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

摘要

El Nino-Southern Oscillation (ENSO) has large socioeconomic impacts worldwide. The positive phase of ENSO, El Nino, 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 Nino the precipitation anomaly over most of East Africa during the short rains season was less intense than experienced during previous El Ninos, linked to less intense easterlies over the Indian Ocean. This moderate impactwas 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 Nino events may have a bias toward high probabilities of wet conditions.