Abstract
While El Niño is known to cause failure of Kiremt (boreal summer) rainfall in Ethiopia, the mechanisms are not fully understood. Here we use the ECHAM5 Atmospheric General Circulation Model to investigate the physical link between Pacific sea surface temperature (SST) anomalies and Kiremt rainfall. We compare ECHAM5 simulations forced with reconstructed SST data, to gauge-based rainfall observations and atmospheric reanalysis for the time period of 1961–2009. We perform composite analysis and sensitivity experiments driven only with equatorial Pacific SST anomalies. Our results show warm SST anomalies in the equatorial Pacific drive a corresponding large-scale circulation anomaly with subsidence over Ethiopia in dry Kiremt seasons. Horizontal wind fields show a slow-down of the whole Indian monsoon system with a weaker Tropical Easterly Jet and a weaker East African Low-Level Jet in these summers. These changes can be seen as an anomalous circulation cell over northern Africa with westerlies at 100–200 hPa and easterlies below 500 hPa. Surface easterlies might reduce the moisture inflow from the Atlantic and Congo basin into Ethiopia. This and the general subsidence over the region could explain the reduction in Kiremt rainfall. Our results suggest up to 50% of the Kiremt rainfall anomalies is driven by equatorial Pacific SST variability.
Highlights
Ethiopia is strongly dependent on agriculture, which supplies employment for 81% of the population and accounts for 40% of its gross domestic product (Äthiopien - Wirtschaft)
We assume Gstation to be closest to reality and Zstation gives a very good approximation of the inter-annual variability of All-Ethiopian Kiremt rainfall, even though the spatial coverage of the rain gauges is much sparser
To ensure the suitability of ECHAM5 for an analysis of Kiremt rainfall we compare the seasonal cycle of rainfall in the ensemble mean of the ECHAM5 historical runs to Gstation, as its spatial coverage is much higher than Zstation
Summary
Ethiopia is strongly dependent on agriculture, which supplies employment for 81% of the population and accounts for 40% of its gross domestic product (Äthiopien - Wirtschaft). These reduce the moisture influx from the Atlantic and Congo basin into the Ethiopia highlands, which is an important moisture source for the region (Diro et al 2011b; Korecha and Barnston 2007; Mohamed et al 2005; Segele et al 2009a) In addition to these two main mechanisms, Diro et al (2011b) find southward shift of the African Easterly Jet (AEJ) related to warm SST anomalies in the Pacific and a Kiremt rainfall deficit. A summary and conclusions are presented in a final section
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