Evaporation induced convection increases upper ocean turbulence and mixing.

The upper ocean surface layer is directly affected by the air-sea fluxes which vary spatiotemporally. The diurnal timescale (24 hr) is one of the most prominent sources of the variability of upper ocean turbulence and mixing, in response to these air-sea fluxes. In this study, we use large-eddy simulation to quantify the role of surface evaporation (haline fluxes) in the modulation of diurnal mixed layer turbulence and mixing. During daytime when the upper ocean boundary layer becomes thermally stratified, a salinity inversion layer forms near the surface, leading to double diffusive salt-fingering instability. During nighttime, convection due to surface buoyancy loss from both surface cooling and evaporation deepens the mixed layer. During transitions between day to night, the flow instability changes from salt-fingering to gravitationally unstable convection and vice versa. In a total sense, surface evaporation increases the mixed layer depth and irreversible mixing through convection, both during nighttime and daytime, and leads to better prediction of dynamical variables like sea surface salinity (SSS) and sea surface temperature (SST).