Influence of ENSO, Southern Annular Mode, and IOD on the interdecadal change of the East Africa ‘short rains’

This study investigates the abrupt shift in the Southern Annular Mode (SAM), El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) and East Africa ‘short rains’ (EASR) during 1961, to understand the associated physical mechanisms. The mechanisms are examined through composite, correlation and regression analysis for the period 1901—2020, using multiple datasets. Findings show that in the pre-1961 period, SAM was associated with the EASR variability through its connection with the variability of the South Indian Ocean (SIO) sea surface temperature (SST) anomaly. The September SAM is related to a change in surface wind speed, which drive meridional oceanic Ekman transport and influence the surface heat exchange, redistributing heat near the surface. Warm SIO SST induces ascending motion throughout the atmosphere, favouring deep convection and Congo Basin westerlies, leading to enhanced EASR. The findings suggest that the September SAM provide an important forecasting signal for the EASR. However, post 1961, the SAM is decoupled from the EASR, and the IOD demonstrates a significant role in influencing the short rains. The anomalous Indian Ocean Walker circulation related to the IOD provides an essential driving process for anomalous moisture transport and convection in East Africa (EA), leading to EASR variability. The El Niño events predispose the Indian Ocean to positive IOD events, and thus, the absence of the former has ramifications to the EASR variability. The observational results show that prior to 1961, there was virtually no significant relation between the EASR and the zonal winds in the central equatorial Indian Ocean. The findings of this study have important implications for regional season climate prediction.