The role of turbulence and double-diffusion in the exchange of central waters at the Cape Verde Frontal Zone

The Cape Verde Frontal Zone (CVFZ) separates North and South Atlantic Central Waters (NCAW and SACW, respectively) in the eastern North Atlantic Subtropical Gyre. This front is described as a strong meandering thermohaline front near Cape Blanc at latitudes close to 20ºN. It shows sharp gradients in temperature and salinity in the upper 600 m with the presence of large lateral intrusions. One important aspect of the CVFZ is the compensating character of the temperature and salinity fields, which cause horizontal density gradients to be relatively small across the front. This frontal feature is an important factor in reducing vertical shear of horizontal velocity in some parts of the frontal region, allowing double diffusion processes to be one of the main causes of the observed diapycnal mixing. However, the presence of large lateral intrusions could favor diapycnal mixing induced by vertical shear instabilities which could overcome double diffusion effects. Despite its importance, studies in the CVFZ with direct turbulence measurements focused on diapycnal mixing and its relation with lateral thermohaline intrusions are scarce. In this study, we use microstructure measurements from a vertical free-falling profiler together with CTD-O and SADCP records of two high spatial resolution (each oceanographic stations ~9 km apart) oceanographic transects along and across the CVFZ (~300 and 100 km, respectively) during November of 2017. An assessment of the turbulent and double-diffusive mixing related to the lateral intrusions was made, identifying the latter through the diapycnal spiciness curvature method. Lateral intrusions ranging from ~20-100 km at subsurface and central levels of the water column showed relative increments in dissipation and diapycnal diffusivity. Therefore, at their boundaries occur the exchange of properties between the NACW and SACW.