Large‐scale atmospheric circulation and global sea surface temperature associations with Horn of Africa June–September rainfall

Abstract

This study uses correlation, regression, and composite analyses for the period 1970-1999 to explore the relationships between the June-September rainfall in the Horn of Africa (especially Ethiopian) and large-scale regional atmospheric circulation patterns across Africa and the Atlantic and Indian Oceans, and global sea surface temperature (SST) anomalies. Abundant rainfall in the Horn of Africa is associated with enhanced westerlies across western and central Africa. These westerlies are produced by a stronger north-east directed mean sea level pressure (MSLP) gradient resulting from MSLP intensification over the Gulf of Guinea and deepening of the monsoon trough across the Arabian Peninsula. This is reflected by a strong correlation (−0.71) between 5-day (pentad) Ethiopian rainfall and the Gulf of Guinea minus the Arabian Peninsula MSLP difference. This correlation decreases to −0.39 when the seasonal cycles are removed from both time series. A wet Horn of Africa monsoon is also associated with deep moist air extending up to mid-troposphere and large water vapour transport convergence across much of Ethiopia, a strong Somali low-level jet, and a strong tropical easterly jet (TEJ). Although there are large changes in TEJ strength, the position of the jet axis shows little variation between wet and dry events. Associated with the TEJ, the strongest upper level divergence occurs at 100 hPa, where the raw/de-seasonalized zonal wind speed correlates negatively (−0.71/−0.23) with the corresponding Ethiopian rainfall at the pentad time-scale. Furthermore, SSTs over the equatorial Pacific, Indian, and southern Atlantic Oceans correlate strongly with contemporary Ethiopian summer rainfall. In general, Ethiopian rainfall is suppressed during El Niño and enhanced during La Niña. This identification and documentation of the regional atmospheric circulation patterns and global SST anomalies directly linked to rainfall variability over Ethiopia/Horn of Africa, are crucial for developing statistical prediction schemes and designing climate model simulations for the region on intra-seasonal to inter-annual time-scales.

Evaluation of satellite rainfall estimates over Ethiopian river basins

“…The ITCZ, which is the main rain producing mechanism during Kiremt, is centered to the North of Ethiopia and extends into the northwest region, the Blue Nile River basin (Segele et al, 2008). In this region, elevation influences the bias in the satellite rainfall estimates: TMPA 3B42RT and CMORPH tend to overestimate at low elevations but give reasonably accurate results at high elevations, whereas PERSIANN gives reasonably accurate values at low elevations but underestimates at high elevations.…”

“…The Kiremt, approximately defined by June to the end of September, is considered the major rainy season. The seasonal oscillation of the Intertropical Convergence Zone (ITCZ) is the predominant mechanism for the rainfall during the Kiremt (Seleshi and Zanke, 2004;Segele et al, 2008). The Bega is the dry season and is approximately defined by October through the end of February.…”

CMIP5 model simulations of Ethiopian Kiremt-season precipitation: current climate and future changes

“…A stronger EALLJ contributes to above-normal Kiremt-season precipitation (Fig. 5a, d) by increasing moisture transport from the tropical oceans (Segele et al 2009b). Meanwhile, the intensification of the TEJ can enhance the upward motion southward of the jet core (Segele and Lamb 2005;Korecha and Barnston 2007).…”

Impact of conservation agriculture on catchment runoff and soil loss under changing climate conditions in May Zeg-zeg (Ethiopia)

“…Most rainfall occurs in the period June-September (kiremt rain) during localized but intense convective storm events in the afternoon (Nyssen et al, 2005), which is due to the seasonal movement of the intertropical convergence zone (Goebel and Odenyo, 1984) and due to enhanced westerlies, caused by the stronger north-east pressure gradient between the Guinea anticyclone and the Arab monsoon trough (Segele et al, 2009). Some rain can be observed in April-May (belg or azmera), due to the early formation of the meridional branch of the Somali jet along the East African coast (Riddle and Cook, 2008), and due to a high-amplitude Madden-Julian Oscillation (Williams and Funk, 2011).…”

“…Yet, yearly rainfall is very variable (Fig. 4), since high yearly rainfall is also depending on a strong Somali low-level jet, a strong tropical easterly jet, and the occurrence of La Niña events (Segele et al, 2009). At local scale, rainy cloud development is highly dependent on topographic slope (Nyssen et al, 2005).…”