Oyashio Southward Intrusion and Cross-Gyre Transport Related to Diapycnal Upwelling in the Okhotsk Sea

Abstract

The intrusion of western boundary currents (WBC) across the wind-driven gyre boundary and associated cross-gyre transport are examined using layered numerical models with realistic topography and annual mean wind stress, with special emphasis on the Oyashio southward intrusion and North Pacific Intermediate Water (NPIW) formation. Diapycnal transport due to strong tidal mixing around the Kuril Islands in the subarctic North Pacific Ocean is modeled by source/sink thickness restoring terms in the continuity equation to form the deep pycnocline observed in the Okhotsk Sea. About 3 Sv (Sv ≡ 106 m3 s?1) of the model diapycnal upwelling occurring along the Kuril Islands induces a meridional?diapycnal overturning circulation, and the transport is consistent with the estimates from recent observations with chemical tracers, GCM results, and the present study using observed density fields with enhanced diapycnal diffusivity around the Kuril Islands. The model with the diapycnal transport reproduced the realistic Oyashio southward intrusion, cross-gyre transport, and current distributions in the ?Mixed Water Region.? These features can be simply explained by a linear 2.5-layer reduced-gravity analytical model composed of a Sverdrup interior and WBC in geostrophic balance with the source/sink distribution. In this analytical model, the southern limit of the Oyashio can be defined as the latitude where the coastal equivalent depth along the Japan coast (Dc) is equal to the depth at the western edge of the interior (Dw). Here, Dc at an arbitrary latitude y can be written as a function of total amount of the diapycnal transport from the deeper layer through the pycnocline in the whole area north of y and total meridional Ekman transport across y; Dw is mainly determined by Ekman pumping velocity at the sea surface based on the linear Sverdrup relation. As the diapycnal transport becomes larger, Dc and the net cross-gyre transport increase, compensating vertical convergence of the diapycnal transport; the Oyashio can cross the wind-driven gyre boundary and intrude more southward because the difference between Dc and Dw at the latitude of the wind-driven gyre boundary causes southward geostrophic transport along the east coast of Japan. These results explain the observed features that about 3?5 Sv of the cross-gyre Oyashio transport along the western boundary participates in the formation of NPIW.