[A-CG36] Extratropical oceans and atmosphere

[ACG36-P13] Observed changes between ocean-driven and atmosphere-driven sea surface heat flux variations in the Kuroshio-Oyashio confluence region

*Haruto Fujishima¹, Fumiaki Kobashi¹, Naoto Iwasaka¹, Hiroyuki Tomita², Shusaku Sugimoto³ (1.Tokyo University of Marine Science and Technology, 2.Hokkaido University, 3.Tohoku University)

Keywords:Kuroshio-Oyashio confluence region, sea surface flux, Kuroshio Extension

The Kuroshio-Oyashio confluence (KOC) region releases an enormous amount of heat and moisture from the ocean to the atmosphere during the winter. The heat release occurs due to the interaction between the warm seawater associated with the Kuroshio Extension (KE) and the cold strong winter monsoon wind from the continent. In addition, the KOC region is characterized by a strong sea surface temperature (SST) front associated with the KE. Numerous studies based on observations and model simulations have demonstrated that the variations in the position and intensity of the SST front influence the atmosphere on interannual to decadal timescales. Traditionally, the variations in heat release in the midlatitude ocean have been considered to be controlled primarily by the atmosphere, particularly sea surface wind speed (WND). However, in the KOC region, some studies have shown the important role of SST in the heat flux variations. In this study, we investigate the interannual-to-decadal variations of wintertime turbulent heat flux (THF) in the KOC region from December 1988 to February 2022 using the J-OFURO3 V1.2 dataset. The KOC region was defined as an area of high interannual standard deviation in THF (34°–42°N, 142°–155°E). The empirical orthogonal function (EOF) analysis was adopted to examine the relationships between the THF variations and fluctuations in SST, surface air temperature (SAT), and WND. Additionally, we performed wavelet analysis for the principal component time series of the EOF modes. The results indicate that decadal-scale SST variations predominantly controlled the THF variations in the KOC region approximately in the period from 1995 to 2010, with greater THF release from the ocean in years of higher SST. The SST variations are found to be closely related to the meridional fluctuations of the KE axis. In contrast, approximately during the period from 2005 to 2020, the decadal relationship between the THF and SST variations became unclear in the KOC region except along the KE axis. During these years, the THF variations were primarily driven by large-scale SAT variations around Japan on interannual timescales of 2–5 years. The lower SAT leads to the larger THF from the ocean. These results from the analysis of the J-OFURO3 data reveal the changes in the mechanism driving the THF variations in the KOC region.