Salt-induced convection in warm wakes

Cátia Azevedo, Dom Luiz Institute, Oceanic Observatory of Madeira, Funchal, Portugal, Carolina Camargo, NIOZ Royal Netherlands Institute for Sea Research, Department Estuarine and Delta Systems, Netherlands, Jose Alves, Dom Luiz Institute, Oceanic Observatory of Madeira, Portugal and Rui Caldeira, ARDITI - Agência Regional para o Desenvolvimento da Investigação Tecnologia e Inovação, Oceanic Observatory of Madeira, Funchal, Madeira, Portugal
The interaction between winds with mountainous islands induces the generation of vortex shedding in the atmosphere, which contributes to weaker winds and the clearing of clouds on the leeward side, exposing the sea surface to intense solar radiation. This in turn results in an increase in the Sea Surface Temperature in the lee, forming what is known as a ‘warm wake’. Convections processes can develop due to a loss of heat from the surface to the atmosphere and/or an increase in salinity caused by increased evaporation rates. Shortwave radiation heats the surface in cloud free conditions i.e. warm wake region. Approximately 700 profiles (CTD; Rosette; UCTD) of temperature and salinity were used for the characterization and quantification of a diffusive (oceanic) layer (25-70 m) and its variation within the warm wake. With respect to temperature, the water column is stratified, with higher temperature at the surface layers and decreasing with depth. Regarding the salinity, the sinking/rising of distinct water parcels with more/less salinity is observable. This vertical transport of saltwater occurs up to 60 m depth on the southwest coast of the island (17.3⁰W; 16.9⁰W), 30 km leeward. These regimes were verified with Turner-angle and density ratio (Rp) calculations. Turbulence and shear profiles were also considered. In the first 50m, the vertical shear was higher during the day than at night, foreseeing a stronger mixing of the surface layer forced by intense solar radiation. This postulates that the daily thermal mixing results in the deepening of the thermocline at night. In exposed regions, particularly in the southeast and north coast of the island, the process was not so evident, suggesting that the warm wake effect is a significant contributor to vertical mixing in the open-ocean, leeward of these islands.