Diurnal Surface Flux Variability Over Western Boundary Currents

An analysis of a satellite ocean surface turbulent flux product demonstrated that, as expected, the western boundary current regions dominate the seasonal cycle amplitude. Surprisingly, our analysis of the global ocean diurnal flux variability also demonstrated a regional maximum in the winter over the western boundary current regions. We conducted comparisons with in situ data from several buoys located in these regions. The buoy data were in general agreement with the relative magnitude, timing, and importance of each of the bulk parameters driving the latent and sensible heat fluxes. Further analysis demonstrated that the strength and timing of the diurnal signal is related to the location of the buoy relative to the region of maximum heat flux and sea surface temperature gradient. In both regions, the timing of the higher winds coincides with the moistest surface layer, indicating that surface fluxes rather than entrainment mixing play a key role in this phenomenon. Plain Language Summary The Gulf Stream and Kuroshio are perhaps the most well-known surface currents in the global ocean system, strongly affecting the weather and climate in the Atlantic and Pacific regions and the surrounding continents. The wintertime exchanges of heat and water between the ocean and atmosphere in these regions, often associated with storms, are some of the largest observed. They affect not only the strongly coupled local ocean and atmosphere but also large-scale weather systems. Understanding these linkages between the ocean and atmosphere is a key part of understanding the evolution of storms. These warm water, strong-current regions typically experience no wintertime daily warming of the upper ocean, as there is less sunlight and stronger winds during this season. As a result, no consistent daily change in the air temperature or the surface exchanges of heat is expected over this region. However, when we analyzed wintertime variability of the surface and lower atmosphere over these regions from satellite records, we found a consistent daily variability of the lower atmosphere. This result was verified by comparisons with buoys in the region. This unexpected result implies that there are some physical processes in the lower atmosphere that still remain for us to understand.