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Oceanic eddies play a crucial role in transporting heat from the subsurface to surface ocean. However, dynamics responsible for the vertical eddy heat transport  Q   T   have not been systematically understood, especially in the mixed layer of western boundary current extensions characterized by the coincidence of strong eddy activities and air–sea interactions. In this paper, the winter (December–March)  Q   T   in the Kuroshio Extension is simulated using a 1-km regional ocean model. An omega equation based on the geostrophic momentum approximation and generalized to include the viscous and diabatic effects is derived and used to decompose the contribution of  Q   T   from different dynamics. The simulated  Q   T   exhibits a pronounced positive peak around the center of the mixed layer (~60 m). The value of  Q   T   there exhibits multi-time-scale variations with irregularly occurring extreme events superimposed on a slowly varying seasonal cycle. The proposed omega equation shows good skills in reproducing  Q   T  , capturing its spatial and temporal variations. Geostrophic deformation and vertical mixing of momentum are found to be the two major processes generating  Q   T   in the mixed layer with the former and the latter accounting for its seasonal variation and extreme events, respectively. The mixed layer instability and the net effect of frontogenesis/frontolysis contribute comparably to the geostrophic deformation induced  Q   T  . The contribution of  Q   T   from vertical mixing of momentum can be understood on the basis of turbulent thermal wind balance.

On the Upper-Ocean Vertical Eddy Heat Transport in the Kuroshio Extension. Part I: Variability and Dynamics