Interannual Modulations of Mesoscale Imprints of the Kuroshio Extension and Oyashio Fronts on the Wintertime Atmospheric Boundary Layer

The Kuroshio Extension (KE) fluctuates between its different dynamic regimes on decadal timescales. In its stable (unstable) regime, the KE jet is strengthened (weakened) and less (more) meandering. The present study investigates wintertime mesoscale atmospheric structures modulated with the changing KE regimes based mainly on the ERA-Interim atmospheric reanalysis data for 2002-2014, during which the resolution of sea surface temperature (SST) data prescribed is high enough to represent the KE regime fluctuations.

In the unstable KE regime, positive anomalies in SST to the north of the climatological-mean KE jet accompany local enhancement in upward heat fluxes from the ocean, surface wind convergence and cloudiness. Furthermore, the enhancement coincide with local lowering of sea level pressure, warming and thickening of the marine atmospheric boundary layer (MABL), anomalous ascent and convective precipitation. In the stable KE regime, by contrast, the corresponding imprints of sharp SST gradients across the KE and subarctic fronts on the wintertime MABL are separated more distinctly. These results are overall consistent with high-resolution satellite observations. In the ERA-Interim reanalysis, such mesoscale imprints of the KE variability as above are not well represented in a period during which the resolution of prescribed SST data is relatively low (1979-2001). This result indicates the importance of high-resolution SST data prescribed for atmospheric reanalysis in representing modulations of the MABL structure and air-sea fluxes by the variability of oceanic fronts and/or jets, including the modulations occurring with the changing KE regimes. The robustness of the above results will be verified by comparing two additional products for the new Japanese reanalysis of the global atmosphere (JRA-55): one with high-resolution (0.25°) SST and the other with low-resolution (1.0°) SST prescribed for the same data assimilation system.