An assessment of the surface turbulent heat fluxes from the NCEP-NCAR reanalysis over western boundary currents
With the completion of the NCEP-NCAR and ECMWF reanalyses there are now global representations of air-sea surface heat fluxes with sufficient spatial and temporal resolution to be useful in characterizing the air-sea interaction associated with individual weather systems, as well as in developing global-scale oceanic heat and moisture budgets. However, these fluxes are strongly dependent on the numerical models used, and, as a result, there is a clear need to validate them against observations. Accurate air-sea heat flux estimates require a realistic representation of the atmospheric boundary layer, and the implementation of an appropriate surface flux parameterization. Previous work at high latitudes has highlighted the shortcomings of the surface turbulent heat flux parameterization used in the NCEP-NCAR reanalysis during high wind speed conditions, especially when combined with large air-sea temperature differences. Here the authors extend this result through an examination of the air-sea heat fluxes over the western boundary currents of the North Atlantic and North Pacific Oceans. These are also regions where large transfers of heat and moisture from the ocean to the atmosphere take place. A comparison with in situ data shows that the surface layer meteorological fields are reasonably well represented in the NCEP-NCAR reanalysis, but the turbulent heat flux fields contain significant systematic errors. It is argued that these errors are associated with shortcomings in the bulk flux algorithm employed in the reanalysis. Using the NCEP-NCAR reanalysis surface layer meteorological fields and a more appropriate bulk flux algorithm, "adjusted'' fields for the sensible and latent heat fluxes are presented that more accurately represent the air-sea exchange of heat and moisture over the western boundary currents.