An observational study of the variability of East African rainfall linked to sea surface temperatures and soil moisture

In this work, Empirical Orthogonal Function (EOF) analysis of East Africa (EA, 5S-20N, 28E- 52E) daily accumulated satellite rainfall (African Rainfall Climatology version 2, ARC v2 at 0.25° spatial resolution) is applied to extract the dominant spatial and temporal patterns of variability during the short rain season (October-November-December, OND). Additionally, the connections among EA's precipitation, sea surface temperature (HadISST1-SST), soil moisture (ECV-SM), and wind (NCEP-NCAR) are investigated (through correlation and composite analysis) to disentangle the different factors influencing rainfall variability over the region. In general, there is a strong positive correlation between EA short rains and the positive phase of the Indian Ocean Dipole (IOD+) with enhanced rainfall response in the years of joint IOD+ and El Niño events, specifically during the years of Eastern Pacific El Niño episodes. The physical mechanism explaining the IOD+ link to EA rainfall consists of a Gill-type response to warm Western IO SST anomalies that induces a westerly low-level flow anomaly over equatorial Africa and leads to moisture convergence over EA (Bahaga et al. 2014). Rainfall's response is opposite during the negative phase of the events (IOD- and La Niña). SM seems to be responding locally to rainfall variability with some time delay but seems also to exert a kind of memory effect on rainfall the following year (positive feedback).