Global observational estimates of thermohaline transformations by interior ocean mixing

Abstract

Small-scale diapycnal mixing and mesoscale isopycnal stirring redistribute heat and freshwater and other solutes in the ocean. These processes shape regional and global circulation patterns and impact the climate system. Due to the scarcity of mixing observations, appraising such a critical role remains a major challenge. Here, we revisit and expand a recent reformulation of the water-mass transformation framework to derive global thermohaline transformation rates from estimates of variance dissipation rates of temperature χΘ and salinity χS in the ocean interior. To estimate χΘ and χS, we leverage new global, vertically resolved maps of diapycnal and isopycnal diffusivity. Global diathermal and diahaline water-mass transformations by interior mixing show respective double circulation cells transporting 600 TW of heat and 20 × 106 kg s−1 of salt to waters cooler than 20°C and fresher than 35.1 g kg−1. Diathermal transformations are dominated by diapycnal mixing, involving the formation of 36 Sv (1 Sv ≡ 106 m3 s−1) of subtropical thermocline waters (10°–25°C) from warmer tropical waters and colder, deeper waters. Isopycnal mixing is the main driver of global diahaline transformations, forming 34 Sv of waters with intermediate salinity (34.4–35.7 g kg−1), and of diathermal transformations in cold waters (<5°C). The Antarctic Circumpolar Current is the global hotspot for isopycnal mixing, controlling the redistribution of heat and freshwater between the Southern Ocean and the rest of the global ocean. Climatological estimations were validated against regional observational microstructure datasets, demonstrating that even a modest number of microstructure profiles can yield meaningful regional estimates of water-mass transformations.

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ORCID for Bieito Fernández Castro: orcid.org/0000-0001-7797-854X

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