Anomalous northern hemispheric summer mid-tropospheric circulation over West Africa

The 400-500 hPa isobaric surfaces of the midtroposphere, generally acknowledged as those levels where maximum ascent is located, play a critical role in weather and climate. Knowledge of the circulation in these levels in relation to the dominant modes of precipitation is of paramount importance in providing useful insights for the improvement of our understanding of precipitation variability over this subcontinent. The goal of this study is to characterize and describe the circulation anomalies in the middle level represented by 500 hPa, and their relationships with the surface anomalies, on the basis of the dominant precipitation modes spanning 1948 to 2006 during northern hemisphere summer (June-July-August-September; JJAS) season. Monthly unfiltered (raw) University of Delware terrestrial precipitation and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) data are used for this study. The primary analysis tools utilized are empirical orthogonal function (EOF) and composite analyses. EOF analysis is performed on precipitation over West Africa, out of which the four leading and physically realistic modes are retained. The associated standardized expansion coefficients (ECs) are decomposed into very wet and dry years, the selections of which are based on ECs whose magnitudes are at least one standard deviation (in absolute terms) from the mean. The decompositions yield four distinct modes each for the wet and dry years, a total of eight precipitation events. Anomalies from horizontal winds fields (1000 hPa and 500 hPa) are computed and then blended with the decomposed ECs, to produce composites. From the wind field composites, velocity potential (divergent) and streamfunction (rotational) anomalies are computed at the two levels, for all the events associated with the eight precipitation ECs. The results show that the centers of action of the divergent circulation anomalies at the middle and surface levels are characterized by spatial variability, highlighting their distinctiveness in relationship to the precipitation modes during the very wet and dry epochs. The anomalous streamfunction and rotational flows in the two levels reveal isolated, varied, and, in some cases, large-scale, co-located, or, juxtaposed, cyclonic and anticyclonic flows generated and maintained by diabatic heating or cooling anomalies. These flows are seen to be directly or remotely linked to West African, Sahelian, and subtropical Saharan highs and heat lows in the Northern Hemisphere (NH) Africa, which play a critical role in the West African monsoon (WAM). In the wet years associated with the leading JJAS West African precipitation mode, a large-scale anomalous surface level cyclonic flow is detected traversing the NH tropical Africa, from east to west. This flow, confined to latitudinal bands between 10oN and 30oN, captures the geographic boundaries of the entire Sahel belt, and escapes off the west coast of Africa, through the exit region of African easterly waves (the precursors of tropical cyclones), and extends into the tropical North Atlantic (TNA) Ocean, in proximity to the main development region (MDR), which is the active site of tropical cyclogenesis. The large-scale low level cyclonic flow, which coincides with the North Atlantic Hurricane season, suggests a strong NH tropical Africa summertime monsoonal flow that accompanies anomalously wet season wherein the humid Gulf of Guinea coast (GOGC) receives a relatively minimal precipitation during the pluvial phase of the Sahelian season. The anomalously wet Sahel is seen to be driven by moisture flux convergence depicted by the low level divergent circulation anomalies. The 500 hPa cyclonic flow appears to be stronger and of much more spatial extent aloft, as compared to the surface one. A juxtaposed subtropical North Atlantic high/low is also found to be closely associated with large-scale tropical Atlantic low, which is an extension of the cyclonic flow from the Sahel belt during the season. In the dry years, a large-scale anomalous low level anticyclonic flow is seen to occupy the same geographical region previously occupied by the wet season flow over land and TNA Ocean. This flow appears to advect atmospheric moisture over land into the ocean thereby creating precipitation deficit, which is corroborated by low level large-scale divergent circulation perturbations. The surface and midtropospheric anomalies associated with the other precipitation modes display features during the wet and dry epochs of the WAM, which are generally consistent with thermally direct circulation model.