Indian Ocean impact on ENSO evolution 2014-2016 in a set of seasonal forecasting experiments

In 2014 the scientific community and forecasters were expecting a major El Nino event, which was suggested by physical indicators and predicted by several seasonal forecasting systems. However, only moderately warm El Nino – Southern-Oscillation (ENSO) conditions materialized in 2014, but one year later in boreal winter 2015/16, one of the strongest El Ninos on record occurred. Moreover, the 2015/16 El Nino exhibited very unusual energetics: Despite warm conditions in the tropical Pacific in 2014 and especially 2015, its ocean heat content (OHC) did not decrease during that period, which usually is the case during El Nino events. Overall, the 2014-16 evolution of the tropical Pacific was quite different from the evolution during the 1997/98 El Nino, which exhibited exceptionally strong Pacific OHC discharge. This discrepancy was attributed at least partly to the anomalously warm Indian Ocean and the exceptionally weak Indonesian Throughflow transports during 2015-16 that kept Pacific OHC at high levels.

This contribution aims to elucidate the role of the Indian Ocean in the tropical Pacific Ocean evolution during ENSO for the two periods February 1997-1999 and February 2014-2016. For this purpose, we perform initialized two-year predictions using the ECMWF seasonal forecasting system. To isolate the role of the Indian Ocean, we carry out hindcasts with unperturbed ocean initial conditions and hindcasts with swapped Indian Ocean initial conditions, where the 2014 (1997) hindcasts use Indian Ocean initial conditions from 1997 (2014). We first investigate the impact of the Indian Ocean on the strength of the Indonesian Throughflow and the evolution of the tropical Pacific heat budget. Second, we seize these experiments to explore the impact of the Indian Ocean state on two-yearly ENSO evolution, especially on the probability of extreme events, and which role the atmospheric bridge plays versus the oceanic bridge.