Study on Diurnal Sea Surface Warming and a Local Atmospheric Circulation over Mutsu Bay

Abstract

This study aims to investigate diurnal sea surface temperature (SST) rise in Mutsu Bay, and its relationship with the local atmospheric field. First, the characteristics of diurnal SST warming is revealed, using satellite and buoy data during 1995-2000. Satellite observations can clearly capture large diurnal SST rise that reaches a few degrees, and clarify the spatial distribution of the SST rise. The diurnal variation of the satellite-derived SST will reflect that of skin SST, which is defined as the temperature of the top layer with the thickness of a few tens micrometers. The analysis of the bulk SST observed with buoys at 1-m depth reveals that the frequency of large diurnal amplitude of SST (ΔSST) appearance becomes high from April to August, and the bulk ΔSST exceeds 1.0 K in some places in the bay for 71% of the days in July. Over the bay, surface air temperature in the daytime is higher than the bulk SST on average in April-August. From spring to summer the wind becomes weaker, and the easterly wind becomes frequent. The weak wind plays a role in promoting the diurnal SST rise, and the warm air-temperaturemay also secondarily contribute to large ΔSST.
A model experiment is then performed to examine a local atmospheric circulation that develops around the bay under a clear and calm condition in summer, and to address the interaction between the circulation, and the diurnal SST warming. A local circulation forced by the land surface solar heating of this area produces the weak easterly surface wind and the subsidence of the air, and makes the air warmer over the bay. This is consistent with the observational results. In such a case, the diurnal rise of skin SST simulated by an ocean model exceeds 2 K all over the bay, and it is especially large in the northeastern area. If the diurnal skin SST rise is considered in the atmospheric model, although the circulation pattern does not change, the circulation becomes weaker and the surface air temperature increases. This result suggests that the diurnal SST variation will be important for the practical prediction of air temperature in coastal areas.