East African Rainfall and the Tropical Circulation/Convection on Intraseasonal to Interannual Timescales

Abstract

It is known that the East African short rains (October?December, OND) have a positive correlation with El Niño?Southern Oscillation (ENSO). A prediction scheme based on sea surface temperature eigenvectors (SST EOFs) including Pacific, Indian, and Atlantic Ocean variability showed skill higher than one based on ENSO alone. First the authors assess the extent to which the large-scale SST predictors correlate with rainfall averaged over smaller subregions of East Africa. Most regions correlate consistently well, though some pockets of lower correlations suggest that interaction with orographic features may modulate the large-scale ENSO and other coupled ocean?atmosphere signals in the region. Next, to evaluate the atmospheric teleconnections giving rise to the SST correlations, the circulation anomalies associated with East African rainfall are investigated using National Centers for Environmental Prediction?National Center for Atmospheric Research reanalysis data and, as a proxy for large-scale tropical convection anomalies, outgoing longwave radiation (OLR). In the seasonal OND mean, a sequence of three horseshoe structures are evident in the OLR anomalies, with anomaly sign in phase over the central Pacific and East Africa, and out of phase over the Maritime Continent. This gives rise to the positive ENSO association with East African rainfall. The horseshoe structures in the Indian Ocean are absent in September and through much of the long rains in March?May, though weakly evident again in May. It is suggested that the presence or absence of this teleconnection structure is related to the state of the background annual cycle. When the ENSO variance is removed (by linear regression) from the datasets, there emerges more strongly a positive correlation between East African rainfall in OND and enhanced convection through equatorial Africa and into the equatorial Atlantic and Amazon region, in turn associated with warm equatorial and tropical South Atlantic SST. The lead?lag structure of intraseasonal teleconnections with East African rainfall suggests that 5?10 days before the rainfall event, low-level dynamics start to develop in the equatorial Atlantic, and these penetrate across equatorial Africa and into East Africa during the event itself, at which time anomalies in the Pacific Ocean, which were strong 15 days before the event, are now weaker. Five days after the rainfall event, 200-hPa divergence over East Africa pulls off the east into the Indian Ocean and shows structures that resemble the Madden?Julian oscillation. For above-normal rainfall in East Africa, the seasonal mean teleconnection across the Indian Ocean resembles this intraseasonal picture with strong convection particularly just off the East African coast, prompting discussion of the interaction between the intraseasonal and seasonal anomalies.