Diurnal and seasonal variation of convection in the wind‐mixed layer of the ocean

Abstract

AbstractBuoyant convection in the wind‐mixed layer of the ocean provides a source of turbulent kinetic energy that is restricted to a layer whose depth, C, is equal to, or less than, H, the depth of the mixed layer. At night C = H, and convection contributes significantly to deepening the mixed layer by turbulent entrainment. During most of the day, when the rate of heat input by the Sun exceeds the rate of surface heat loss by evaporation, conduction and long wave radiation, C < H and convection does not contribute to entrainment. When C < H, C = D, the thermal compensation depth, whose magnitude is independent of any consequences of turbulent mixing or entrainment. D can be calculated from a model of solar energy absorption in the ocean. Such a model is used to determine the diurnal variation of D at selected locations. The shape of this variation (a ‘top hat’ distribution) suggests the use of two parameters Dmin (the noon value) and P10 (the proportion of each day for which D < 10m). The seasonal and meridional variations of these parameters reveal the corresponding variations in the contribution of convectively generated turbulent kinetic energy to mixed layer deepening by turbulent entrainment. The parameter P10 can be used to take account of the daytime quenching of convection by the Sun in climate models that do not resolve the diurnal cycle.