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

• Published in: Scientific Reports
• Main Authors: Shu, Hung-Wei, Mitsudera, Humio, Yamazaki, Kaihe, Nakamura, Tomohiro, Kawasaki, Takao, Nakanowatari, Takuya, Nishikawa, Hatsumi, Sasaki, Hideharu
• Format: Text
• Language: English
• Published: Nature Publishing Group UK 2021
• Subjects: Article; Okhotsk; Pacific; Subarctic
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8187450/; https://doi.org/10.1038/s41598-021-91412-y

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.