Mechanisms of the interannual to decadal thermal variability of the upper ocean in the western boundary region of the North Pacific

Abstract

Winter upper-ocean heat content variability and its causal mechanisms are investigated using observational and reanalysis products in the western North Pacific. The relationship between the East Asian winter monsoon (EAWM) and the North Pacific Oscillation (NPO) and their impact on the sea surface temperature (SST) are nonstationary, with a sudden change at 1987/1988. During the 197387, the EAWM and NPO were significantly correlated to each other, but their correlation practically vanishes during the 19882002. This nonstationary relationship is related to the pronounced decadal weakening of the Siberian high after the 1988 regime shift as well as the concomitant positive NPO-like dipole change. The influence of EAWM and NPO to the winter SST in the study region is significantly decreased after the sudden change near-1990. The upper 400 m heat budget in the western North Pacific is analyzed using outputs from a high resolution ocean general circulation model. Winter heat storage rate on interannual to decadal time scales is mainly determined by oceanic heat advection rather than by net air-sea heat flux. The role of heat advection becomes particularly prominent after the 1990 regime shift in association with the reduced variability of surface heat flux caused by weakened SST variability. The net heat flux acts to dampen temperature anomalies caused by the ocean dynamics principally associated with the meridional shift of the Oyashio Extension front, which is significantly correlated with the West Pacific (WP) and Pacific-North America (PNA) teleconnection patterns.