Impact of March North Atlantic Oscillation on Indian Ocean Dipole: role of air–sea interaction over the Western North Pacific

Abstract

We investigated the relationship between the North Atlantic Oscillation (NAO) and Indian Ocean Dipole (IOD), which has remained unknown to date. Reanalysis data and linear baroclinic model experiments were employed in our study. The results showed significant correlation between the March NAO and the boreal summer and autumn IOD, independent of the El Niño–Southern Oscillation signal, verified by partial correlation analysis. Air–sea interaction over the western North Pacific (WNP) is a significant aspect of the physical mechanism through which the March NAO affects the subsequent IOD. A strong positive March NAO induces equivalent barotropic cyclonic circulation over the WNP through a steady Rossby wave, accompanied by a local tripole sea surface temperature (SST) anomaly pattern. Facilitated by local air–sea positive feedback, the low-level cyclonic circulation and associated precipitation anomalies over the WNP persist from early spring to summer and shift equatorward. During May–June, the WNP anomalous cyclone strengthens the southeasterly wind and enhances cooling off Sumatra–Java through local meridional circulation. Such circulation ascends over the WNP and descends over the tropical southeastern Indian Ocean and Maritime Continent. Subsequently, wind–evaporation–SST and wind–thermocline–SST positive feedback in the tropical Indian Ocean contribute to IOD development. A diagnosis of ocean mixed-layer heat budget indicated that the ocean dynamic process associated with the NAO contributes more to IOD development than does atmospheric thermal forcing. Determining the influence mechanism of the March NAO on the subsequent IOD is considered useful in advancing the seasonal prediction of IOD.