularly intriguing is the observed relationship between summer monsoon rainfall over Sahel and the previous second rainy season over the Guinean Coast, suggesting the possibility of a soil moisture memory beyond the seasonal timescale. The present study is aimed at revisiting this question through a detailed analysis of the instrumental record and a set of numerical sensitivity experiments. Three ensembles of global atmospheric simulations have been designed to assess the relative influence of SST and SM boundary conditions on the West African monsoon predictability over the 1986–1995 period. On the one hand, the results indicate that SM contributes to rainfall predictability at the end and just after the rainy season over the Sahel, through a positive soil-precipitation feedback that is consistent with the “hot spot” hypothesis. On the other hand, SM memory decreases very rapidly during the dry season and does not contribute to the predictability of the all-summer monsoon rainfall. Though possibly model dependent, this conclusion is reinforced by the statistical analysis of the summer monsoon rainfall variability over the Sahel and its link with tropical SSTs. Our results indeed suggest that the apparent relationship with the previous second rainy season over the Guinean Coast is mainly an artefact of rainfall teleconnections with tropical modes of SST variability both at interannual and multi-decadal timescales.     

Interannual spring Wyrtki jet variability and its regional impacts

The role of spring Wyrtki jets in modulating the equatorial Indian Ocean and the regional climate is an unexplored problem. The source of interannual variability in the spring Wyrtki jets is explored in this study. The relationship between intraseasonal and interannual variability from 1958 to 2008 and its relation with Indian Summer Monsoon is further addressed. Analysis reveals that the interannual variability in spring Wyrtki jets is controlled significantly by their intraseasonal variations. These are mostly defined by a single intraseasonal event of duration 20 days or more which either strengthens or weakens the seasonal mean jet depending on its phase. The strong spring jets are driven by such intraseasonal westerly wind bursts lasting for 20-days or more, whereas the weak jets are driven by weaker intraseasonal westerlies. During the years of strong jets, the conventional westward phase propagation of Wyrtki jets is absent and instead there is an eastward phase propagation indicating the possible role of Madden Julian Oscillation (MJO) in strengthening the spring Wyrtki jets. These strong intraseasonal westerly wind bursts with eastward phase propagation during strong years are observed mainly in late spring and have implications on June precipitation over the Indian and adjoining land mass. Anomalously strong eastward jets accumulate warm water in the eastern equatorial Indian Ocean (EIO), leading to anomalous positive upper ocean heat content and supporting more local convection in the east. This induces subsidence over the Indian landmass and alters monsoon rainfall by modulating monsoon Hadley circulation. In case of weak current years such warm anomalies are absent over the eastern EIO. Variations in the jet strength are found to have strong impact on sea level anomalies, heat content, salinity and sea surface temperature over the equatorial and north Indian Ocean making it a potentially important player in the north Indian Ocean climate variability.     

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.     

A multi-model analysis of the role of the ocean on the African and Indian monsoon during the mid-Holocene

We investigate the role of the ocean feedback on the climate in response to insolation forcing during the mid-Holocene (6,000 year BP) using results from seven coupled ocean–atmosphere general circulation models. We examine how the dipole in late summer sea-surface temperature (SST) anomalies in the tropical Atlantic increases the length of the African monsoon, how this dipole structure is created and maintained, and how the late summer SST warming in the northwest Indian Ocean affects the monsoon retreat in this sector. Similar mechanisms are found in all of the models, including a strong wind evaporation feedback and changes in the mixed layer depth that enhance the insolation forcing, as well as increased Ekman transport in the Atlantic that sharpens the Atlantic dipole pattern. We also consider changes in interannual variability over West Africa and the Indian Ocean. The teleconnection between variations in SST and Sahelian precipitation favor a larger impact of the Atlantic dipole mode in this region. In the Indian Ocean, the strengthening of the Indian dipole structure in autumn has a damping effect on the Indian dipole mode at the interannual time scale.     

Understanding and simulating the link between African easterly waves and Atlantic tropical cyclones using a regional climate model: the role of domain size and lateral boundary conditions

Using a suite of lateral boundary conditions, we investigate the impact of domain size and boundary conditions on the Atlantic tropical cyclone and african easterly Wave activity simulated by a regional climate model. Irrespective of boundary conditions, simulations closest to observed climatology are obtained using a domain covering both the entire tropical Atlantic and northern African region. There is a clear degradation when the high-resolution model domain is diminished to cover only part of the African continent or only the tropical Atlantic. This is found to be the result of biases in the boundary data, which for the smaller domains, have a large impact on TC activity. In this series of simulations, the large-scale Atlantic atmospheric environment appears to be the primary control on simulated TC activity. Weaker wave activity is usually accompanied by a shift in cyclogenesis location, from the MDR to the subtropics. All ERA40-driven integrations manage to capture the observed interannual variability and to reproduce most of the upward trend in tropical cyclone activity observed during that period. When driven by low-resolution global climate model (GCM) integrations, the regional climate model captures interannual variability (albeit with lower correlation coefficients) only if tropical cyclones form in sufficient numbers in the main development region. However, all GCM-driven integrations fail to capture the upward trend in Atlantic tropical cyclone activity. In most integrations, variations in Atlantic tropical cyclone activity appear uncorrelated with variations in African easterly wave activity.     

印度洋海气相互作用对热带夏季大气环流气候态的影响

利用分辨率较高的SINTEX-F(Scale INTeraction EXperiment-FRCGC) 海气耦合模式, 进行多组长时间积分模拟和理想试验, 分析研究热带印度洋海气耦合对夏季大气环流气候态的影响。主要结果有: (1) 热带印度洋海气相互作用使热带东印度洋产生明显的东风变化, 使热带中西太平洋赤道北部产生气旋性切变变化。 (2) 印度洋海气相互作用对大气环流气候态的影响绝大部分由于大气对海气相互作用的响应存在年际变化正负距平不对称性造成, 这种年际变化不对称性包括正偶极子与负偶极子的不对称、 海盆宽度正异常与海盆宽度负异常的不对称。 (3) 年际和季节内两种时间尺度海气相互作用对印度洋关键区大气环流平均态都有影响, 约各占60%、 40%; 季节内尺度海气相互作用对太平洋近赤道区大气环流平均态有重要影响; 年际尺度海气相互作用对太平洋赤道外地区大气环流平均态有重要影响。热带印度洋年际尺度、 季节内尺度海气相互作用对大气环流气候态的影响, 都存在年际变化以及年际变化正负距平不对称性。这两种尺度海气相互作用主要通过年际变化正负距平不对称性而对大气环流平均态产生影响。     

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     

WRF/ARPEGE-CLIMAT simulated climate trends over West Africa

The Weather Regional Forecast (WRF) model is used in this study to downscale low-resolution data over West Africa. First, the performance of the regional model is estimated through contemporary period experiments (1981?C1990) forced by ARPEGE-CLIMAT GCM output (ARPEGE) and ERA-40 re-analyses. Key features of the West African monsoon circulation are reasonably well represented. WRF atmospheric dynamics and summer rainfall compare better to observations than ARPEGE forcing data. WRF simulated moisture transport over West Africa is also consistent in both structure and variability with re-analyses, emphasizing the substantial role played by the West African Monsoon (WAM) and African Easterly Jet (AEJ) flows. The statistical significance of potential climate changes for the A2 scenario between 2032 and 2041 is enhanced in the downscaling from ARPEGE by the regional experiments, with substantial rainfall increases over the Guinea Gulf and eastern Sahel. Future scenario WRF simulations are characterized by higher temperatures over the eastern Tropical Atlantic suggesting more evaporation available locally. This leads to increased moisture advection towards eastern regions of the Guinea Gulf where rainfall is enhanced through a strengthened WAM flow, supporting surface moisture convergence over West Africa. Warmer conditions over both the Mediterranean region and northeastern Sahel could also participate in enhancing moisture transport within the AEJ. The strengthening of the thermal gradient between the Sahara and Guinean regions, particularly pronounced north of 10°N, would support an intensification of the AEJ northwards, given the dependance of the jet to the position/intensity of the meridional gradient. In turn, mid-tropospheric moisture divergence tends to be favored within the AEJ region supporting southwards deflection of moist air and contributing to deep moist convection over the Sahel where late summer rainfall regimes are sustained in the context of the A2 scenario regional projections. In conclusion, WRF proved to be a valuable and efficient tool to help downscaling GCM projections over West Africa, and thus assessing issues such as water resources vulnerability locally.     

The South Atlantic dipole and variations in the characteristics of the South American Monsoon in the WCRP-CMIP3 multi-model simulations

This study investigates relationships between Atlantic sea surface temperature (SST) and the variability of the characteristics of the South American Monsoon System (SAMS), such as the onset dates and total precipitation over central eastern Brazil. The observed onset and total summer monsoon precipitation are estimated for the period 1979?C2007. SST patterns are obtained from the Empirical Orthogonal Function. It is shown that variations in SST on interannual timescales over the South Atlantic Ocean play an important role in the total summer monsoon precipitation. Negative (positive) SST anomalies over the topical South Atlantic along with positive (negative) SST anomalies over the extratropical South Atlantic are associated with early (late) onsets and wet (dry) summers over southeastern Brazil and late (early) onset and dry (wet) summers over northeastern Brazil. Simulations from Phase 3 of the World Climate Research Programme Coupled Model Intercomparison Project (CMIP-3) are assessed for the 20th century climate scenario (1971?C2000). Most CMIP3 coupled models reproduce the main modes of variability of the South Atlantic Ocean. GFDL2.0 and MIROC-M are the models that best represent the SST variability over the South Atlantic. On the other hand, these models do not succeed in representing the relationship between SST and SAMS variability.     

Projected Changes in Asian Summer Monsoon in RCP Scenarios of CMIP5

Responses of the Asian Summer Monsoon(ASM) in future projections have been studied based on two core future projections of phase five of the Coupled Model Intercomparison Project(CMIP5) coordinated experiments with the IAP-coupled model FGOALS_s2(the Flexible Global Ocean-Atmosphere-Land System Model).The projected changes of the ASM in climatological mean and interannual variability were respectively reported.Both the South Asian Summer Monsoon(SASM) and the East Asian Summer Monsoon(EASM) were intensified in their climatology,featuring increased monsoon precipitation and an enhanced monsoon lower-level westerly jet flow.Accordingly,the amplitude of the annual cycle of rainfall over East Asia(EA) is enhanced,thereby indicating a more abrupt monsoon onset.After the EA monsoon onset,the EASM marched farther northward in the future scenarios than in the historical runs.In the interannual variability,the leading pattern of the EASM,defined by the first multi-variable EOF analysis over EA,explains more of the total variances in the warmest future scenario,specifically,Representative Concentration Pathway(RCP8.5).Also,the correlation coefficients analysis suggests that the relationship between the EASM interannual variations and ENSO was significantly strengthened in the future projections,which may indicate improved predictability of the EASM interannual variations.     

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.     

Interannual variability of rainfall over the Sahel based on multiple regional climate models simulations

We analyse the interannual variability of the averaged summer monsoon rainfall over the Sahel from multiple regional climate models driven by the ERA-interim reanalysis and seek to provide effective information for future modelling work. We find that the majority of the models are able to reproduce the rainfall variability with correlation coefficient exceeding 0.5 compared with observations. This is due to a good representation of the dynamics of the main monsoon features of the West African climate such as the monsoon flux, African Easterly Jet (AEJ) and Tropical Easterly Jet (TEJ). Among the models, only HIRHAM fails to reproduce the rainfall variability exhibiting hence a correlation coefficient of ?0.2. This deficiency originates from the fact that HIRHAM does not properly capture the variability of monsoon flow and the relationship between rainfall and the AEJ dynamic. We conclude that a good performance of a regional climate model in simulating the monsoon dynamical features variability is of primary importance for a better representation of the interannual variability of rainfall over the Sahel.     

东亚季风系统的动力过程和准定常行星波活动的研究进展

本文系统地回顾了近几年来关于东亚季风系统的动力过程与机理方面的研究,特别是关于东亚季风系统年际和年代际变异与准定常行星波活动关系的研究。最近的许多研究表明东亚夏季风系统变异的动力过程主要与东亚／太平洋型(即EAP型)遥相关有关,利用EAP型遥相关理论不仅可以说明东亚夏季风系统各成员之间内在联系的机理,而且可以揭示热带西太平洋热力和菲律宾周围对流活动影响东亚夏季风系统季节内、年际变化及其异常的经向三极子结构的动力过程;除了EAP型遥相关外,研究还表明北半球夏季从北非到东亚的对流层上层经向风异常存在一个沿急流传播的遥相关型,它对东亚夏季风系统异常的经向三极子型分布也有重要影响。并且,最近关于东亚冬季风变异与行星波活动的关系已做出许多研究,并获得很大进展。这些研究表明:北半球冬季准定常行星波传播波导在年际和年代际变化上存在着反相振荡特征,即若“极地波导”加强,则“低纬波导”将减弱,反之亦然;准定常行星波两支波导的反相振荡与北半球环状模(NAM)的年际和年代际振荡有紧密联系,而NAM的变化通过行星波活动的异常可以导致东亚冬季风的年际和年代际变化;此外,准定常行星波活动的年际变化与东亚冬季风异常之间的关系明显地受热带平流层纬向风准两年周期振荡(QBO)的调制,进一步的研究还提出了可能的机理。最后本文还指出:2005～2007年冬季东亚冬季风的异常不仅与西伯利亚高压和阿留申低压的变异有关,而且与极涡的演变和准定常行星波活动密切相关。     

Daily atmospheric variability in the South American monsoon system

The space–time structure of the daily atmospheric variability in the South American monsoon system has been studied using multichannel singular spectrum analysis of daily outgoing longwave radiation. The three leading eigenmodes are found to have low-frequency variability while four other modes form higher frequency oscillations. The first mode has the same time variability as that of El Nino-Southern Oscillation (ENSO) and exhibits strong correlation with the Pacific sea surface temperature (SST). The second mode varies on a decadal time scale with significant correlation with the Atlantic SST suggesting an association with the Atlantic multidecadal oscillation (AMO). The third mode also has decadal variability but shows an association with the SST of the Pacific decadal oscillation (PDO). The fourth and fifth modes describe an oscillation that has a period of about 165 days and is associated with the North Atlantic oscillation (NAO). The sixth and seventh modes describe an intraseasonal oscillation with a period of 52 days which shows strong relation with the Madden-Julian oscillation. There exists an important difference in the variability of convection between Amazon River Basin (ARB) and central-east South America (CESA). Both regions have similar variations due to ENSO though with higher magnitude in ARB. The AMO-related mode has almost identical variations in the two regions, whereas the PDO-related mode has opposite variations. The interseasonal NAO-related mode also has variations of opposite sign with comparable magnitudes in the two regions. The intraseasonal variability over the CESA is robust while it is very weak over the ARB region. The relative contributions from the low-frequency modes mainly determine the interannual variability of the seasonal mean monsoon although the interseasonal oscillation may contribute in a subtle way during certain years. The intraseasonal variability does not seem to influence the interannual variability in either region.