Intraseasonal surface fluxes in the tropical western pacific and Indian oceans from NCEP reanalyses

Reliability of the surface fluxes from National Centers for Environmental P rediction (NCEP) reanalyses is assessed across the warm pool of the western Pacific and Indian Oceans. Emphasis is given to the spatial distribution a nd coherence of the fluxes on intraseasonal (25-100 day) periods, as intras easonal variability predominates the subseasonal variability across the war m pool. Comparison is made with surface fluxes estimated from data collecte d at a mooring during the Coupled Ocean-Atmosphere Response Experiment and with independent gridded estimates based on operational wind and surface pr essure analyses and satellite observations of rainfall, shortwave radiation , and outgoing longwave radiation. In general, fluxes that depend primarily on surface wind variations (e.g., stress and latent heat flux) agree more favorably than fluxes that are largely dependent on fluctuations of convect ion (e.g., surface shortwave radiation and freshwater or precipitation). In particular, the intraseasonal variance of shortwave radiation and precipit ation in the NCEP reanalyses is about half of that estimated from in situ o bservations and from satellite observations. Composite surface flux variati ons for the Madden-Julian oscillation, which is the dominant mode of intras easonal variability in the warm pool, are also constructed. Again, the comp osite variations of wind stress and latent heat flux from the NCEP reanalys es agree reasonably well, both in magnitude and phasing, with the composite fluxes from the independent gridded data. However, the composite intraseas onal shortwave radiation and precipitation from the NCEP reanalyses, while agreeing in phase, exhibit less than half the amplitude of the satellite-ba sed estimates. The impact of the underestimation of these surface flux variations in the N CEP reanalyses on the intraseasonal evolution of sea surface temperature (S ST) in the warm pool is investigated in the context of a one-dimensional mi xed layer model. When forced with the intraseasonal surface fluxes from the NCEP reanalyses, the amplitude of the intraseasonal SST variation is some 30%-40% smaller than observed or than that from forcing with the independen t gridded fluxes. This reduced amplitude is primarily caused by the underes timation of the intraseasonal shortwave radiation variations in the NCEP re analyses.