Global Observational Estimates of Thermohaline Transformations by Interior Ocean Mixing

Castro, BF ORCID: 0000-0001-7797-854X, Groeskamp, S, Broullón, E, Clément, L, Evans, DG, Garabato, ACN and Williams, RG (2026) Global Observational Estimates of Thermohaline Transformations by Interior Ocean Mixing Journal of Physical Oceanography, 56 (5). pp. 1051-1076. ISSN 0022-3670, 1520-0485

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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 x<inf>Q</inf> and salinity x<inf>S</inf> in the ocean interior. To estimate x<inf>Q</inf> and x<inf>S</inf>, 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 3 10⁶ kg s²¹ of salt to waters cooler than 208C andfresherthan35.1gkg²¹. Diathermal transformations are dominated by diapycnal mixing, involving the formation of 36 Sv (1 Sv; 10⁶ m³ s²¹) of subtropical thermocline waters (108–258C) 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 kg²¹), and of diathermal transformations in cold waters (,58C). 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.

Item Type:	Article
Uncontrolled Keywords:	4012 Fluid Mechanics and Thermal Engineering, 3708 Oceanography, 40 Engineering, 37 Earth Sciences, 14 Life Below Water, 13 Climate Action
Divisions:	Faculty of Science & Engineering Faculty of Science & Engineering > School of Environmental Sciences Faculty of Science & Engineering > School of Environmental Sciences > Earth, Ocean and Ecological Sciences
Depositing User:	Symplectic Admin
Date Deposited:	04 Mar 2026 16:47
Last Modified:	07 May 2026 17:45
DOI:	10.1175/JPO-D-25-0265.1
Related Websites:	Publisher
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3197362
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