Eastern Africa experiences two wet seasons per year; the long rains in March-May, and the short rains in October-December. For much of Eastern Africa the long rains are the major rainfall and primary agricultural season, thus, the decline in the long rains since 1985 has had major socio-economic consequences. In a recent study, within the HyCRISTAL project, specific characteristics of this decline were investigated, and a regional mechanism that explains the decline is proposed.

Analysis of satellite-based rainfall datasets shows that the decline in rainfall during the long rains was associated with a later start to the season (later onset) and earlier end of the season (earlier cessation). Since the late 2000s there has been some recovery of the long rains, although with high inter-annual variability; this recovery is associated with the onset recovering to pre-decline values, and some recovery in the cessation dates.

Concurrently with the rainfall decline over Eastern Africa, reanalysis data shows a decrease in the surface pressure over Arabia in May. This indicates a strengthening of the Arabian Heat Low, which is a dominant feature over the Arabian Peninsula in spring and summer, that interacts with local weather phenomena including the Somali Jet and the Indian Monsoon. The southerly winds, forming part of the Somali Jet, are also found to strengthen in May. Furthermore, over the period of the decline, May sea surface temperature increased more over the very north Arabian Sea than further south in the Indian Ocean.

The increase in wind speed, strengthening of the Arabian Heat Low, and change in sea surface temperature gradient across the Arabian Sea act to draw the rain-band northwards faster and further, away from the Eastern Africa region, consistent with the decline in rainfall and earlier end to the long rains.

Considering the later onset, warming sea surface temperatures south of Madagascar are associated with reduced March rainfall and later onsets, as warmer temperatures to the south delay the northward progression of the rain-band.

Read more and download the paper “‘Eastern African Paradox’ rainfall decline due to shorter not less intense Long Rains” here.

This article was written by FCFA HyCRISTAL researcher and lead author of the paper Caroline Wainwright.