[A-OS09] Climate variability and predictability on subseasonal to multidecadal timescales

[AOS09-09] Changes and future predictions of Oyashio Intermediate Water: Weakened overturning vs. enhanced tidal mixing

*Vigan Mensah¹、Kay I. Ohshima¹ (1.Institute of Low Temperature Science)

キーワード：Oyashio, Climate change, Sea of Okhotsk, Western subarctic Pacific, 18.6 year tidal cycle, Pacific overturning

The western subarctic Pacific exhibits major biological productivity and possesses one of the largest biological CO2 drawdown in the world. This biological productivity is fed by the Oyashio Current and its two source waters: Western Subarctic Water, which supplies nutrients from the subarctic Pacific, and cold Okhotsk Sea Intermediate Water (OSIW), which supplies iron from the Sea of Okhotsk. As iron content is a limiting factor for biological productivity, it is essential to understand and predict the variability in OYW properties in a changing climate, and specifically the content of OSIW in OYW (mixing ratio).

Here, we created seasonal climatologies of water properties to understand how the long-term trend (~50 years) and 18.6-year tidal cycle affect the Oyashio Intermediate Water (OYW). We found that the “recipe” of OYW is modified mainly in winter via decreased OSIW outflow over the trend and increased (decreased) outflow in high-tide (low-tide) years. Based on this, we provide a 20-year prediction of OYW temperature and mixing ratio. We predict that the opposite effects of the trend and high tide in the 2020s will lead to stagnation of the OYW properties, followed by accelerated warming in the 2030s (low-tide years). Besides, we found that OYW temperature will increase by 1 °C, and the mixing ratio of OSIW in OYW will decrease by 50% between 1960 and 2040, which might greatly impact biological productivity and carbon drawdown in the North Pacific.