OLR and SST characteristics over Africa during years of deficient and surplus rainfall in East Africa

Abstract

Accurate rainfall forecasts would greatly improve the quality and quantity of agricultural products and other rain dependent activities, while allowing for the amelioration and integration of the catastrophic socio-economic impacts of extreme rainfall events. Such forecasts are as yet unavailable and although numerical models could Improve contemporary forecasts, their potentials remain unexploited in this region. The objective of this study was to establish the ocean-atmosphere anomalies which have been associated with some of the periods of surplus and deficient rainfall over the whole of East Africa, with a view of understanding some of the responsible mechanisms Monthly rainfall data for 50 East African stations were used to select years of extremely dry and wet rainfall events, using an empirically derived rainfall index. NOAA Outgoing long wave radiation (OLR) data for 1974-1988 at 5° x 5° lat/longitudes and Sea surface temperatures (SST) data for 1970-1984 at 2° x 2° from the UK Meteorological office were then subjected to a number of statistical analyses including Empirical Orthogonal Function Analysis (EOF). The December-February, March-May, June-August and September-November seasons were each investigated independently during the years of extreme rainfall anomalies. The most distinct feature of the OLR anomalies was the opposite response depicted between the west Indian Ocean and the eastern side extending into Indonesia. During the wet years, enhanced cloudiness dominate East Africa and west Indian Ocean, while clear conditions prevail over the east Indian Ocean and the Indonesian region. The corresponding SST anomalies depict tendencies for warm (cold) SST anomalies over west Indian Ocean to occur during the wet (dry) years. The east Atlantic SST anomaly patterns were not very consistent with cold anomalies occurring in the wet years of some seasons while warm anomalies occur in others. In the dry year the OLR dipole between east and west Indian Ocean is reversed. Results from peA indicated that, the first OLR eigen mode which is associated with the Saharan anticyclone was strongly correlated with seasonal rainfall anomalies over East Africa. Which IS reflective of the strong influence of the synoptic scale systems. The study has shown that ocean-atmosphere anomalies over the Indian Ocean significantly influence rainfall events in East Africa.