ESTIMATING GLOBAL AIR-SEA FLUXES FROM SURFACE-PROPERTIES AND FROM CLIMATOLOGICAL FLUX DATA USING AN OCEANIC GENERAL-CIRCULATION MODEL

A simple method is presented and demonstrated for estimating air-sea f luxes of heat and fresh water with the aid of a general circulation mo del (GCM), using both sea surface temperature and salinity data and cl imatological air-sea flux data. The approach is motivated by a least s quares optimization problem in which the various data sets are combine d to form an optimal solution for the air-sea fluxes. The method provi des estimates of the surface properties and air-sea flux data that are as consistent as possible with the original data sets and with the mo del physics. The calculation of these estimates involves adding a simp le equation for calculating the air-sea fluxes during the model run an d then running the model to a steady state. The proposed method was ap plied to a coarse resolution global primitive equation model and annua lly averaged data sets. Both the spatial distribution of the global ai r-sea fluxes and the meridional fluxes carried by the ocean were estim ated. The resulting air-sea fluxes seem smoother and significantly clo ser to the climatological flux estimates than do the air-sea fluxes ob tained from the GCM by simply specifying the surface temperature and s alinity. The better fit to the climatological fluxes was balanced by a larger deviation from the surface temperature and salinity. These sur face fields were still close to the observations within the measuremen t error in most regions, except western boundary areas. The inconsiste ncy of the model and data in western boundary areas is probably relate d to the inability of the coarse resolution GCM to appropriately simul ate the large transports there. The meridional fluxes calculated by th e proposed method differ very little from those obtained by simply spe cifying the surface temperature and salinity. We suggest therefore tha t these meridional fluxes are strongly influenced by the interior mode l dynamics; in particular, the too-weak model meridional circulation c ell seems to be the reason for differences between the meridional tran sports in the model and those estimated from other sources. We discuss the implications for the calculation of air-sea fluxes by inverse mod els.