Interannual to decadal variability of phosphate in the Oyashio region: Roles of wind-driven ocean current and tidally induced vertical mixing in the Sea of Okhotsk

In the Oyashio region, remarkable climatic signals are observed in biogeochemical parameters such as phosphate (PO4) concentration and debate continues regarding possible causes. Using a regional ice-ocean coupled model with a simple biogeochemical cycle, this study investigated the mechanisms controlling interannual-decadal variations in surface PO4 in the Oyashio region and their relationships to climate change. Hindcast experiments forced with atmospheric reanalysis data for 1980-2010 and 18.6-year tidal mixing strength in the Kuril Straits qualitatively simulated interannual-decadal variations of surface PO4, including a realistic seasonal cycle. Interannual fluctuations of simulated PO4 in the Oyashio region are prominent in winter and characterized by year-to-year variability. Budget analysis of PO4 in the mixed layer showed that the wintertime increase in PO4 is caused by lateral advection as well as by local vertical convection. The geostrophic current variability responsible for lateral advection of PO4 is related primarily to the barotropic response of arrested topographic waves in the Sea of Okhotsk as well as the wind-driven gyre in the North Pacific, both of which are regulated by the strength of the wintertime monsoon atmospheric pattern. On a decadal timescale (>7 years), temporal variations of surface PO4 in the Oyashio region are characterized by decadal-scale fluctuation with positive (negative) peaks around 1985, 1995, and 2005 (1990 and 2000). A series of sensitivity experiments demonstrated that the decadal variability of PO4 is largely explained by atmospheric wind conditions; however, modulation by 18.6-year tidal mixing is not negligible. Diagnostic analysis of wind-forced-experiment data revealed that the decadal PO4 signal is advected from the Sea of Okhotsk, where 8-year leading wintertime Ekman upwelling supplies PO4-rich water in the northern shelf region, and that the responsible atmospheric circulation is related to the West Pacific pattern. Our model simulation suggests that the wintertime wind-driven current system in the Sea of Okhotsk is important to the system feeding surface nutrients into the Oyashio region on an interannual-decadal timescale.