Validation of forecasted surface sensible and latent heat fluxes by GFS and GEFS against saildrone observations in the Arctic

Air-sea fluxes have rarely or never been estimated from in situ observations in many parts of the global oceans, especially in the Arctic, despite their critical roles in weather and climate. In consequence, their reproductions by numerical models have seldomly been validated against observations. In this study, observations from Saildrone Explorer uncrewed surface vehicles are used to validate surface sensible and latent heat fluxes from GFS deterministic forecasts and GEFS ensemble forecasts in the Arctic during May – October 2019. The most striking result from this study is the low biases in sea surface temperature (SST) in the initial conditions of both the deterministic and ensemble forecasts. Excessively cold predictions of SST lead to reversed signs in air-sea differences in temperature and humidity in comparison to the observations. Consequently, surface sensible and latent heat fluxes in the forecast can be negative (from air into the water), while observed fluxes are positive. The larger SST biases at the initial time og the GEFS ensemble forecasts is the main reason for their underperformance in comparison to the GFS deterministic forecasts. The results clearly demonstrate the vital step of improving forecasts in the Arctic is to prepare for accurate initial conditions of SST.