Potential Predictabilty of Precipitation in the Horn of Africa in ECHAM5

Abstract:

In the Horn of Africa (HoA), agriculture, economy and human health are very sensitive to extremes in precipitation. Models for seasonal forecasts of precipitation in this region are mostly based on statistical connections between precipitation and slowly-varying atmospheric variables like sea surface temperature (SST) and large-scale atmospheric circulation. Here we examine such connections with the atmosphere general circulation model ECHAM5 in order to explore the seasonal predictability of HoA rainfall with a dynamical model.

A historic run with a T106 horizontal resolution (~ 1.125°) was forced with reconstructed sea surface temperature data from 1870 to 2009. Comparisons to observation data from the Global Precipitation Climatology Project showed that ECHAM5 captures the diverse seasonal cycles of precipitation throughout the HoA. The results of an analysis of variance indicate that the season with the highest potential for seasonal predictability of rainfall in this region is the Northern Hemisphere summer.

In agreement with observational studies, the model simulations show an El Niño-like SST pattern with a warm tongue in the equatorial East and Central Pacific in summers of low precipitation in the Horn of Africa. Accordingly, the large-scale atmospheric circulation anomalies show a weak Tropical Walker Circulation in summers of low precipitation.

We conducted a set of sensitivity experiments with ECHAM5. Results so far show that warm SST anomalies in the Eastern Equatorial Pacific have to persist until summer to provoke a dry summer in the HoA. We do not find a difference in the HoA rainfall pattern when driving the model with evolving East Pacific or Central Pacific El Niños.