Seasonal forecasting of the Ethiopian summer rains

Abstract

We present a new seasonal forecasting model for the June-September rains in Ethiopia. It has previously been found that the total June-September rainfall over the whole country is difficult to predict using statistical methods. A detailed study of all available data shows the rainfall seasonality varies greatly from one region to another, which would explain why the total June-September rainfall over all regions is a difficult property to forecast. In addition, the correlation between rainfall and the southern oscillation index varies spatially, with a strong teleconnection present only in some regions. This study accounts for the spatial variability in rainfall by grouping the rain gauge stations into four geographical clusters based on seasonality and cross-correlation of rainfall anomalies. Linear regression equations are then developed separately for each cluster. The variables we use for the regressions are sea-surface temperature anomalies in the preceding March, April and May of the tropical western Indian Ocean, the tropical eastern Indian Ocean, and Niño3.4. Formal skill testing of the equations shows that the new forecasting scheme is more effective in central western Ethiopia than either climatology or persistence -the methods currently used by the Ethiopian National Meteorological Services Agency.

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

“…Warm equatorial central and eastern Pacific Ocean SSTs tend to be associated with early Kiremt onset but shorter growing season duration, while warm SSTs in the Indian Ocean and Arabian Sea tend to delay Kiremt cessation and prolong rain. This equatorial Pacific Ocean SST influence on the Ethiopian Kiremt is consistent with findings of other recent studies (Gissila et al, 2004;Block and Rajagopalan, 2007;Korecha and Barnston, 2007), which largely focused on the predictability of Ethiopian summer rainfall.…”

“…They found moderate to strong correlations between the commencement and duration of the season with Indian Ocean and equatorial Pacific Ocean SSTs (+0.45 to +0.55, significant at the 1% level according to an ordinary Student's t-test). In similar vein, Gissila et al (2004) and Korecha and Barnston (2007) used global SSTs to predict June-September Ethiopian rainfall, and found significant associations with Indian and Pacific Ocean SSTs. In this section, the relationships between Ethiopian rainfall and global SST (including their temporal evolution) are examined in greater detail.…”

“…The spatial extent and intensity of the Mascarene and St Helena subtropical high-pressure cells are generally considered to be linked to Ethiopian rainfall (Conway, 2000;Gissila et al, 2004;Seleshi and Zanke, 2004;Korecha and Barnston, 2007). The correlation between raw pentad MSLP and Ethiopian pentad average summer rainfall ( Figure 5, top) shows that the stronger positive correlations involve the intensity of the aforementioned meridional ridge along the Mozambique Channel (+0.35) and the northward extent of the St Helena high over the Gulf of Guinea (+0.44).…”

“…The authors noted that improvements in understanding the basic circulation patterns across Africa are essential for improving the management of socio-economic activities affected by climate variability and future climate change, not only in Africa but also on a more global scale. Because the large-scale atmospheric circulation is directly linked to many aspects of regional climate variation (Beck et al, 2007), investigation of the links between atmospheric circulation patterns over Africa and rainfall there and elsewhere is important for understanding For example, although the large-scale circulation systems during the Indian summer (south-west) monsoon generally are associated with concurrent rainfall over the Horn of Africa (Krishnamurti and Bhalme, 1976;Conway, 2000;Gissila et al, 2004;Seleshi and Zanke, 2004;Korecha and Barnston, 2007), only a few quantitative studies have linked those circulation systems specifically to local weather and climate patterns over the Horn of Africa (e.g. Beltrando and Camberlin, 1993;Camberlin, 1997;Shanko and Camberlin, 1998;Block and Rajagopalan, 2007).…”

Evaluation of reanalysis rainfall estimates over Ethiopia

“…The standard division of Ethiopia into three zones (Figure 1) was felt to be too imprecise and a new classification scheme was devised based on data from the 33 gauges mentioned above. The method follows Gissila et al (2004) and is summarized briefly as follows. Subjective comparison of the annual cycles from the available gauges gave an initial classification of six homogeneous zones as shown in Figure 2.…”