Remote Drivers of Interannual Variability of the East African Long Rains

The March-May East African Long Rains season is unusual in that its year-to-year variations are relatively insensitive to the main modes of tropical SST variability (ENSO, IOD). Various alternative drivers have been proposed in the past but these remain poorly understood. Here we present a detailed analysis of three important drivers: local SST, MJO and QBO. We use MERRA2 reanalyses and other instrumental and reanalysis datasets to understand how these drivers affect March-April mean rainfall from 1979-present. Common to all three drivers is that they modify the large-scale subsidence over the East African region during boreal spring. SST in the Indian Ocean is able to modify subsidence through direct thermodynamical forcing. The MJO is non-linear in that its effect is uni-directional, being more effective at enhancing than at reducing seasonal rainfall. Understanding the QBO is complicated by the limited number of events over the period considered. Each driver individually has a relatively weak effect on the Long Rains, but together they explain ⅓- ⅔ of the total variance of yearly variability depending on dataset. Representation of these drivers in the UK Met Office latest climate models is investigated with an aim to enhance our capability to model the Long Rains and, ultimately, predict this season better.