Why was the Indian Ocean Dipole weak in the context of the extreme El Niño in 2015?

Abstract

he Indian Ocean witnessed a weak positive Indian Ocean Dipole (IOD) event from the boreal summer to autumn in 2015, while an extreme El Niño occurred over the tropical Pacific. This was different from the case in 1997/1998 when an extreme El Niño and the strongest IOD took place simultaneously. Our analysis suggests the unique sea surface temperature anomaly (SSTA) pattern of El Niño in 2015 might have contributed to the weak IOD that year. The El Niño in 2015 had a complex SSTA pattern, with positive warming over the central and eastern tropical Pacific. Such a combination of the classic El Niño (also known as cold-tongue El Niño) and the recently identified central Pacific El Niño (also known as El Niño Modoki II ) had opposite remote influences on the tropical Indian Ocean. The classic El Niño reduced the strength of Walker circulation over the tropical Indian Ocean, but this was offset by El Niño Modoki II. This study points out that the IOD can be strongly modulated by combined El Niño types in some circumstance, as in 2015.