Dynamical and Synoptic Characteristics of 'Wet' and 'Dry' Seasons Across Northern Sub-Saharan Africa (NSSA)

The dynamical and synoptic characteristics that distinguish 'wet' and 'dry' seasons in NSSA are discussed. The main goal of this paper is to use the composite time series of meteorological fields from NCEP-NCAR 50-year reanalysis and CPC rainfall estimate to determine the spatial and temporal distribution of precipitation across the NSSA. Teleconnections between climate indices and precipitation over the region shows strong and positive correlation with North Atlantic Multidecadal Oscillation (AMO), Northern Oscillation Index (NOI) and Southern Oscillation Index to the west and Multivariate ENSO Index (MEI) to the highlands of Eastern Africa. One of the synoptic-scale features that encourage dynamic lifting in the region appears to be the differential distribution of temperature and relative humidity that generates a temperature-moisture gradient. This variation creates a strong baroclinic zone that triggers the generation of African waves that is intricately tied to the African Easterly Jet (AEJ). There is significant correlation between rainfall anomaly for relatively 'wet' (2009) and 'dry' (2004) years and the southward displacement of AEJ. This confirms AEJ as responsible for most of the rainfall in the region. The large negative values of vertical velocity (omega) for JAS is a noticeable feature with its core between 750mb and 250mb (lat. 50N and 150N) and appear to capture the convective precipitation processes in the region. In JFM however, the situation is reversed. Meteorological fields of zonal and meridional winds, OLR, surface temperature and relative humidity from reanalysis is correlated with observed precipitation across the NSSA and discussed. At the intersection of the dry Sahel and relatively 'wetter' savannah region of NSSA lies the 'shrinking' Lake Chad affected by climate variability and subjected to immense anthropogenic pressure. Thus, the implications of the findings of this paper in relation to Lake Chad are also discussed.; climatological vertical velocity (omega) (Pa/s) (bottom) and relative humidity (top) for MAM (left) and JAS (right) 1948 -2012 averaged for latitude by height