DETERMINATION OF OCEAN SURFACE HEAT FLUXES BY A VARIATIONAL METHOD

A new technique of determination of the ''nonsolar'' heat flux (sum of the latent, sensible, and net infrared fluxes) at the ocean surface i s proposed. It applies when oceanic advection remains weak and thus re lies on a one-dimensional modeling approach. It is based on a variatio nal data assimilation scheme using the adjoint equation formalism. Thi s allows to take advantage of all observed data with their error estim ates. Results from experiments performed with station Papa (Gulf of Al aska) and Long-Term Upper Ocean Study (LOTUS, Sargasso Sea) data sets are discussed. The temperature profiles assimilation allows the one-di mensional model to reproduce correctly the temperature evolution at th e surface and under the oceanic mixed layer at the two sites. The retr ieved fluxes are compared to the fluxes calculated through classical e mpirical formulae. The diurnal dependence of the fluxes at the LOTUS s ite is particularly investigated. The results are also compared with t hose obtained using a simpler technique based on an iterative shooting method and allowing the assimilation of the only sea surface temperat ure. This second comparison reveals that the variability of the retrie ved fluxes is damped when temperature in the inner ocean are assimilat ed. This is the case for the diurnal cycle at the LOTUS mooring. When the available current data at this site are assimilated, the diurnal v ariability of the retrieved fluxes is further decreased. This points o ut a model discrepancy in the representation of mixing processes assoc iated to internal wave activity. The remaining part of the diurnal cyc le is significant and could be due to a direct effect of air-sea tempe rature difference.