Variability of currents and mixing processes during the onset of the equatorial cold tongue in the Atlantic

The heat content of the surface water masses is important for the circulation of the oceans and the atmosphere. The surface layer serves as a medium for the ocean-atmosphere exchange and thereby significantly affects the dynamics of our climate. Despite this, many of the mechanisms that allow exchange between the surface layer and the ocean interior are still elusive. Measurements of the turbulent dissipation rate of kinetic energy in May and June 2011 in the Atlantic Cold Tongue region depict mixing events that extend below the surface mixed layer. These mixing processes show a distinct diurnal cycle with a nighttime maximum, which is preserved until some hours after sunset. These deep mixing events occur when the stratification is weak and high vertical shear is present. The diurnal cycle is superimposed on a ow-frequent variation of the turbulent dissipation rate of kinetic energy. Due to this low frequent cycle with a period of 11 days, the increased mixing events below the mixed layer vanish completely during some days while they extend 30 meters below the mixed layer depth on other days.