Separation of Climatological Imprints of the Kuroshio Extension and Oyashio Fronts on the Wintertime Atmospheric Boundary Layer
Mesoscale structures of the wintertime marine atmospheric boundary layer (MABL) as imprints of the Kuroshio Extension (KE) front and Oyashio front east of Japan are investigated, by taking advantage of high horizontal resolution data based on satellite measurements and the ERA-Interim global atmospheric reanalysis. The resolution of SST data prescribed for the ERA-Interim has been substantially improved in 2002 with grid intervals changed from 1.0 deg. to 0.5 deg. The high-resolution SST data can represent the KE front and Oyashio front, separately. By contrast, the low-resolution SST data prescribed until 2001 cannot separate these oceanic fronts and represent the Kuroshio-Oyashio Extension (KOE) region as a broad single frontal zone. Wintertime ERA-Interim climatologies constructed for the high-resolution SST period (2002 through 2013) are compared with those for the low-resolution SST period (1979 through 2001), to assess the imprints of the KE and Oyashio fronts on the MABL.
Climatologically, satellite observed data capture two local maxima of upward turbulent heat fluxes at the surface, surface wind convergence, cloudiness and precipitation on the warmer flanks of both the KE and the Oyashio fronts. These dual-peak atmospheric features are reasonably reproduced in the ERA-Interim in its high-resolution SST period. Correspondingly, dual-peaks are evident in meridionally high-pass-filtered fields of SLP, potential temperature and upward motions. In its low-resolution SST period, however, these dual-peak features are totally missing in the ERA-Interim, where each of those atmospheric features exhibits a broader single peak within the KOE region. These results indicate that not only the KE front but also the Oyashio front can climatologically leave mesoscale imprints on the overlying MABL.
Furthermore, both the satellite data and the ERA-Interim reveal that these mesoscale atmospheric distributions in the KOE region are varying with SST variations associated with the variability of the KE system, namely, fluctuation between stable and unstable regime of its path and strength. We also pointed out that it is stable KE that leads to clear dual-peak atmospheric features as observed in wintertime climatologies.