On dynamics of vertical eddy heat transport

Peiran Yang, Zhao Jing, Bingrong Sun and Lixin Wu, Ocean University of China, Qingdao, China
Oceanic eddies play a crucial role in transporting heat and nutrients from the subsurface to surface ocean. However, dynamics responsible for the vertical eddy heat transport has not been systematically understood, especially in realistic scenarios. In this study, the wintertime vertical eddy heat transport in the Kuroshio-Oyashio extension region is simulated based on a 1-km regional ocean model and its underlying dynamics is analyzed using a semi-geostrophic omega equation (SG-Omega) generalized by including the viscous and diabatic effects.

Simulations results reveal low coherence (<0.3) between eddy vertical velocity and temperature anomalies in the upper 100 m, suggesting that bulk of the eddy motions is irrelevant to the vertical heat transport. The generalized SG-Omega equation shows good skills in reproducing the eddy vertical velocity anomalies responsible for the vertical heat transport. Frontogenesis and vertical friction are found to be the two dominant mechanisms generating the vertical eddy heat transport. By destructing the vertical geostrophic shear, the latter induces a secondary circulation that acts to restore the geostrophic balance and transport heat upwards.

The vertical eddy heat transport in the upper ocean is strongly modulated by atmospheric synoptic variabilities, exhibiting pronounced enhancement under strong surface cooling associated with intense winter storms. This enhancement is mainly attributed to the intensified vertical mixing and negative surface wind power on eddies that promote the vertical friction effect.