Serge Janicot

The West African Monsoon Dynamics. Part II: The “Preonset” and “Onset” of the Summer Monsoon

The arrival of the summer monsoon over West Africa has been documented by using daily gridded rainfall data and NCEP‐NCAR reanalyses during the period 1968‐90, and OLR data over the period 1979‐90. Two steps have been characterized through a composite approach: the preonset and the onset of the summer monsoon. The preonset stage corresponds to the arrival in the intertropical front (ITF) at 15 8N, that is, the confluence line between moist southwesterly monsoon winds and dry northeasterly Harmattan, bringing sufficient moisture for isolated convective systems to develop in the Sudano‐Sahelian zone while the intertropical convergence zone (ITCZ) is centered at 58N. The mean date for the preonset occurrence is 14 May and its standard deviation is 9.5 days during the period 1968‐90. This leads to a first clear increase of the positive rainfall slope corresponding to the beginning of the rainy season over this Sudano‐Sahelian area. The onset stage of the summer monsoon over West Africa is linked to an abrupt latitudinal shift of the ITCZ from a quasi-stationary location at 58N in May‐June to another quasi-stationary location at 108N in July‐August. The mean date for the onset occurrence is 24 June and its standard deviation is 8 days during the period 1968‐ 90. This leads to a second increase of the positive rainfall slope over the Sudano‐Sahelian zone signing the northernmost location of the ITCZ and the beginning of the monsoon season. This abrupt shift occurs mostly between 108W and 58E, where a meridional land‐sea contrast exists, and it is characterized by a temporary rainfall and convection decrease over West Africa. Preonset dates, onset dates, and summer rainfall amount over the Sahel are uncorrelated during the period 1968‐90. The atmospheric dynamics associated with the abrupt ITCZ shift has been investigated. Between the preonset and the onset stages, the heat low dynamics associated with the ITF controls the circulation in the low and midlevels. Its meridional circulation intensity is the highest at the beginning of the monsoon onset. This can lead to 1) increased convective inhibition in the ITCZ through intrusion of dry and subsiding air from the north, and 2) increased potential instability through a greater inland moisture advection and a higher monsoon depth induced by a stronger cyclonic circulation in the low levels, through higher vertical wind shear due to westerly monsoon wind and midlevel African easterly jet (AEJ) increases, through enhancement of the instability character of the AEJ, and through increased shortwave radiation received at the surface. During the monsoon onset, once the rainfall minimum occurred due to the convective inhibition, the accumulated potential instability breaks the convective inhibition, the inertial instability of the monsoon circulation is released, and the associated regionalscale circulation increases, leading to the abrupt shift of the ITCZ. Then the ITCZ moves north up to 108N, where thermodynamical conditions are favorable. It is suggested by the authors that the abrupt shift of the ITCZ, initiated by the amplification of the heat low dynamics, could be due to an interaction with the northern orography of the Atlas‐Ahaggar Mountains. Subsidence over and north of this orography, due to both the northern branches of the heat low and of the northern Hadleytype cell, contributes to enhance the high geopotentials north of these mountains and the associated northeasterly winds. This leads to the development of a leeward trough that reinforces the heat low dynamics, maintaining an active convective ITCZ through enhanced moist air advection from the ocean, increasing the northern Hadley circulation, which reinforces the high geopotentials and the interaction with the orography through a positive feedback. The fact that an abrupt shift of the ITCZ is only observed on the western part of West Africa may result from the enhancement of moisture advection, which comes from the west and has a stronger impact west of the Greenwich meridian. The northwest‐southeast orientation of the Atlas‐Ahaggar crest can induce the interaction with the heat low, first in the east where the mountains are nearer to the ITF than in the west, and second in the west. Another consequence of the possible orography-induced interaction with the atmospheric circulation is that the induced leeward trough, increasing the cyclonic vorticity in the heat low, may stimulate moisture convergence in the oceanic ITCZ near the western coast of West Africa.

Abrupt shift of the ITCZ over West Africa and intra‐seasonal variability

The onset of the monsoon system over West Africa is linked to the northward migration of the Inter-Tropical Convergence Zone (ITCZ) during the northern spring and summer. By using daily gridded rainfall data and NCEP/NCAR wind reanalyses over the period 1968–1990, we show that this migration is characterised by an abrupt latitudinal shift of the ITCZ in late June from a quasi-stationary location at 5N in May–June to another quasi-stationary location at 10N in July–August. A composite analysis based on the shift dates shows that this northward shift is associated with the occurrence of a westward-travelling monsoon depression pattern over the Sahel with characteristic periodicities of 20–40 days.

The West African Monsoon Dynamics. Part I: Documentation of Intraseasonal Variability

Abstract Intraseasonal variability in the West African monsoon is documented by using daily gridded datasets of rainfall and convection, and reanalyzed atmospheric fields, over the period 1968–90. Rainfall and convection over West Africa are significantly modulated at two intraseasonal timescales, 10–25 and 25–60 day, leading to variations of more than 30% of the seasonal signal. A composite analysis based on the dates of the maximum (minimum) of a regional rainfall index in wet (dry) sequences shows that these sequences last, on average, 9 days and belong to a main quasiperiodic signal of about 15 days. A secondary periodicity of 38 days is present but leads to a weaker modulation. During a wet (dry) sequence, convection in the ITCZ is enhanced (weakened) and its northern boundary moves to the north (south), while the speed of the African easterly jet decreases (increases), the speed of the tropical easterly jet increases (decreases), and the monsoon flow becomes stronger (weaker), all these features bei...

From GCM grid cell to agricultural plot: scale issues affecting modelling of climate impact

General circulation models (GCM) are increasingly capable of making relevant predictions of seasonal and long-term climate variability, thus improving prospects of predicting impact on crop yields. This is particularly important for semi-arid West Africa where climate variability and drought threaten food security. Translating GCM outputs into attainable crop yields is difficult because GCM grid boxes are of larger scale than the processes governing yield, involving partitioning of rain among runoff, evaporation, transpiration, drainage and storage at plot scale. This study analyses the bias introduced to crop simulation when climatic data is aggregated spatially or in time, resulting in loss of relevant variation. A detailed case study was conducted using historical weather data for Senegal, applied to the crop model SARRA-H (version for millet). The study was then extended to a 10°N–17° N climatic gradient and a 31 year climate sequence to evaluate yield sensitivity to the variability of solar radiation and rainfall. Finally, a down-scaling model called LGO (Lebel–Guillot–Onibon), generating local rain patterns from grid cell means, was used to restore the variability lost by aggregation. Results indicate that forcing the crop model with spatially aggregated rainfall causes yield overestimations of 10–50% in dry latitudes, but nearly none in humid zones, due to a biased fraction of rainfall available for crop transpiration. Aggregation of solar radiation data caused significant bias in wetter zones where radiation was limiting yield. Where climatic gradients are steep, these two situations can occur within the same GCM grid cell. Disaggregation of grid cell means into a pattern of virtual synoptic stations having high-resolution rainfall distribution removed much of the bias caused by aggregation and gave realistic simulations of yield. It is concluded that coupling of GCM outputs with plot level crop models can cause large systematic errors due to scale incompatibility. These errors can be avoided by transforming GCM outputs, especially rainfall, to simulate the variability found at plot level.

West African Monsoon Dynamics and Eastern Equatorial Atlantic and Pacific SST Anomalies (1970–88)

Abstract The Laboratoire de Meteorologie Dynamique atmospheric GCM is used to investigate relationships between West African monsoon dynamics and SST anomalies in the eastern equatorial Atlantic and Pacific for the period 1970–88. Positive SST anomalies in the eastern equatorial Pacific, mainly associated with a larger east–west divergent circulation over the tropical Atlantic, are found to coincide with negative rainfall anomalies over West Africa. This is the case for the composite ENSO warm episodes of 1972, 1976, 1982, and 1983. By contrast, positive SST anomalies in the eastern equatorial Atlantic are accompanied by a southward shift of the intertropical convergence zone along with negative rainfall anomalies in the Sahel and positive rainfall anomalies in the Guinean region. This was the case in 1987. The ENSO warm event during this year had apparently no significant impact on West African monsoon dynamics. A zonal atmospheric coupling associated with differences of SST anomalies between the eastern...

Sea Surface Temperature Fields Associated with West African Rainfall Anomaly Types

Abstract Four West African rainfall anomaly types are defined in relation to the northern summer rainfall departure signs in the Sahel and in the Guinean region in order to investigate the statistical links between interannual variability of West African rainfall and sea surface temperature (SST) through the period 1950–90. Composite analysis depicts the setup of four different mean SST anomaly fields. Drought over all of West Africa is associated with the growth of positive SST anomalies in the eastern Pacific and in the Indian Ocean, and negative SST anomalies in the northern Atlantic and in the Gulf of Guinea. In contrast, drought limited to the Sahel corresponds mostly to a northward expansion of positive SST anomalies in the southern Atlantic, and negative SST anomalies in the northern Atlantic. Northward expansion of negative SST anomalies in the southern Atlantic, positive SST departures in the northern Atlantic, and development of negative SST anomalies in the eastern Pacific appear to be synchron...

Rainfall anomaly patterns and wind field signals over West Africa in August (1958-1989)

Abstract Relationships between monthly West African rainfall anomaly patterns and monthly tropospheric wind changes are investigated for the 32 months of August (1958–1989) at an interannual time step. Regarding the Sahelian belt alone, results show that wet (dry) months are significantly linked to an increase (decrease) in both upper easterlies and lower southwesterlies, along with weaker (stronger) than usual midtropospheric easterlies south and under the main axis of the African Easterly Jet (AEJ). However, the most interesting signals are obtained with consideration of the West African rainfall anomaly patterns. Large droughts over Sahelian and Guinean areas are significantly associated with a less southward extension of upper easterlies and a decrease of the upper-meridional diffluence in the Hadley circulation. Contrasted rainfall anomaly patterns exhibiting both Sahelian droughts and Guinean floods are mainly linked to easterly anomalies in the equatorial stratosphere and enhanced easterlies south ...

THE AMMA RADIOSONDE PROGRAM AND ITS IMPLICATIONS FOR THE FUTURE OF ATMOSPHERIC MONITORING OVER AFRICA

In the face of long-term decline, the AMMA research program has reactivated the radiosonde network over West Africa. The lessons learned in AMMA have significance for the upper-air network throughout the continent.

Spatiotemporal variability of west African rainfall. Part I. Regionalizations and typings

Abstract Space and time scales for analysis of the interannual variability of Sahelian rainfall are determined. Regionalizations of annual and monthly rainfall fields are performed in West Africa for the period 1948–78. Four coherent regions and a less coherent one are identified. Different type classifications derived from the regionalization results are built. The monthly type based on the rainfall anomaly signs north and south of 10°N suggests two major causes of the rainfall pattern variability, one resulting from an anomaly of rainfall amount and the other from a displacement of the intertropical convergence zone (ITCZ). The type based solely on the anomaly sign north of 10°N blends these factors and may give misleading analyses. The use of monthly rainfall fields over all of West Africa is then recommended.

Spatiotemporal Variability of West African Rainfall. Part II: Associated Surface and Airmass Characteristics

Abstract In the second part of this study, the variability of surface and airmass characteristics associated with West African rainfall fields is investigated. Ship observations over the tropical Atlantic and rawinsonde records over West Africa are used for the months of July, August, and September from 1948 to 1978. A composite analysis based on the sign of monthly rainfall departures north of 10°N indicates that the Sahelian drought may result from a more southern than normal position of the intertropical convergence zone (ITCZ) over the tropical Atlantic. A composite analysis into four monthly rainfall types is performed by using rainfall information north and south of 10°N. It shows two types of Sahelian drought. The first is linked to an enhanced surface atmospheric circulation over the northern tropical Atlantic, higher sea surface temperatures in the Gulf of Guinea, a convergent moisture flux anomaly south of 10°N along the western African coast, and a lower (larger) water content in the monsoon fl...