Effects of the Non-breaking Surface Wave-induced Vertical Mixing on Winter Mixed Layer Depth in Subtropical Regions

Abstract

Compared to observations, the simulated multi-model mean surface oceanic mixed layer depth (MLD) during winter in the subtropical regions of both hemispheres shows deep bias from 45 CMIP5 climate models. Our results from two numerical experiments using one of CMIP5 models show that the non-breaking surface wave-induced vertical mixing can serve as a remedy. The enhanced vertical mixing increases the heat content of the upper ocean and reduces the oceanic potential density in winter which then stabilize the upper ocean and shallow the simulated winter MLD in subtropical regions. This heat content increase is not directly induced by air-sea heat fluxes during winter, but is the legacy of temperature increase during summer, when the additional vertical mixing induces an enhanced surface heating. The simulation biases of the annually averaged water temperatures in the upper 400 m reduced by 43% and 28% in south and north latitude bands between 20°and 40°, respectively. The non-breaking surface wave-induced vertical mixing shallows both boreal and austral winter MLDs by 2–11 m (a change of 5–20%) in both northern and southern subtropical regions. These results indicate that the incorporation of vertical mixing induced by the non-breaking surface waves in our experiments can improve the simulation of boreal and austral winter MLDs in northern and southern subtropical regions.