Tidally modified western boundary current drives interbasin exchange between the Sea of Okhotsk and the North Pacific

Abstract

The interbasin exchange between the Sea of Okhotsk and the North Pacific governs the intermediate water ventilation and fertilization of the nutrient-rich subpolar Pacific, and thus has an enormous influence on the North Pacific. However, the mechanism of this exchange is puzzling; current studies have not explained how the western boundary current (WBC) of the subarctic North Pacific intrudes only partially into the Sea of Okhotsk. High-resolution models often exhibit unrealistically small exchanges, as the WBC overshoots passing by deep straits and does not induce exchange flows. Therefore, partial intrusion cannot be solely explained by large-scale, wind-driven circulation. Here, we demonstrate that tidal forcing is the missing mechanism that drives the exchange by steering the WBC pathway. Upstream of the deep straits, tidally-generated topographically trapped waves over a bank lead to cross-slope upwelling. This upwelling enhances bottom pressure, thereby steering the WBC pathway toward the deep straits. The upwelling is identified as the source of joint-effect-of-baroclinicity-and-relief (JEBAR) in the potential vorticity equation, which is caused by tidal oscillation instead of tidally-enhanced vertical mixing. The WBC then hits the island chain and induces exchange flows. This tidal control of WBC pathways is applicable on subpolar and polar regions globally.

Weakened overturning and tide control the properties of Oyashio Intermediate Water, a key water mass in the North Pacific

“…The 5–6 years of lag in the maximum layer thickness to the maximum mixing ratio means that thickness variations cannot explain changes in OSIW outflow. Instead, we suggest that the tidal current influences the exchange system between the Sea of Okhotsk and the northern Pacific and leads to variations in OSIW outflow 15 , as explained in a recent modelling study 30 . The 4-year lag in layer thickness to the tidal cycle may relate to how OSIW properties are remotely affected by slow advection of surface salinity anomalies from the Bering Sea to the Sea of Okhotsk 15 , 37 , 38 .…”

“…In particular, we adopt the concept delineated in Ref. 30 that the tidal current is essential for water exchange between the Sea of Okhotsk and the northern Pacific. As the intensity of the tidal current should increase during the strong period of the 18.6-year tidal cycle, so should the outflow of OSIW into the northern Pacific.…”

Dissolved rare earth elements in the Northwest Pacific: Sources, water mass tracing, and cross‐shelf fluxes

“…Where, F 1; Hill et al, 2003;Shu et al, 2021). Because the water flow flux into the Sea of Okhotsk through the Kruzenshtern Strait is thought to be nearly equal to the outflow flux from the Bussol' Strait (Ohshima et al, 2010), we assume that both the inflow and outflow water fluxes are 8.2~8.8 Sv (Katsumata, 2004) (excluding the water exchange caused by tidal processes).…”

Cross-shelf overturning in geostrophic-stress-dominant coastal fronts

“…The COCO model uses the hydrostatic and Boussinesq approximations, incorporates curvilinear horizontal coordinates (referred to as COCO's Mercator grids) and couples the sea ice model for diverse applications (Hasumi 2000;Matsuda et al 2015;Shu et al 2021). The brine rejection process is included in the sea ice module.…”