| The intermediate-water circulation in the North Pacific subarctic and northern subtropical regions is investigated, focusing on the formation mechanism of the mesothermal (intermediate temperature maximum)structure in the subarctic region.Through the analysis of climatological and WOCE/SAGE hydrographic data,it is suggested that the water originating from North Pacific Intermediate Water (NPIW) in the area east of Japan is transported to the northern Gulf of Alaska in the density range of 26.7-27.2σθ.NPIW is suggested to be formed in the Kuroshio-Oyashio inter-frontal zonethrough mixing between the Kuroshio and Oyashio watersand is warmer and more saline than the waters in the western and central part of the subarctic region in this density range.Therefore, this warm and saline water transport is suggested to provide the mesothermal water in the subarctic region with the heat and salt.Through inverse analysis, we further examined the intermediate-water circulation in the North Pacific subarctic and northern subtropical regions.The isopycnal velocities estimated through the inverse analysis are mostly consistent with the oxygen distribution and support the hypothesis that warm and saline intermediate water influenced by the Kuroshio water massis transported from the area east of Japan to the northern Gulf of Alaska.The northward isopycnal advective and diffusive heat transport across 46°N in the entire open North Pacific is estimated to be 3.6-21.4TW at 26.7-27.2σθ in total.This value is larger than the cooling of the intermediate layer in the open North Pacific subarctic region through diapycnal mixing, and comparable to the heat loss in the Okhotsk Sea intermediate layer estimated in previous studies.This suggests that the cooling of the intermediate layer in the subarctic region occurs mainly in the marginal seas,i.e. outside the area analyzed with inverse method.These results quantitatively indicate that the heat transport from the south in the intermediate layer plays an essential role in maintaining the heat of the mesothermal water in the North Pacific subarctic region. |
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