Surface fluxes in the North Atlantic current during CATCH/FASTEX

CATCH (Couplage avec 1' Atmosphere en Conditions Hivernales) was the oceani c component of FASTEX (Fronts and Atlantic Storm-Track EXperiment). It took place in January and February 1997, in the Newfoundland Basin near 47 degr ees N, 40 degrees W, a region characterized by the presence of the warm Nor th Atlantic Current and cold surrounding waters. CATCH was devoted to the s tudy of the parametrization of surface turbulent fluxes in strong winds and changing directions, the surface-flux variability related to the passage o f atmospheric fronts and the influence on fluxes of the strong sea surface temperature gradients associated with the North Atlantic Current. This pape r presents first results of ship data analysis. A large range of wind and s tratification conditions were experienced: 5% of measured winds were higher than 20 m s(-1); 30% of unstable stratification (air-sea temperature diffe rences lower than --5 degC) and 30% of very dry conditions lair-sea moistur e differences lower than -2.5 g kg(-1) were sampled. Surface turbulent heat and momentum fluxes were obtained using the inertial-dissipative method fr om which a bulk algorithm was derived. A significant increase of latent-hea t and momentum-transfer coefficients with increasing wind is obtained, This parametrization is compared to others published using the CATCH dataset. F or high winds and unstable stratifications, differences between schemes rea ch 200 W m(-2) for latent-heat flux values of 600 W m(-2). Radiative and tu rbulent ship-measured fluxes are compared with modelled fluxes from the Eur opean Centre for Medium-Range Forecasts (ECMWF) along the ship's trajectory : each component of the net heat budget is higher in the ECMWF model, conse quently 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 an alysed by evaluating the contribution of the various terms in the Aux varia tions. showing a significant impact of the surface temperature change in al l unperturbed cases.