Impacts of Midlatutide Oceanic Fronts on the Atmosphere as Revealed in a New Japanese Atmospheric Reanalysis

Impacts of frontal SST gradients associated with midlatitude western boundary currents on the overlying atmosphere are assessed through comparison between two products of a new Japanese reanalysis (JRA-55). One is JRA-55C in which all the observational data except satellite data have been assimilated in a forecast system with horizontal resolution of ~60km and the COBE SST data on 1-degree resolution prescribed over 55 years. The other is JRA-55CHS, which is the same as JRA-55C but with MGDSST data with a quarter-degree resolution but only over 28 recent years. This data set is prepared jointly by the Meteorological Research Institute and University of Tokyo as a legacy of the Japanese “hot-spot” project, which was concluded in March 2015. The comparison reveals substantial differences in midlatitude atmospheric processes around the SST fronts. For example, enhancement of cloudiness and precipitation in the mixed-water region east of Japan during the unstable regime of Kuroshio Extension (KE) relative to its stable regime is represented well in JRA-55CHS, as in satellite observations, but not in JRA55C. The enhancement arises from augmented heat/moisture release from the warmer ocean with more active warm-core eddies. This oceanic thermal forcing onto the atmosphere is manifested as positive correlation in anomalies between SST and heat/moisture release, which is represented only with the high-resolution MGDSST but not with the COBE SST. Other examples include stormtrack response to meridional displacement of the Oyashio front. 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. Convective diabatic heating may be essential for the development of synoptic-scale cyclones, which may enhance the sensitivity of a stormtrack to the variability of the SST front. Convective precipitation is also enhanced climatologically along the Gulf Stream in JRA-55CHS. Note that the main JRA-55 product, in which satellite data are also incorporated with the COBE SST, exhibits almost the same characteristics as the JRA-55C.