Global Observational Estimates of Thermohaline Transformations by Interior Ocean Mixing

Castro, Bieito Fernández ORCID: 0000-0001-7797-854X, Groeskamp, Sjoerd, Broullón, Espe, Clément, Louis, Evans, D Gwyn, Garabato, Alberto C Naveira and Williams, Richard G (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

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 χ Θ and salinity χ S in the ocean interior. To estimate χ Θ and χ S , 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 × 10 6 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 ≡ 10 6 m 3 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. Significance Statement Oceanic turbulence mixes water masses of varying temperature and salinity, shaping global circulation and redistributing heat and freshwater—critical for climate. Quantifying turbulent fluxes at large scales is challenging because direct measurements require centimeter-scale observations, which are costly and scarce. Here, we combine recent global estimates of turbulent diffusivity from hydrographic data and theory with a new water-mass transformation framework to quantify—based on observations—the roles of diapycnal and isopycnal mixing in ocean circulation. We find diapycnal mixing sustains shallow low-latitude circulations, while isopycnal mixing in high latitudes is crucial for deep overturning. This challenges the traditional emphasis on diapycnal mixing. Applying the framework to localized direct microscale mixing observations shows that even sparse data can yield valuable regional insights.

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:	14 Apr 2026 11:44
DOI:	10.1175/jpo-d-25-0265.1
Related Websites:	Publisher
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3197362
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