Enhanced Influences of Tropical Atlantic SST on WNP–NIO Atmosphere–Ocean Coupling since the Early 1980s

Abstract

Abstract This paper reports a new finding and related mechanism: the forcing effect of the tropical Atlantic (TA) sea surface temperature (SST) on the atmosphere–ocean coupling in the western North Pacific (WNP) and northern Indian Ocean (NIO). Since the early 1980s, the TA SST has increased and, notably, exhibited an enhanced interannual statistical relationship with the WNP subtropical high and NIO SST in boreal summer. Empirical diagnostics reveal the following spatial pattern linking the TA SST and the atmosphere–ocean coupling in the Pacific and Indian Ocean: 1) a cyclonic (anticyclonic) circulation pair straddling the equator over the eastern Pacific, 2) an anticyclonic (cyclonic) circulation pair straddling the equator in the WNP and Indian Ocean, 3) overturning circulation with ascending (descending) and descending (ascending) anomalies over the TA and tropical western Pacific, respectively, and 4) positive (negative) SST anomaly in the TA and NIO. The characteristics of this pattern are consistent with those of a WNP–NIO coupling pattern identified in a previous study. Empirical diagnostics and numerical simulations indicate that the TA SST serves as a forcing to induce low-level divergence and streamfunction anomalies in the Indian Ocean and the western Pacific. The latter in turn induces anomalous heat storage in the NIO and enhances the WNP–NIO coupling system, which is an intrinsic pattern engendered by the atmosphere–ocean interaction in the region. Without the remote influence of the TA SST forcing, the WNP–NIO coupling pattern and its impacts on the summer monsoon and TC variability in South Asia, East Asia, and the WNP would be considerably less significant than observed.