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

Thursday, 18 August 2016: 11:15 AM
Lecture Hall (Monona Terrace Community and Convention Center)
Hisashi Nakamura, Univ. of Tokyo, Tokyo, Japan

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.
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