Quantifying Air–Sea Fluxes from the Tropics to the Ice Edge: Atmospheric Reanalyses versus Conventional and Autonomous Observing Platforms

Air-sea exchange of heat (latent, sensible, shortwave, longwave), moisture (evaporation and precipitation), and momentum is a key link in virtually all mechanisms that explain ocean-atmosphere coupling and feedback. They play a central rolein understanding and attributing causes of climate change and variability and in assessing the imbalance of the global energy and water cycle budgets associated with the changing climate.However, air-sea fluxes are not typically obtained directly from either in situ or remote sensing platforms. Global flux products are constructed from bulk aerodynamic formulae that parameterize the fluxes as a function of surface meteorological variables (e.g. wind speed and direction, sea surface temperature, air temperature and humidity, surface pressure). The flux-related variables can be mostly retrieved from satellite remote sensing. The latest versions of atmospheric reanalyses, with improved spatial and temporal resolutions and improved quality, have become a useful alternative. Since neither data sources are not error free, the climate trends and variability that come out from the gridded global flux products need to be fully evaluated. In most tropical-subtropical oceans air-sea flux products are better evaluated thanks to the sustained buoy measurements in climatically important locations. At high latitudes and near the ice edges where air-sea conditions differ significantly from those at lower latitudes, measurements from moored buoys are limited. We used observations from icebreakers and found that reanalyses show a high-level agreement among themselves, but this agreement reflects a universal bias, not a “truth”. Downward shortwave radiation is overestimated (warm biased) and downward longwave radiation is underestimated (cold biased), an indication that the cloud amount in all models is too low. The important role of sensible heat flux at high latitudes is also significantly underestimated by reanalyses. For ocean variability ranging from mesoscales to submesoscales, our understanding of air-sea coupling is improving but the representation of the flux products is poorly known. With the recent availability of air-sea measurements from the Saildrone unmanned surface vehicles, some of the understanding gaps may start to be filled. In this study we provide a comprehensive assessment of air-sea flux variability using both conventional and autonomous measurements. Three issues are addressed: differences in air-sea flux variability between low and high latitudes; characterization of air-sea exchanges from meso to basin scales; and the functionality of the unmanned vehicles in comparison with buoys and ships.