Surface fluxes in the North Atlantic current during CATCH/FASTEX

Abstract

AbstractCATCH (Couplage avec l'Atmosphère en Conditions Hivernales) was the oceanic component of FASTEX (Fronts and Atlantic Storm‐Track Experiment). It took place in January and February 1997, in the Newfoundland Basin near 47°N, 40°W, a region characterized by the presence of the warm North Atlantic Current and cold surrounding waters. CATCH was devoted to the study of the parametrization of surface turbulent fluxes in strong winds and changing directions, the surface‐flux variability related to the passage of atmospheric fronts and the influence on fluxes of the strong sea surface temperature gradients associated with the North Atlantic Current. This paper presents first results of ship data analysis. A large range of wind and stratification conditions were experienced: 5% of measured winds were higher than 20 m s‐1; 30% of unstable stratification (air‐sea temperature differences lower than −5 °C) and 30% of very dry conditions (air‐sea moisture differences lower than −2.5 g kg−1) were sampled. Surface turbulent heat and momentum fluxes were obtained using the inertial‐dissipative method from which a bulk algorithm was derived. A significant increase of latent‐heat and momentum‐transfer coefficients with increasing wind is obtained. This parametrization is compared to others published using the CATCH dataset. For high winds and unstable stratifications, differences between schemes reach 200 W m−2 for latent‐heat flux values of 600 W m−2. Radiative and turbulent ship‐measured fluxes are compared with modelled fluxes from the European Centre for Medium‐Range Forecasts (ECMWF) along the ship's trajectory: each component of the net heat budget is higher in the ECMWF model, consequently the heat loss of the ocean is 35% higher in the model. Finally, the effect of sea surface temperature fronts on surface turbulent fluxes is analysed by evaluating the contribution of the various terms in the flux variations, showing a significant impact of the surface temperature change in all unperturbed cases.