The Japan Sea is the East Asia marginal sea located in the monsoon region subjected to strong wintertime winds from continental Asia. The Kuroshio Branch enters the sea and carries subtropical water from the Korean (Tsushima) Strait northward to Tsugaru and La Perouse (Soya) Straits. Excessive wintertime surface cooling in the sea has to be balanced by throughflow of warm water (Hirose et al., 1996). In summer precipitation is high due to summer monsoon, especially in the southern and coastal areas. Fresh surface water is also supplied in summer by continental runoff and inflow from Tsushima and Tatarsky Straits. The purpose of this study is to simulate surface heat and freshwater fluxes in the Japan Sea, as affected by simulated circulation patterns and temperature/salinity fields and prescribed external atmospheric characteristics.
The MHI model (Shapiro, 1998) is applied for simulation. It is a primitive equation quasi-isopycnal model under hydrostatic, Boussinesq, beta-plane, and free surface approaches. The upper mixed layer model is incorporated, coupled to inner layers through entrainment and subduction. In the MHI model not only seawater temperature and salinity but also buoyancy are allowed to vary horizontally in any layer, facilitating simulation of subduction and of temperature and salinity fields. Stablity of water column is maintained by introduction of ÅÃase¡|buoyancy constraining buoyancy variations in inner layers; buoyancy in an upper layer is unbounded. If buoyancy gets out of its base limits, an inner layer vanishes and water goes to an adjacent layer of appropriate base buoyancy. At the sea surface, complete (non-linear) heat balance condition providing vertical heat flux to upper layer is applied, taking into account prescribed atmosphere characteristics and simulated water temperature in the upper layer. Surface freshwater flux affecting upper layer salinity is estimated from prescribed GPCP (Global Precipitation Climatology Project; 1979-2001) precipitation data and simulated evaporation. Circulation is also forced by wind stress from NCEP reanalysis for the period from January 1998 through June 2001.
Surface heat and freshwater fluxes and also upper layer temperature and salinity are realistically simulated by the MHI model; restoration to prescribed surface temperature and salinity is not applied, with the exception of dumping in the outflow ports. Comparison is made with observation data. The simulation results confirm that in summer heat budget of the Japan Sea is mostly determined by local surface heating, while it is determined by throughflow of subtropical water in winter and on the average for a year. Throughflow of subtropical water in the sea subjected to intensive wintertime cooling results in excess of evaporation from the sea surface over precipitation in winter and on the average for a year, while in summer precipitation prevails. |
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