Diurnal Variability in Air-Sea Fluxes over Western Boundary Currents

Ocean surface turbulent latent (LHF) and sensible heat (SHF) fluxes are an integral component of the Earth’s energy and water cycles. Satellite-based estimates of turbulent fluxes provide a unique vantage point from which to evaluate regional and global variability, from sub-daily to interannual. This presentation will focus on the SeaFlux-CDR, a 3-hourly, quarter-degree, 25+year satellite dataset, in responding to the two externally-forced modes, the diurnal and the seasonal cycles.

An analysis of the dataset indicates that, like previous studies of reanalysis and other blended products, the western boundary currents dominate the seasonal cycle amplitudes. The largest differences in the yearly cycle of the satellite-derived LHF occur over the western boundary currents, with values over 300 W m^-2. Typical values over the rest of the global ocean are closer to 100 W m^-2. Previous studies have noted the importance of episodic high flux events in defining the mean winter fluxes over the western boundary currents.

More surprisingly, an analysis of the mean diurnal amplitude of the fluxes and near-surface parameters, computed by comparing the maximum value during the daytime minus minimum value during the nighttime, also highlights the western boundary current regions. The mean diurnal cycle of LHF ranges from near zero in the polar latitudes and eastern tropical regions to values of over 100 W m^-2 during winter over the Gulf Stream and Kuroshio currents. In this presentation we will show that this variability is a result of diurnal wind and air-sea humidity difference variability. Unlike the tropical regions, where diurnal SST variability is a large factor in diurnal flux variability, the humidity difference over the current regions is largely a result of diurnal variability in the air specific humidity, and is much higher in the cold seasons than in the warm seasons. The patterns of variability of the SHF are similar to the LHF.

These results will be compared with data from the CLIMODE and KEO buoys, as well as the MERRA-2 reanalysis dataset. We will also present an analysis of some possible reasons for this strong diurnal variability in the western boundary current regions.