Boreal winter Arctic Oscillation as an indicator of summer SST anomalies over the western tropical Indian Ocean

Abstract

The inter-annual relationship between the boreal winter Arctic Oscillation (AO) and summer sea surface temperature (SST) over the western tropical Indian Ocean (TIO) for the period from 1979 to 2015 is investigated. The results show that the January？February？March AO is significantly correlated with the June？July？August SST and SST tendency. When both El Nino/Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) variance are excluded, the winter AO is significantly correlated with the regional mean SST of the western TIO (40？？60？E and 10？ S？10？N), r = 0.71. The multi-month SST tendency, i.e., the SST difference of June？July？August minus April？May, is correlated with the winter AO at r = 0.75. Composite analysis indicates similar warming over the western TIO. Two statistical models are established to predict the subsequent summer's SST and SST tendency. The models use the winter AO, the winter ENSO and the autumn-winter IOD indexes as predictors and explain 65 and 62 % of the variance of the subsequent summer's SST and SST tendency, respectively. Investigation of the regional air？sea fluxes and oceanic dynamics reveals that the net surface heat flux cannot account for the warming, whereas the oceanic Rossby wave plays a predominant role. During positive AO winters, the enhanced Arabian High causes stronger northern winds in the northern Indian Ocean and leads to anomalous cross-equatorial air-flow. The Ekman pumping in association with the anomalous wind stress curl in the central TIO generates a significantly deeper thermocline and abovenormal sea surface height at 60？？75？E and 5？？10？S. The winter AO-forced Rossby wave propagates westward and arrives at the western coast in summer, resulting in the significant SST increase. Forced by the observed winter AOrelated wind stress anomalies over the Indian Ocean, the ocean model reasonably reproduces the Rossby wave as well as the resulting surface ocean warming over the western TIO in the subsequent summer. Observational analysis and numerical experiments suggest the importance of the oceanic dynamics in connecting the winter AO and summer SST anomalies.