资源环境科技发展态势分析平台

The Indian Ocean witnessed a weak positive Indian Ocean dipole (IOD) event from the boreal summer to autumn in 2015, while an extreme El Nino occurred over the tropical Pacific. This was different from the case in 1997/98, when an extreme El Nino and the strongest IOD took place simultaneously. The analysis here suggests that the unique sea surface temperature anomaly (SSTA) pattern of El Nino in 2015 might have contributed to the weak IOD that year. El Nino in 2015 had a complex SSTA pattern, with positive warming over the central and eastern tropical Pacific. Such a combination of the classic El Nino (also known as cold-tongue El Nino) and the recently identified central Pacific El Nino (also known as El Nino Modoki II) had opposite remote influences on the tropical Indian Ocean. The classic El Nino reduced the strength of the Walker circulation over the tropical Indian Ocean, but this was offset by El Nino Modoki II. This study points out that the IOD can be strongly modulated by combined El Nino types in some circumstances, as in 2015.