The Observed Synoptic scale precipitation relationship between Western Equatorial Africa and Eastern Equatorial Africa
Charles JR. Williams
Department of Meteorology, University of Reading, Reading, RG6 6BB, United Kingdom, School of Geographical Sciences, University of Bristol, Bristol, BS8 1SS, United Kingdom
An improvement to subseasonal (i.e. days to weeks) rainfall prediction across Equatorial Africa is an important area of current research because most countries there are highly dependent on rain-fed agriculture, with millions of livelihoods at risk in the event of an unexpected harvest failure. This study examines 16 years of daily precipitation anomalies to investigate the relationship in precipitation between Western Equatorial Africa (WEA) and Eastern Equatorial Africa (EEA). Using lead/lag correlation and spatio-temporal correlation patterns over various sub-regions, a synoptic-scale relationship in precipitation is presented between WEA and EEA in which precipitation over EEA lags precipitation over WEA by 1-2 days. In addition, central WEA and sub-regions in the Sudan highlands display a synoptic-timescale precipitation contrast, suggesting a weak precipitation dipole. Consistent with the known heterogeneity characteristic of Equatorial Africa’s precipitation, our findings suggest that the 1-2 day precipitation relationship is dependent upon the sub-region under investigation.
Furthermore, our results indicate a coherent synoptic-timescale eastward/northeastward propagating signal with a speed of approximately 12 m s-1. Composite and correlation analyses of precipitation anomalies and a novel equatorial wave dataset show an apparent connection between eastward/northeastward propagating wet anomalies and Kelvin wave lower-tropospheric convergence. This suggests that Convectively Coupled Kelvin Waves (CCKWs) play a role in modulating the 1-2 day convection and precipitation oscillation between EEA and WEA. These results imply that monitoring the propagation characteristics of CCKWs may be important in synoptic-timescale forecasting over Equatorial Africa.