Role of ocean-atmosphere interactions in modulating the 2016 La Niña like pattern over the tropical Pacific

Despite having strong El Niño conditions in 2015, the year 2016 witnessed weak La Niña conditions in the equatorial Pacific. Though the El Niños of 1997 and 2015 were observed to be similar in strength, the following years (1998 and 2016 respectively) experienced contrasting La Niña characteristics. The factors responsible for this contrast are investigated in this study. It is found that the presence of anomalous westerlies over the equatorial Pacific without any typical eastward phase propagation from central to eastern Pacific is a trade mark of the 2015 El Niño. The latitudinal structure of zonal wind anomalies over the central Pacific in 1997 was symmetric to the equator from June to November. On the contrary, anomalous winds mostly persisted towards the north of the equator in 2015, with asymmetry. The meridional asymmetry in zonal wind anomalies, and the contrast in the wind stress curl throughout 2015-16, was seen to affect the recharge-discharge processes and off equatorial wave dynamics, thereby modulating the evolution of 2016 La Niña. This study highlights the role of these modulated ocean dynamics in weakening the 2016 La Niña as compared to 1998. The hypothesis is successfully tested using ocean model sensitivity experiments. The experiments reveal that, wind forcing and the associated wave dynamics, and discharge from the year 2015 onwards, are the primary factors responsible for weakening the 2016 La Niña. This study paves the way for providing accurate La Niña forecasts, months before its actual formation.