Analysis of Subtropical Anticyclones as Synoptic Controls of Seasonal Rainfall Extremes in East Africa

Abstract

This study aims to investigate the influence of anticyclones adjacent to Africa on seasonal rainfall extremes over East Africa, focusing on the March to May (MAM) and October to December (OND) rainy seasons. Specific objectives for the study include identifying years with extreme rainfall, determining spatial distribution of rainfall for the identified years with rainfall extremes, comparing the pressure characteristics of the anticyclones during the extreme rainfall years with climatological patterns, and analysing moisture transport and circulation patterns during extreme rainfall years. Precipitation data and reanalysis datasets of mean sea level pressure (MSLP), zonal and meridional wind and specific humidity from 1981 to 2020 were used for analysis. Four years of extremely high and extremely low rainfall were identified for each season using the tercile approach. Spatial distribution of rainfall anomalies depicted areas of concentration of ‘wetness’ and ‘dryness’ due to rainfall extremes. The pressure patterns and subsequently the moisture and circulation patterns were crucial in showing the impact of anticyclones on moisture flux during extreme rainfall years. During the MAM season, equatorward displacement of the Azores high and St. Helena high contributed to extremely high rainfall, while for dry years, the poleward displacement of these anticyclones resulted in less effective moisture influx. In the OND season, the Mascarene and Siberian highs' eastward displacement with reference to the climatological evidence caused an influx of moist easterlies, contributing to wet years. Conversely, wet years experienced westward (compared to the climatology) displacement of these anticyclones, leading to converging easterlies and westerlies in the Indian Ocean, leaving East Africa devoid of moisture for rain formation. The study highlights the significant role of subtropical anticyclones in influencing moisture patterns and controlling seasonal rainfall extremes in East Africa. The findings highlight the need for improved forecasting techniques, especially for extreme events like flooding and droughts. Furthermore, the study underscores the importance of developing and enhancing general circulation models for Africa to advance weather forecasting and analyse weather drivers and teleconnections effectively.