Processes Shaping the Time-Mean Surface Wind Convergence Pattern around the Wintertime Kuroshio Extension and Oyashio Fronts

In time-mean fields, surface wind convergence and divergence are locally enhanced along the warm western boundary currents and to their poleward across sea-surface temperature (SST) fronts, respectively. This particular pattern has been interpreted as a manifestation of local boundary layer response to frontal SST gradient via “hydrostatic effect” and/or “vertical mixing effect”. The surface convergence and divergence coincide with local enhancement and reduction, respectively, of upward motion, cloudiness, and precipitation. Recent studies argue, however, that recurrent development of synoptic-scale disturbances, which accompany extreme convergence and divergence, can be essential in shaping this time-mean atmospheric structure. It is still an open question as to the relative contributions from these potential processes to the time-mean pattern, which is investigated in this study with main focus on the wintertime the Kuroshio Extension (KE) and Oyashio fronts. Comparison is made between two additional products of the Japanese atmospheric reanalysis JRA-55 family, which use 25km and 100km resolution SST for lower boundary condition for the same data assimilation system. In the product with higher-resolution SST, moderately strong convergence tends to occur frequently even on daily scales right over the KE and moderate divergence slightly to the north, which are not directly related to atmospheric disturbances. Cyclonic disturbances give rise to enhanced convergence even to the north of the KE. Comparison of the two products suggests that not only the extreme convergence or divergence associated with synoptic-scale disturbances but also the moderate but more persistent contributions from the frontal-scale SST pattern can be important in shaping the time-mean meso-scale surface wind pattern and associated distributions of cloud and precipitation.