Mesoscale Imprints of the Kuroshio Extension and Oyashio Fronts on the Wintertime Atmospheric Boundary Layer as Revealed in a New Additional Product of the JRA-55 Reanalysis

Recent studies have identified distinct influence of the warm western boundary currents, such as the Gulf Stream and Kuroshio, and associated midlatitude frontal zones on the overlying atmosphere, including boundary-layer wind/thermal structure, vertical motion and cloud-precipitation systems. Their representation in reanalysis should be sensitive to the resolution of SST data prescribed at the lower boundary of a forecast model. This sensitivity is assessed through comparison of two products of a new Japanese reanalysis (JRA-55). One is JRA-55C, in which observed data have been assimilated in a forecast system with horizontal resolution of ~60km and the COBE SST data with 1-deg. resolution prescribed over 55 years. The other is an additional product (JRA-55CHS) with MGDSST data with a 25-km resolution only over 28 recent years. The comparison reveals substantial differences in midlatitude atmospheric processes around the western boundary currents and associated SST fronts. As a typical example, atmospheric response to variability of the Kuroshio Extension (KE) is examined. As in satellite observations, enhancement of cloudiness and precipitation in the oceanic frontal zone east of Japan during the unstable regime of KE relative to its stable regime is represented well in JRA-55CHS but not in JRA-55C product. The enhancement, which results from augmented heat/moisture release from the warmer ocean with more active warm-core eddies, comes to be better represented in ERA-Interim after 2001, when the SST resolution was improved. This oceanic thermal forcing onto the atmosphere is manifested as positive correlation in anomalies between SST and heat/moisture release, which is represented only in the high-resolution MGDSST but not in COBE SST. As another example, stormtrack response to meridional displacement of the Oyashio front is examined. Again, the positive correlation between anomalous SST and heat/moisture release is much stronger in JRA-55CHS, and so is the enhancement of convective precipitation over warm SST anomalies.