Sea level pressure minimum and ageostrophic surface winds over the Kuroshio and its extension

We examine effects of sea surface temperature (SST) front along the Kuroshio and its extension (K-KE) onto the surface wind and marine atmospheric boundary layer (MABL), based on diagnosis of an in-situ observation and operationally analyzed data sets, and numerical experiments of the regional atmospheric model. Owing to large temperature and moisture differences between ocean and atmosphere, the SST fronts along the K-KE anchor the local maximum of upward surface heat flux that can act to induce hydrostatic and vertical mixing effects in the MABL. The hydrostatic effect characterized by a westward-extending wedge-shaped trough of sea level pressure (SLP) is observed over the region with the intense surface heat flux during autumn and winter when the monsoonal northwesterlies are predominant. On the same time, ageostrophic surface westerlies toward the trough center are found north of low-SLP wedge but not south of it. This asymmetric feature of the ageostrophic wind may be the result of different effects on a combination of surface friction and vertical mixing in the momentum balance in either side of the front. Vertical structures of the ageostrophic winds in the MABL based on the operational meso-scale analysis provided by Japan Meteorological Agency will be discussed. Since the observational diagnosis alone cannot conclusively distinguish these ocean-to-atmosphere effects from mid-latitude weather noise, we conducted numerical experiments using the regional atmospheric model. In the control experiment prescribed with high resolution SST, low-SLP wedge and ageostrophic surface westerlies are significantly reproduced only along the K-KE where surface heat flux is intense, as observed. However these features are weakened when the SST is smoothed out to reduce the SST gradient. The results of the RAM experiments support the ideas that the observed SLP and surface winds along the K-KE are formed by the hydrostatic and vertical mixing adjustments.