Role of buoyancy in driving upper ocean mixing across timescales

Abstract

This thesis explores the response of the upper ocean to surface buoyancy fluxes on different timescales using theory, numerical simulations and observation-based datasets. First, we investigate the role of evaporation in the diurnal (24 hour) variability of upper ocean mixing, using large-eddy simulations (LES). We find that surface evaporation increases the depth of the turbulent boundary layer and enhances irreversible mixing through convection, both during nighttime and daytime, leading to improved prediction of the diurnal cycle of sea surface salinity (SSS) and sea surface temperature (SST). Next we investigate upper ocean mixing caused by the passage of a tropical cyclone (timescale ∼ O(5) days). Using high-fidelity LES simulations and moored observations away from the storm track, we show that mutually interacting shear and convective processes, govern the upper ocean evolution. Surface buoyancy loss enhanced ocean mixing via convective entrainment and shear driven turbulence eroded stratification and deepened the mixed layer. Lastly, we focus on a climate change scenario (timescale ∼ O(10) years or more), particularly the effect of intensifying hydrological cycle on subtropical mode waters. The patterns of global SSS reveal the fingerprint of such a process, where salty subtropical waters have become saltier and fresh tropical waters have become fresher. Using various observation-based datasets of upper ocean stratification, we find salinity driven compensation within the upper ocean, which decreases stratification and increases upper ocean mixing.