Swell and UVCN regimes- competing mechamisms influencing the exchange of scalars over the sea

In very slightly unstable conditions (UVCN regime) when wind speed is high and the Obukhov length is much greater than the surface layer depth, it has been observed from studies in the Baltic Sea and Lake Ontario that the structure of the surface layer turbulence does not accord with standard similarity theory. In particular the efficiency of the turbulent exchange of sensible and latent heat is observed to be more strongly enhanced than is consistent with the standard model. Also the profiles of dissipation of turbulent kinetic energy and temperature fluctuation variance are found to depend on the structure higher up in boundary layer (i.e. are non-local), indicating that a large-scale transport process is at work. All these features are found to be similar over flat land sites and marine sites during growing sea conditions. But in situations when relatively strong wind (> c. 10 m/s) and pronounced swell coexist, as observed in an experiment from the Pacific, the UVCN mechanism is being disrupted. This is caused by deep vertical circulations in the atmospheric boundary layer induced by the swell waves, which erase the sharp velocity gradients near the surface which, in turn, are a pre-requisite for the UVCN mechanism to develop. In this way, the flux of sensible heat and water vapour becomes lower than observed in situations when the UVCN-regime prevails. As the effect of the UVCN-regime on these fluxes can be appreciable, it is important to find out the exact criteria for occurrence/non-occurrence of this mechanism in oceanic and coastal areas.