Sub‐seasonal teleconnections between convection over the Indian Ocean, the East African long rains and tropical Pacific surface temperatures

Abstract

Since 1999, the increased frequency of dry conditions over East Africa, particularly during the March–May (MAM) season, has heightened concerns in a region already highly insecure about food. The underlying mechanisms, however, are still not yet fully understood. This article analyses a proxy for daily convection variations over a large region encompassing East Africa and the whole Indian Ocean basin by applying a cluster analysis to more than 30 years of daily outgoing longwave radiation (OLR). Focusing on the MAM season to investigate relationships with East African long rains, four recurrent convection regimes associated with wet/dry conditions in East Africa are identified. Interestingly, all four regimes are related to western/central Pacific sea surface temperatures (SSTs) and rainfall. Wet regimes are associated with cool and dry/warm and wet conditions over the Maritime Continent (MC)/tropical Pacific east of the date line. Dry regimes exhibit opposite SST/rainfall dipole patterns in the Pacific compared to wet regimes, with the Indian Ocean found to modulate impacts on East African rainfall. Significant relationships between off‐equatorial warming in the west Pacific and a more frequent dry regime in May since 1998–1999 suggest an earlier onset of the monsoon and Somali jet, consistent with the recent abrupt shift observed in East African long rains and their modulation at multi‐decadal time scales of the Pacific.

Trend and periodicity of drought over Ethiopia

“…It has to be noted that the actual amount of precipitation decrease per year is larger in spring as it is the main rainy season for the southern region. Similar drying trend are also found over East equatorial Africa, which has climatic features similar to the southern part of Ethiopia (Williams and Funk, ; Lyon and DeWitt, ; Viste et al, ; Funk et al, ; Liebmann et al, ; Yang et al, ; Funk and Hoell, ; Vigaud et al, ).…”

“…This phenomenon (variability of ocean basins) has significant impact on the displacement and weakening of the rain‐producing mechanisms in Ethiopia (Korecha and Barnston, ; Segele et al, ; Diro et al, , ). Overall, SST anomalies in the equatorial Pacific Ocean are significantly correlated with East African seasonal rainfall variations, but the signs of the correlations and their phase vary from region to region (Camberlin, ; Nicholson, ; Segele et al, ; Diro et al, ; Lyon and DeWitt, ; Zeleke, ; Funk et al, ; Liebmann et al, ; Yang et al, ; Funk and Hoell, ; Vigaud et al, ).…”

“…Studies have also emphasized the influence of other oceanic basins on Ethiopian seasonal/annual rainfall (Kassahun, ; Camberlin, , ; Camberlin and Philippon, ; Giannini et al, ; Williams and Funk, ; Williams et al, ; Berhane et al, ; Funk and Hoell, ; Vigaud et al, ). For example, the model‐based study of Giannini et al .…”

Revisiting interannual to decadal teleconnections influencing seasonal rainfall in the Greater Horn of Africa during the 20th century

“…A similar finding has been noted by Diatta and Fink () for the West African monsoon. Further studies should consider individual months (Vigaud et al, ) and the spatial coherence of the signals within the three regions considered (e.g., Camberlin and Philippon, ). Furthermore, our results can be taken to design modelling studies to gain insights into the physical mechanisms responsible for the internanual and decadal teleconnections and their non‐stationarities.…”

“…Similarly, Vigaud et al . () suggest that a more frequent dry regime in May since 1998/1999, associated with an earlier onset of the Indian summer monsoon and Somali jet, could account for a recent abrupt shift observed in GHA Long Rains. In contrast, other studies link the drying trend to anthropogenic and aerosol forced Indo‐PO warming, resulting in the westward extension of the Indo‐Pacific warm pool, causing a westward shift of the Walker circulation and a subsidence anomaly and drying over East Africa (Williams and Funk, ; Funk et al, ; Liebmann et al, ; Funk and Hoell, ; Hoell et al, ).…”