The Moderate Impact of the 2015 El Niño over East Africa and Its Representation in Seasonal Reforecasts

David MacLeod,Cyril Caminade

doi:10.1175/jcli-d-19-0201.1

David MacLeod, Cyril Caminade

Open Access

https://doi.org/10.1175/jcli-d-19-0201.1

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Abstract

Abstract El Niño–Southern Oscillation (ENSO) has large socioeconomic impacts worldwide. The positive phase of ENSO, El Niño, has been linked to intense rainfall over East Africa during the short rains season (October–December). However, we show here that during the extremely strong 2015 El Niño the precipitation anomaly over most of East Africa during the short rains season was less intense than experienced during previous El Niños, linked to less intense easterlies over the Indian Ocean. This moderate impact was not indicated by reforecasts from the ECMWF operational seasonal forecasting system, SEAS5, which instead forecast large probabilities of an extreme wet signal, with stronger easterly anomalies over the surface of the Indian Ocean and a colder eastern Indian Ocean/western Pacific than was observed. To confirm the relationship of the eastern Indian Ocean to East African rainfall in the forecast for 2015, atmospheric relaxation experiments are carried out that constrain the east Indian Ocean lower troposphere to reanalysis. By doing so the strong wet forecast signal is reduced. These results raise the possibility that link between ENSO and Indian Ocean dipole events is too strong in the ECMWF dynamical seasonal forecast system and that model predictions for the East African short rains rainfall during strong El Niño events may have a bias toward high probabilities of wet conditions.

Highlights

Extreme climate hazards are associated with El Niño events and 2015/16 saw one of the strongest events ever recorded (Huang et al 2016)
Given that the Indian Ocean dipole (IOD) has been shown to be a key driver of the East African short rains (Black et al 2003; Black 2005), it is possible that this phenomenon played a role in the difference between 2015 and 1997 impacts
Regional climate anomalies related to the 1982 (Figs. 3c,h,m) and 2015 events (Figs. 3e,j,o) tend to be in closer agreement with regression patterns calculated using the Niño-3.4 index (Figs. 3a,e,j), with weaker easterly wind anomalies over the Indian Ocean and more moderate rainfall anomalies reaching the coasts of East Africa. These results suggest that the moderate impact of the 2015 El Niño on East African rainfall compared to 1997 is linked to weaker IOD-like activity in the Indian Ocean, with no strong cold SST in the east and weaker easterly wind anomalies at 850 mb crossing the Indian basin

Summary

Introduction

Extreme climate hazards are associated with El Niño events and 2015/16 saw one of the strongest events ever recorded (Huang et al 2016). The regression of climate fields onto an East Africa OND precipitation index (see Fig. S1 in the online supplemental material) confirms the strong association between OND rainfall and the zonal wind pattern associated with the IOD.

Results

Conclusion