A hydrological onset and withdrawal index for the West African monsoon

We have developed a hydrological prognostic index, HOWI (hydrological onset and withdrawal index), for the onset and the withdrawal of the West African monsoon (WAM), based on the vertically integrated moisture transport (VIMT). The regions of West Africa with the same climatological onset (withdrawal) date are characterized by a large change of the VIMT around the onset (withdrawal) date. By analyzing the variability of the VIMT, we determine the extension and the geographical position of these regions, which we take sufficiently large to filter out the fast weather variability. It turns out that the regions with the same climatological onset date do not usually coincide with the regions with the same climatological withdrawal date, the areas with the maximum variability of the VIMT during the onset phase are usually a fraction of the area where the variability of the VIMT is large during the withdrawal phase. This is because the onset has active phases and pauses in time and it is fragmented in space, while the withdrawal is rather rapid and almost uniformly distributed through the entire monsoonal region. When the monsoon moves inland, the rainfall slightly trails behind the arrival of the moisture, and, when the monsoon moves back towards the gulf of Guinea, the moisture slightly precedes the retreating rainfall. In a specific region, we say that the onset (withdrawal) of the monsoon occurs when the moisture reaches (declines to) half of its climatological value. The level of the moisture relatively to its climatological value is evaluated through the HOWI, i.e., at the onset (withdrawal) the HOWI is zero with a positive (negative) tendency. We find that the dates of the onset of the monsoon determined using the HOWI, computed in the region where the VIMT has its maximum variability during the onset phase of WAM, well agree with the dates of the sudden transition of the ITCZ (Intertropical Convergence Zone) from 5 to 10°N. The uncertainty on the onset date is of the order of 2 pentads, which is comparable to the uncertainty on the date of the sudden transition of the ITCZ. We, then, use the HOWI to determine the onset and the withdrawal dates of the monsoon for the period 1979–2004, finding that an early (late) onset usually preludes to a longer (shorter) monsoonal season with more (less) cumulated rain. Finally, we compare the onset dates in the Sahelian region, for the period 1979–2004, with those determined using methods based on rainfall.     

Seasonal forecasts for regional onset of the West African monsoon

The West African monsoon has over the years proven difficult to represent in global coupled models. The current operational seasonal forecasting system of the UK Met Office (GloSea4) has a good representation of monsoon rainfall over West Africa. It reproduces the various stages of the monsoon: a coastal phase in May and June, followed by onset of the Sahelian phase in July when rainfall maxima shift northward of 10N until September; and a secondary coastal rainfall maximum in October. We explore the dynamics of monsoon onset in GloSea4 and compare it to reanalyses. An important difference is the change in the Saharan heat low around the time of Sahelian onset. In Glosea4 the deepening heat low introduces moisture convergence across an east-west Sahelian band, whereas in the reanalyses such an east-west organisation of moisture does not occur and moisture is transported northwards to the Sahara. Lack of observations in the southern Sahara makes it difficult to verify this process in GloSea4 and also suggests that reanalyses may not be strongly constrained by station observations in an area key to Sahelian onset. Timing of monsoon onset has socio-economic importance for many countries in West Africa and we explore onset predictability in GloSea4. We use tercile categories to calculate probabilities for onset occurring before, near and after average in four different onset indicators. Glosea4 has modest skill at 2–3 months’ lead time, with ROC scores of 0.6–0.8. Similar skill is seen in hindcasts with models from the ENSEMBLES project, even in models with large rainfall biases over the Sahel. Forecast skill derives from tropical SST in June and many models capture at least the influence of the tropical Atlantic. This suggests that long-range skill for onset could be present in other seasonal forecasting systems in spite of mean rainfall biases.     

Long-term variability in the date of monsoon onset over western India

The date of onset of the southwest monsoon in western India is critical for farmers as it influences the timing of crop plantation and the duration of the summer rainy season. Identifying long-term variability in the date of monsoon onset is difficult, however, as onset dates derived from the reanalysis of instrumental rainfall data are only available for the region from 1879. This study uses documentary evidence and newly uncovered instrumental data to reconstruct annual monsoon onset dates for western India for the period 1781–1878, extending the existing record by 97 years. The mean date of monsoon onset over the Mumbai (Bombay) area during the reconstruction period was 10 June with a standard deviation of 6.9 days. This is similar to the mean and standard deviation of the date of monsoon onset derived from instrumental data for the twentieth century. The earliest identified onset date was 23 May (in 1802 and 1839) and the latest 22 June (in 1825). The longer-term perspective provided by this study suggests that the climatic regime that governs monsoon advance over western India did not change substantially from 1781 to 1955. Monsoon onset over Mumbai has occurred at a generally later date since this time. Our results indicate that this change is unprecedented during the last 230 years. Following a discussion of the results, the nature of the relationship between the date of monsoon onset and the El Niño-Southern Oscillation is discussed. This relationship is shown to have been stable since 1781.     

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.     

Precipitation sensitivity to regional SST in a regional climate simulation during the West African monsoon for two dry years

The influence on precipitation of regional sea surface temperature (SST) during a drought period of the West African monsoon is determined, using a regional climate model (RCM). The results from three simulations of two realistic dry years are compared. The first two experiments are initialised and nested respectively in 1983 and 1984 reanalysis data sets. The third experiment is a hybrid simulation of 1983 which is the same as the first experiment except that the SST field is the 1984 SST. Precipitation from the RCM is compared with several precipitation data sets and, as in observations, the RCM reasonably simulates the West African monsoon (seasonal cycle and monsoon sub-period) for the two different years. In particular, the model reproduces stage by stage the motion of the monsoon band well: installation phase, high rain period with abrupt northward shift of the rain band, and the retreat southward phase. Interannual variability and wet or dry tendencies are also represented. The most significant effect of SST is shown by the hybrid simulation, when the regional SST appears as a major factor in the seasonal and interannual monsoon precipitation regime over the African continent (up to 12°N) although this influence is modulated both by the surface conditions (soil and vegetation) and by the reanalysis flow introduced at the lateral boundaries. Dynamically, a warmer SST leads to a decrease in the magnitude of the African Easterly Jet and an increase in northward equivalent water content transport (from equator to 12°N).     

The West African monsoon onset in 2006: sensitivity to surface albedo, orography, SST and synoptic scale dry-air intrusions using WRF

In order to test the sensitivity of the transitional phase of the 2006 West African monsoon (WAM) onset to different mechanisms, weather research and forecasting (WRF) model simulations have been carried out addressing the role of the Saharan heat low (SHL) and its sensitivity to the albedo field and to the northern Africa orography, and the role of the sea surface temperature (SST) in the eastern tropical Atlantic and Mediterranean. Lowering albedo over the desert region induces a northward location of the inter-tropical convergence zone (ITCZ), while removing mountains in North Africa reduces rainfall over West Africa. Shifting SST forward by 15?days leads to a northward location of the ITCZ before the WAM onset. However none of these factors modifies the timing of the WAM onset in 2006. The transitional phase of the 2006 WAM onset has been examined in more detail. The enhancement of SHL intensity, combined with the development of the oceanic cold tongue in the Guinea gulf, leads to low-level moisture flux divergence in the ITCZ reducing rainfall and increasing low-level humidity over the Sahel. However, weakening of convection can be clearly attributed to dry-air intrusions in mid-levels, originating from the subtropical westerly jet and associated with Rossby wave pattern over North Africa. Sensitivity tests on the synoptic scale forcing outside of the WRF model domain confirm the dominating role of large-scale dynamics to control the transitional phase of the WAM onset and its timing. However it is shown that the regional factors can modulate this larger scale forcing.     

青藏高原的热力和机械强迫作用以及亚洲季风的爆发I. 爆发地点

使用欧洲中期天气预报中心（ECMWF）的客观分析资料、ECMWF/TOGA补充数据集,美国NMC气候分析中心的向外长波辐射（OLR）资料以及国家气候中心存档的中国336个测站的降水资料,研究了1989年春天青藏高原和邻近地区的热力特征和环流特征,及其对亚洲季风区季节转换的影响。文中集中分析了表面感热和潜热通量的时空分布特征。结果表明:1989年亚洲季风的爆发由三个接续的阶段组成。第一阶段是5月上旬在孟加拉湾东岸,称为孟加拉（BOB）季风爆发阶段。第二阶段是5月20日左右开始的中国南海（SCS）季风爆发阶段。第三阶段是6月10日左右开始的印度上空的南亚季风（或称印度季风）的爆发阶段。分析表明,正是由于青藏高原的热力和机械强迫作用才使亚洲季风首先在孟加拉湾地区出现。BOB季风环流提供了有利的背景条件,使SCS季风接着爆发。最后随着亚洲热带流型的西移,印度季风爆发才发生。     

Climate simulation over CORDEX Africa domain using the fifth-generation Canadian Regional Climate Model (CRCM5)

The new fifth-generation Regional Climate Model (CRCM5) was driven by ERA reanalyses for the period 1984–2008 over the African continent following the CORDEX experimental protocol. Overall the model succeeds in reproducing the main features of the geographical distribution and seasonal cycle of temperature and precipitation, the diurnal cycle of precipitation, and the West African Monsoon (WAM). Biases in surface temperature and precipitation are discussed in relation with some circulation defects noted in the simulation. In the African regions near the equator, the model successfully reproduces the double peak of rainfall due to the double passage of the tropical rainbelt, although it better simulates the magnitude and timing of the second peak of precipitation. CRCM5 captures the timing of the monsoon onset for the Sahel region but underestimates the magnitude of precipitation. The simulated diurnal cycle is quite well simulated for all of the regions, but is always somewhat in advance for the timing of rainfall peak. In boreal summer the CRCM5 simulation exhibits a weak cold bias over the Sahara and the maximum temperature is located too far south, resulting in a southward bias in the position of the Saharan Heat Low. The region of maximum ascent in the deep meridional circulation of the Hadley cell is well located in the CRCM5 simulation, but it is somewhat too narrow. The core of the African Easterly Jet is of the right strength and almost at the right height, but it is displayed slightly southward, as a consequence of the southward bias in the position of the Saharan Heat Low and the thermal wind relationship. These biases appear to be germane to the WAM rainfall band being narrower and not moving far enough northward, resulting in a dry bias in the Sahel.     

Impacts of warm and cold situations in the Mediterranean basins on the West African monsoon: observed connection patterns (1979–2006) and climate simulations

Using both empirical and numerical ensemble approaches this study focuses on the Mediterranean/West African relationship in northern summer. Statistical analyses utilize skin temperature, sea surface temperature, in situ and satellite rainfall, outgoing longwave radiation (OLR) observations and reanalyzed data winds and specific humidity on isobaric surfaces. Numerical investigations are based on a large set of sensitivity experiments performed on four atmospheric general circulation models (AGCM): ARPEGE-Climat3, ECHAM4, LMDZ4 and UCLA7.3. Model outputs are compared to observations, discussed model by model and with an ensemble (multi-model) approach. As in previous studies the anomalous Mediterranean warm events are associated with specific impacts over the African monsoon region, i.e., a more intense monsoon, enhanced flux convergence and ascendances around the ITCZ, a strengthening of low level moisture advection and a more northward location of ascending motion in West Africa. The results show also new features (1) thermal variability observed in the two Mediterranean basins has unalike impacts, i.e. the western Mediterranean covaries with convection in Gulf of Guinea, while the eastern Mediterranean can be interpreted as Sahelian thermal-forcing; (2) although observations show symmetry between warming and cooling, modelling evidences only support the eastern warming influence; (3) anomalous East warm situations are associated with a more northward migration of the monsoon system accompanied by enhanced southwertely flow and weakened northeasterly climatological wind; (4) the multi-model response shows that anomalous East warm surface temperatures generate an enhancement of the overturning circulation in low and high levels, an increase in TEJ (Tropical Eeasterly Jet) and a decrease in AEJ (African Eeasterly Jet).     

Simulation of the West African monsoon onset using the HadGEM3-RA regional climate model

The performance of the Hadley Centre Global Environmental Model version 3 regional climate model (HadGEM3-RA) in simulating the West African monsoon (WAM) is investigated. We focus on performance for monsoon onset timing and for rainfall totals over the June–July–August (JJA) season and on the model’s representation of the underlying dynamical processes. Experiments are driven by the ERA-Interim reanalysis and follow the CORDEX experimental protocol. Simulations with the HadGEM3 global model, which shares a common physical formulation with HadGEM3-RA, are used to gain insight into the causes of HadGEM3-RA simulation errors. It is found that HadGEM3-RA simulations of monsoon onset timing are realistic, with an error in mean onset date of two pentads. However, the model has a dry bias over the Sahel during JJA of 15–20 %. Analysis suggests that this is related to errors in the positioning of the Saharan heat low, which is too far south in HadGEM3-RA and associated with an insufficient northward reach of the south-westerly low-level monsoon flow and weaker moisture convergence over the Sahel. Despite these biases HadGEM3-RA’s representation of the general rainfall distribution during the WAM appears superior to that of ERA-Interim when using Global Precipitation Climatology Project or Tropical Rain Measurement Mission data as reference. This suggests that the associated dynamical features seen in HadGEM3-RA can complement the physical picture available from ERA-Interim. This approach is supported by the fact that the global HadGEM3 model generates realistic simulations of the WAM without the benefit of pseudo-observational forcing at the lateral boundaries; suggesting that the physical formulation shared with HadGEM3-RA, is able to represent the driving processes. HadGEM3-RA simulations confirm previous findings that the main rainfall peak near 10°N during June–August is maintained by a region of mid-tropospheric ascent located, latitudinally, between the cores of the African Easterly Jet and Tropical Easterly Jet that intensifies around the time of onset. This region of ascent is weaker and located further south near 5°N in the driving ERA-Interim reanalysis, for reasons that may be related to the coarser resolution or the physics of the underlying model, and this is consistent with a less realistic latitudinal rainfall profile than found in the HadGEM3-RA simulations.     

LOW-FREQUENCY OSCILLATION AND MECHANISM OF VERTICAL CIRCULATION OF EASTERN ASIAN MONSOON*

Analysis is performed of low-frequency oscillation (LFO) and its relation to monsoon by means of ECMWF numerical prediction data in the period 1 June to 30 September 1984,indicating that remarkable local LFO exists in the vertical meridional and equatorial zonal circulations.And preliminary discussion is made of the origin of the LFO of the East-Asian summer monsoon meridional circulation in the LFO of the mid and upper troposphere vertical motion around 30°S.The LFOs in the meridional circulations of both hemispheres are linked together by the LFO of the meridional circulation.Finally the possible relation between the tropical monsoon LFO and Meiyu (plum rain).     

晚春初夏西太平洋副热带高压南撤过程的气候学特征

利用1979—2006年多年平均逐日NCEP/NCAR再分析资料、NOAA的OLR和逐候CAMP降水资料,从气候学角度探讨了晚春初夏季节转换时期,西太平洋副热带高压(副高)脊线位置变化及其与亚洲夏季风爆发的关系。发现晚春初夏时期西太平洋副高在向北移动过程中存在一次显著的南撤过程,之后西太平洋副高发生第一次北跳,南撤主要发生在对流层高层和低层,南撤生命期可达2周,且高层的南撤过程结束时间比低层的南撤过程开始时间早约1旬,这为预测低层副高南撤及其第一次北跳提供了有意义的前期信号。低层西太平洋副高南撤的同时伴随着一次显著东退过程。在低层副高南撤结束后(约5月底),由于气温经向梯度的变化使副高脊轴倾斜发生反转。晚春初夏的西太平洋副高南撤过程与亚州夏季风爆发、强对流活动和降雨带的移动变化关系密切。在对流层高层西太平洋副高南撤过程的中后期(约4月底),夏季风在安达曼海和临近孟加拉湾爆发。在对流层低层西太平洋副高南撤过程开始后,南海夏季风开始爆发(5月14—15日);南撤过程结束后(6月初),印度夏季风爆发;在副高脊线返回日后(6月中),东亚夏季风爆发。西太平洋副高南撤过程不同阶段的建立时间为预知亚洲不同地区夏季风的爆发时间提供了非常有用的信息。此外,在西太平洋副高主体南北两侧存在两支强的雨带,与副高主体控制的少雨带构成一个典型的"湿干湿"三明治雨型,这个雨型的变化与西太平洋副高脊线移动有关。     

北半球夏季风区大气视热源和视水汽汇的低频振荡

利用１９８６年５－９月ＥＣＭＷＦ／ＷＭＯ资料计算非洲季风区、印度季风区、南海季风区和副热季风区的视热源和视水汽汇。结果表明非洲季风区和印度季风区Ｑ１、Ｑ２的准４０天周期显著；南海季风区准双周振荡明显；副热带季风区盛行８天左右的周期；准４０天周期振荡也是南海季风区和副热带季风区的重要信号，印度季风区Ｑ１，Ｑ２的准４０天周期振荡比其他季风区的更为显著；非洲季风区Ｑ１振荡位相超前于Ｑ２振荡位相，其他季风     

A study on the systematic errors of the tropical forecasts: Influence of physical processes

Summary In this paper, an attempt is made to examine the influence of the physical forcings of an atmospheric general circulation model (AGCM) in the reduction of the systematic errors of the tropical forecasts. A number of major modifications in the parameterization of physical processes were carried out in the operational forecasting system of the European Centre for Medium Range Weather Forecasts (ECMWF) during the period 1984–88 largely in an attempt to reduce the conceptual weaknesses in their formulation. A large number of studies (Slingo et al., 1988; Tiedtke et al., 1988; etc) have demonstrated the positive impact on the reduction of tropical forecast errors to various changes in the treatment of physical processes in the ECMWF model.Keeping in view of these facts, the evaluation of the systematic errors of the ECMWF tropical forecasts is carried out for a period prior to the incorporation of major modifications in the parameterization of physical processes (1984) and corresponding period after such major changes are implemented in the operational AGCM of ECMWF (1988). The paper describes a detailed comparison of the tropical forecast errors for summer monsoon seasons (June-August [JJA]) of 1984 and 1988 in order to bring out the impact on tropical simulation of various improvements in the treatment of physical processes.The results demonstrate a dramatic reduction in the systematic errors of the tropical circulation together with an enhancement of the hydrological cycle to a realistic climatological level with the incorporation of major changes in the treatment of physical processes. Similar improvements are also observed in the winter simulation. In spite of major improvements in the simulation of tropical circulation, the nature of the tropical systematic errors of the ECMWF AGCM, however, remains unchanged. Thus, the inference of the study indicates the requirement of a new approach to the problem of parameterization of physical processes particularly, convection, radiation, boundary layer and their interactions for further reduction of the tropical forecast errors.With 14 Figures     

Fluctuations in annual cycles and inter-seasonal memory in West Africa: rainfall, soil moisture and heat fluxes

Summary Annual cycle and inter-seasonal persistence of surface-atmosphere water and heat fluxes are analyzed at a 5-day time step over the West African Monsoon (WAM) through observational precipitation estimates (CMAP), model datasets (NCEP/DOE level 2 reanalyses) and a Soil Water Index (SWI) from the ERS scatterometer. Coherent fluctuations (30–90 days) distinct from supra-synoptic variability (10–25 day periods) are first detected in the WAM precipitation and heat fluxes over the period 1979–2001. During all the northward excursion of the WAM rain band, a succession of four active phases (abrupt rainfall increases) occurs. They are centered in the first days of March, mid-April, the second half of May and from the last week of June to mid-July (the Sahelian onset). A simple statistical approach shows that the Spring to Summer installation of the monsoon tends to be sensitive to these short periods. Other analyses suggest the existence of lagged relationship between rainfall amounts registered in successive Fall, Spring (active periods) and Summer (top of the rainy season) implying land surface conditions. The spatial extension of the generated soil moisture anomalies reaches one maximum in March, mainly at the Guinean latitudes and over the Sahelian belt where the signal can persist until the next monsoon onset. Typically after abnormal wet conditions in September–October two signals are observed: (1) more marked fluctuations in Spring with less (more) Sahelian rainfall in May (June and after) at the Sahelian-Sudanian latitudes; (2) wetter rainy seasons along the Guinean coast (in Spring and Summer with an advance in the mean date of the ‘little dry season’). The reverse arises after abnormal dry conditions in autumn.     

Seasonal Transition Features of Large-Scale Moisture Transport in the Asian-Australian Monsoon Region

Using NCEP/NCAR reanalysis data for the period of 1957-2001, the climatological seasonal transition features of large-scale vertically integrated moisture transport （VIMT） in the Asian-Australian monsoon region are investigated in this paper. The basic features of the seasonal transition of VIMT from winter to summer are the establishment of the summertime ＂great moisture river＂ pattern （named the GMR pattern） and its eastward expansion, associated with a series of climatological events which occurred in some ＂key periods＂, which include the occurrence of the notable southerly VIMT over the Indochina Peninsula in mid March, the activity of the low VIMT vortex around Sri Lanka in late April, and the onset of the South China Sea summer monsoon in mid May, among others. However, during the transition from summer to winter, the characteristics are mainly exhibited by the establishment of the easterly VIMT belt located in the tropical area, accompanied by some events occurring in ＂key periods＂. Further analyses disclose a great difference between the Indian and East Asian monsoon regions when viewed from the meridional migration of the westerly VIMT during the seasonal change process, according to which the Asian monsoon region can be easily divided into two parts along the western side of the Indochina Peninsula and it may also denote different formation mechanisms between the two regions.     

Multi-stage onset of the summer monsoon over the western North Pacific

 The climatological summer monsoon onset displays a distinct step wise northeastward movement over the South China Sea and the western North Pacific (WNP) (110°–160°E, 10°–20°N). Monsoon rain commences over the South China Sea-Philippines region in mid-May, extends abruptly to the southwestern Philippine Sea in early to mid-June, and finally penetrates to the northeastern part of the domain around mid-July. In association, three abrupt changes are identified in the atmospheric circulation. Specifically, the WNP subtropical high displays a sudden eastward retreat or quick northward displacement and the monsoon trough pushes abruptly eastward or northeastward at the onset of the three stages. The step wise movement of the onset results from the slow northeastward seasonal evolution of large-scale circulation and the phase-locked intraseasonal oscillation (ISO). The seasonal evolution establishes a large-scale background for the development of convection and the ISO triggers deep convection. The ISO over the WNP has a dominant period of about 20–30 days. This determines up the time interval between the consecutive stages of the monsoon onset. From the atmospheric perspective, the seasonal sea surface temperature (SST) change in the WNP plays a critical role in the northeastward advance of the onset. The seasonal northeastward march of the warmest SST tongue (SST exceeding 29.5 °C) favors the northeastward movement of the monsoon trough and the high convective instability region. The seasonal SST change, in turn, is affected by the monsoon through cloud-radiation and wind-evaporation feedbacks. Received: 19 October 1999 / Accepted: 5 June 2000     

Signature of a southern hemisphere extratropical influence on the summer monsoon over India

The weakening relationship of El Nino with Indian summer monsoon reported in recent years is a major issue to be addressed. The altered relationships of Indian monsoon with various parameters excite to search for other dominant modes of variability that can influence the precipitation pattern. Since the Indian summer monsoon circulation originates in the oceanic region of the southern hemisphere, the present study investigates the association of southern extratropical influence on Indian summer monsoon using rainfall and reanalysis parameters. The effect of Southern Annular Mode (SAM) index during the month of June associated with the onset phase of Indian summer monsoon and that during July–August linked with the active phase of the monsoon were analysed separately for a period from 1951 to 2008. The extra-tropical influence over the monsoon is illustrated by using rainfall, specific humidity, vertical velocity, circulation and moisture transport. The June high SAM index enhances the lower level wind flow during the onset phase of monsoon over Indian sub-continent. The area of significant positive correlation between precipitation and SAM in June also shows enhancement in both ascending motion and specific humidity during the strong phase of June SAM. On the other hand, the June high SAM index adversely affects July–August monsoon over Indian subcontinent. The lower level wind flow weakens due to the high SAM. Enhancement of divergence and reduction in moisture transport results in the Indian monsoon region due to the activity of this high southern annular mode. The effect is more pronounced over the southwest region where the precipitation spell has high activity during the period. Significant correlation exists between SAM and ISMR, even after removing the effect of El Nino. It indicates that the signals of Indian summer monsoon characteristics can be envisaged to a certain extend using the June SAM index.     

Thermodynamic budget of the five day wave over the Saharan desert during summer

Summary A spectral analysis of the domain averaged height field at 1000 hPa surface over the Sahara using ECMWF data reveals a major oscillation of about five days. A composite analysis technique has then been developed which permits to emphasize the major characteristics of the evolution of the thermal low over the Sahara during a five day period in the summer season. This analysis shows that the composite structure of the termal field reveals a well mixed layer with an almost constant potential temperature and specific humidity from the surface up to about 650 hPa.The computation of the vertical velocity and the horizontal divergence reveals a presence of a two cell vertical circulation over the thermal low region. Although the heat low over the Saharan desert appears as a shallow low pressure area confined into the mixed layer, the entire troposphere seems to be dynamically active.In the surface layer, the response of the height field to the temperature field has a lag time of about one day.Adjustment of the theoretical net radiative heating rate have been applied assuming dust to have an effect on both shortwave and longwave irradiances. The results compare reasonably well with measurements obtained from the literature, and provide an improvement for the surface heat fluxes as compared to those deduced from conventional radiative transfer model.The thermodynamic budget over the desert region reveals that the contribution of the advective terms is to stabilize the mixed layer by removing the excess of heat from the surface layer. Furthermore, the destabilization of the mixed layer seems to result from a thermal unbalance between the net radiative heating and the convergence of the eddy sensible heat flux during the formative stage of the thermal low.Downward motion developing above the mixed layer during the occurrence of the heat low seems to be responsible for the cooling of the surface layer and a decrease of the surface temperature, which marks the decaying phase of the thermal wave.A qualitative inference of the top of the dust layer from the large scale variables has been attempted, and a simple scheme for the parameterization of the net solar radiation as a function of the large scale apparent heating is proposed. The results appear to be a reasonable approach for the definition of the net radiative heating.The moisture budget reveals that water vapor is supplied to the domain via horizontal advection from the region of the West African monsoon. Furthermore, its vertical distribution appears to be controlled by the complex vertical eddy motions in the dry thermals which develop over the warm Saharan desert during the afternoon hours.With 27 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.