The joint effects of the El Niño-Southern Oscillation and Arctic Oscillation on tropical Indian Ocean heat flux during boreal winter

Abstract

In this study, the joint effects of the El Niño-Southern Oscillation (ENSO) and Arctic Oscillation (AO) on winter net surface heat flux (Q_net) anomalies over the tropical Indian Ocean (TIO) are investigated for the 1979/1980–2017/2018 period. The results show that, in multisource datasets, the Q_net pattern for El Niño plus positive AO cases is the most robust. The major feature of Q_net is dominated by a significant dipole pattern, with oceanic heat gain anomalies appearing over the southeastern TIO and oceanic heat loss near the western TIO. Analysis of the flux components shows that the Q_net anomalies are mainly contributed by changes in latent heat flux and shortwave radiation (Q_LHF and Q_SWR), which are tightly associated with the regional atmospheric and oceanic conditions. In association with positive AO, northerly wind anomalies on the eastern flanks of a low-level anomalous anticyclone in the Arabian Sea enhance the intertropical convergence zone (ITCZ) in the western TIO. Meanwhile, the El Niño-related SST warming occurs in the western TIO. Both enhanced ITCZ and anomalous SST warming favour in situ increased low and middle cloud cover and a reduced Q_SWR. Furthermore, under El Niño's effect, a low-level anomalous anticyclone located in the southeastern TIO leads to a positive Q_LHF through decreased near-surface wind speed and increased near-surface air humidity. As a result, a dipole Q_net pattern tends to be observed for the combination of El Niño and positive AO.