Wintertime Surface Heat and Momentum Fluxes in the Gulf Stream from Saildrone Observations

During winter, the ocean loses a tremendous amount of heat when weather systems transport cold and dry continental air across the warm and moist Gulf Stream. Quantifying this heat loss with conventional moorings is challenging due to harsh winter conditions and strong Gulf Stream currents. A new autonomous observing platform, the Saildrone designed and built by Saildrone Inc. offers a solution. In late January through early March 2019, a Saildrone was deployed in the Gulf Stream. The Saildrone measured surface ocean and atmospheric parameters across five Gulf Stream crossings. The goals of this study were to first compare the surface sensible and latent heat fluxes with ERA5 reanalysis fluxes. Sensible, latent and momentum fluxes were computed from Saildrone measurements using the Coupled Ocean-Atmosphere Response Experiment (COARE) version 3 algorithm, and compared with the appropriate ERA5 dataset. The comparison with ERA5 is favorable, with correlation coefficients between both state variables and air-sea heat fluxes ranging between 0.90 and 0.99. Large heat fluxes (~1200 W/m2) occur during the passage of the 7 winter weather systems during the Saildrone deployment. The annual average flux within the Gulf Stream in this region is about 200 W/m2. Thus, the majority of the annual heat flux not only occurs during winter months but occurs in an event-like manner associated with a handful of extratropical storm systems each year.