Submesoscale Thermohaline Compensation and Its Role in Frontogenesis

Abstract Upper thermohaline properties play a critical role in mediating the transfer of momentum, heat, and biogeochemical tracers, thereby influencing the global carbon cycle and climate system. Thermohaline compensation – where temperature and salinity exert opposing effects on seawater density–is more prevalent in the mixed layer and modulates frontal dynamics. In this study, observations from 12 underwater gliders reveal that thermohaline compensation within salinity fronts becomes more pronounced at submesoscales during a tropical cyclone in the northern South China Sea. Comparative analyses from idealized numerical experiments demonstrate that surface cooling enhances submesoscale activity and thermohaline compensation at salinity fronts, with the compensated ratio reaching approximately 20%. Through intense submesoscale ageostrophic motions and restratification, surface cooling generates temperature perturbations to rapidly form temperature fronts aligned with salinity gradients, thereby enhancing submesoscale thermohaline compensation more effectively. Surface diabatic effects are incorporated into the frontogenesis function by accounting for surface cooling and submesoscale restratification. This process becomes more pronounced and efficient as submesoscale restratification shoals the mixed layer. Atmospheric cooling over submesoscale salinity fronts, together with the accompanying submesoscale compensation, constitutes a sink of oceanic eddy potential energy (EPE). These findings advance our understanding of thermohaline compensation mechanisms and the modulation of submesoscale frontal dynamics by atmospheric forcing, offering further insights into air-sea interactions in dynamically active, salinity-dominated ocean regimes.