Recent warming in the western North Pacific in relation to rapid changes in the atmospheric circulation of the Siberian high and Aleutian low systems

Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/2011JCLI4142.s1International audienceOn the basis of a new East Asian winter monsoon (EAWM) index and by analyzing the relationshipbetween sea surface temperature (SST) anomalies and different atmospheric and oceanic factors in winter,this study investigates the causes of the recent unusual warming in the western North Pacific Ocean.Analyses presented here emphasize the dual contribution from the atmosphere and ocean to the local SSTvariability, with the relative importance of each contributor varying with the period and place. During theperiod 1970–89, the EAWM, controlled mostly by the Siberian high, is predominantly responsible for theSST variability in most of the western North Pacific, whereas in the period 1990–2005 ocean dynamicsbecome increasingly important in most places or even dominant in the Kuroshio–Oyasio Extension (KOE)region. The delayed response of the KOE SST to basinwide wind stress curl forcing via Rossby waves isepoch dependent and is significant at lags of 1, 3, and 4 yr before 1990 but only at 1 yr afterward. This epochdependency of the impact of Rossby waves is related to the different locations of the centers of action ofwind stress curl in the midlatitude North Pacific between the two epochs. In addition, mean advection of theEAWM-driven anomalous SST from the southern East China Sea, which can be transported into the KOEregion in about a year by the Kuroshio, likely affects the KOE SST lagged by 1 yr. The strongest positiveSST trend observed in the western North Pacific results from the combined effects of the abrupt weakeningof the EAWM due to the unprecedented decline of the Siberian high and the increasing role of the ocean.The latter is best evidenced by the 1-yr delayed response of the western North Pacific via the gyre circulation adjustment to the basinwide decadal-scale wind stress curl change associated with the northwardshift of the strengthened Aleutian low