\u2018Eastern African Paradox\u2019 rainfall decline due to shorter not less intense Long Rains

Emily Black,Richard J Keane,Richard P Allan,Caroline M Wainwright,John H Marsham,Declan L Finney,David P Rowell

doi:10.1038/s41612-019-0091-7

Abstract

An observed decline in the Eastern African Long Rains from the 1980s to late 2000s appears contrary to the projected increase under future climate change. This “Eastern African climate paradox” confounds use of climate projections for adaptation planning across Eastern Africa. Here we show the decline corresponds to a later onset and earlier cessation of the long rains, with a similar seasonal maximum in area-averaged daily rainfall. Previous studies have explored the role of remote teleconnections, but those mechanisms do not sufficiently explain the decline or the newly identified change in seasonality. Using a large ensemble of observations, reanalyses and atmospheric simulations, we propose a regional mechanism that explains both the observed decline and the recent partial recovery. A decrease in surface pressure over Arabia and warmer north Arabian Sea is associated with enhanced southerlies and an earlier cessation of the long rains. This is supported by a similar signal in surface pressure in many atmosphere-only models giving lower May rainfall and an earlier cessation. Anomalously warm seas south of Eastern Africa delay the northward movement of the tropical rain-band, giving a later onset. These results are key in understanding the paradox. It is now a priority to establish the balance of mechanisms that have led to these trends, which are partially captured in atmosphere-only simulations.

Highlights

The March, April and May (MAM) ‘Long Rains’ of Eastern Africa have historically been the major rainfall and primary agricultural season for much of the region[1,2]
Shortage of in situ observations makes assessment of regional rainfall trends across Africa challenging, yet the recent drying of the Eastern African long rains appears robust across datasets[4,6,8], with a drying in March, April, and May[6]
Comparison of the observed long rains trend with a range of trends from control runs of Coupled ModelIntercomparison Project Phase 5 (CMIP5) coupled models, in which anthropogenic forcing are absent, indicates that the observed rainfall trend is unlikely to be entirely consistent with natural variability[6]

Summary

INTRODUCTION

The March, April and May (MAM) ‘Long Rains’ of Eastern Africa have historically been the major rainfall and primary agricultural season for much of the region[1,2]. Projections of changes in the onset and cessation of wet seasons across Africa under future climate change, produced using CMIP5 models, show the onset and cessation of the Eastern African October–December “short rains” getting later, while the cessation of the long rains is projected to get earlier[13]. A stronger SHL (amplified by a water vapour—greenhouse gas warming feedback14,15), leads to a northward shift in the tropical rain-band during the boreal summer and a delayed retreat of the rain-band southwards[13]. This delayed retreat may be associated with the later onset and cessation of the short rains. Specific characteristics of the long rains decline are investigated, and a regional mechanism is proposed that explains both the observed decline, and the partial recovery since the late 2000s

RESULTS

DISCUSSION

METHODS