Typology of pentad circulation anomalies over the Eastern Africa–Western Indian Ocean region, and their relationship with rainfall

Abstract

The aim of this study was to classify the most frequently observed atmospheric circula- tion anomaly patterns in eastern Africa and the adjacent Indian Ocean. As an example of the useful- ness of such a classification, the second objective was to test whether these patterns account for intraseasonal rainfall anomalies in the region. A partitioning algorithm, known as dynamical cluster analysis, was therefore applied to the zonal (U) and meridional (V) components of the wind anom- alies, obtained from the NCEP-NCAR Reanalysis R-2 at the pentad (5 d) timescale. The 3 geopoten- tial levels 850, 700 and 200 hPa were combined. Focus is on the transition seasons (March to May and October to December), which correspond to the 2 East African rainy seasons. The classification domain is large enough (40° latitude × 40° longitude) to describe large-scale anomaly patterns, but it focuses on equatorial and subequatorial atmospheric dynamics. The results consist of 7 major wind anomaly clusters, for which associated atmospheric fields (geopotential height anomalies, precipita- tion anomalies) and the seasonal distribution were explored. Among them, 2 clusters strongly resem- ble the anomaly patterns associated with the Madden-Julian Oscillation. The other major configura- tions show similarities with recurrent previously observed patterns involved in wet or dry spells over East or Southern Africa. Statistical tests reveal that this partition is able to significantly discriminate rainfall anomalies over the eastern part of the African continent; 4 of the 7 clusters correspond to abnormally wet or dry events over Equatorial East Africa.