Potential predictability of the sea‐surface temperature forced equatorial East African short rains interannual variability in the 20th century

Abstract

In this article, the predictability of the 20th century sea-surface temperature (SST) forced East African short rains variability is analyzed using observational data and ensembles of long atmospheric general circulation model (AGCM) simulations. To our knowledge, such an analysis for the whole 20th century using a series of AGCM ensemble simulations is carried out here for the first time. The physical mechanisms that govern the influence of SST on East African short rains in the model are also investigated. It is found that there is substantial skill in reproducing the East African short rains variability, given that the SSTs are known. Consistent with previous recent studies, it is found that the Indian Ocean and in particular the western pole of the Indian Ocean dipole (IOD) play a dominant role for the prediction skill, whereas SSTs outside the Indian Ocean play a minor role. The physical mechanism for the influence of the western Indian Ocean on East African rainfall in the model is consistent with previous findings and consists of a gill-type response to a warm (cold) anomaly that induces a westerly (easterly) low-level flow anomaly over equatorial Africa and leads to moisture flux convergence (divergence) over East Africa. On the other hand, a positive El Niño-Southern Oscillation (ENSO) anomaly leads to a spatially noncoherent reducing effect over parts of East Africa, but the relationship is not strong enough to provide any predictive skill in our model. The East African short rains prediction skill is also analyzed within a model-derived potential predictability framework and it is shown that the actual prediction skill is broadly consistent with the model potential prediction skill. Low-frequency variations of the prediction skill are mostly related to SSTs outside the Indian Ocean region and are likely due to an increased interference of ENSO with the Indian Ocean influence on East African short rains after the mid-1970s climate shift.

An extreme negative Indian Ocean Dipole event in 2016: dynamics and predictability

“…The modulation of IOD on East African rainfall has been studied with atmospheric model simulations (Latif et al 1999;Ummenhofer et al 2009). Although other forcings, such as Arctic Oscillation (Gong et al 2016), can also modulate East African short rain, IOD plays a dominant role (Bahaga et al 2015). Here we demonstrate that coupled seasonal predictions also capture the observed connection between East African rainfall and IOD.…”

An observational study of the variability of East African rainfall with respect to sea surface temperature and soil moisture

“…() attributed the positive anomalies during El Niño years especially to the mature phases of the event. Some studies concentrated instead on a negative response of EA short rains to El Niño events (Behera et al, ; Bahaga et al, ). Specifically, Bahaga et al .…”

Spatial Patterns of Sea Surface Temperature Influences on East African Precipitation as Revealed by Empirical Orthogonal Teleconnections

“…The major rainy season, the so-called "long rains" is from March until May (MAM), while the second rainy season from October to December (OND), the "short rains" is more variable but usually centered around November. The modulation of these rainy seasons by regional to global sea surface temperature (SST) anomalies has been the focus of numerous studies in the past (e.g., Rocha and Simmonds, 1997;Mutai et al, 1998;Latif et al, 1999;Plisnier et al, 2000;Behera et al, 2005;Black, 2005;Marchant et al, 2007;Ummenhofer et al, 2009;Manatsa et al, 2012Manatsa et al, , 2014Manatsa and Behera, 2013;Bahaga et al, 2015;Tierney et al, 2015). From these studies it becomes evident that, at least for the "short rains, " the Indian Ocean Dipole (IOD) plays a much bigger role than the El Nino Southern Oscillation (ENSO) in East Africa.…”