Latent heat flux is a primary pathway for ocean-atmosphere exchange of heat and moisture, yet the influence of sea surface temperature variability at fine scales ($\leq$ 100 km) on latent heat flux variability, particularly over the Southern Ocean, remains poorly understood. Here we quantify the scale-dependent drivers of latent heat flux (LHF) variability using a year-long, global, fully coupled ocean-atmosphere simulation with kilometer-scale resolution. Annual-mean LHF in eddy-rich regions reaches $\approx$ 215 W m$^{-2}$, approximately three times larger than in eddy-poor regions. Spectral analyses show that ocean mesoscale [$\mathcal{O}$(100 km)] and submesoscale [$\mathcal{O}$(1-10 km)] variability accounts for up to $\approx$ 80% of the total LHF variance in eddy-rich sectors, but as little as 10% in eddy-poor regions, and increases proportionally with eddy kinetic energy and sea surface temperature (SST) variance. We also find that strong submesoscale SST fronts ($\approx$ 5 $^\circ$C over 10 km) force a localized secondary circulation that extends well above the marine boundary layer into the mid-troposphere. Comparison with ERA5 shows that fine ocean scales, responsible for about 17% of the ocean-driven LHF variance in the simulation, are largely unresolved in the reanalysis, leading to a muted atmospheric response lacking any secondary circulation. Despite a strong heterogeneity in LHF variability, the atmospheric dynamics are mostly uniform across the domain, suggesting a non local atmospheric response to ocean forcing. These results highlight the potential for ocean meso- and submesoscales, commonly under-resolved in climate models and reanalysis, to influence Southern Ocean air-sea coupling and atmosphere both locally and remotely.
ABSTRACT: Latent heat flux is a primary pathway for ocean-atmosphere exchange of heat and moisture, yet the influence of sea surface temperature variability at fine scales (≤ 100 km) on latent heat flux variability, particularly over the Southern Ocean, remains poorly understood. Here we quantify the scale-dependent drivers of latent heat flux (LHF) variability using a year-long, global, fully coupled ocean-atmosphere simulation with kilometer-scale resolution. Annual-mean LHF in eddy-rich regions reaches ≈ 215 W m -2 , approximately three times larger than in eddy-poor regions. Spectral analyses show that ocean mesoscale [O(100 km)] and submesoscale [O(1-10 km)] variability accounts for up to ≈ 80% of the total LHF variance in eddy-rich sectors, but as little as 10% in eddy-poor regions, and increases proportionally with eddy kinetic energy and sea surface temperature (SST) variance. We also find that strong submesoscale SST fronts (≈ 5 • C over 10 km) force a localized secondary circulation that extends well above the marine boundary layer into the mid-troposphere. Comparison with ERA5 shows that fine ocean scales, responsible for about 17% of the ocean-driven LHF variance in the simulation, are largely unresolved in the reanalysis, leading to a muted atmospheric response lacking any secondary circulation. Despite a strong heterogeneity in LHF variability, the atmospheric dynamics are mostly uniform across the domain, suggesting a non local atmospheric response to ocean forcing. These results highlight the potential for ocean meso-and submesoscales, commonly under-resolved in climate models and reanalysis, to influence Southern Ocean air-sea coupling and atmosphere both locally and remotely.
The ocean is a major heat reservoir in the Earth system, and growing evidence has shown that air-sea exchanges at mesoscale can substantially influence large-scale oceanic and atmospheric circulation (Seo et al. 2023). Latent heat fluxes (LHF) provide a primary pathway for the exchange of heat and moisture between the ocean and the atmosphere, as they capture the bulk of turbulent heat fluxes, far exceeding the amplitude of sensible heat fluxes over warm sea surface temperature (SST) anomalies (Small et al. 2019;Tamsitt et al. 2020).
In fact, ocean mesoscales (∼100-500 km scale) account for more than half of the monthly LHF variance in western boundary currents (WBCs) and in the Antarctic Circumpolar Current (ACC) (Bishop et al. 2017;Small et al. 2019). To understand the impact of ocean mesoscales on LHF, it is useful to consider the physical mechanisms at play (see for example Fig. 1 in Seo et al. 2023). When an air mass flows across a SST gradient, it creates air-sea differences in temperature and humidity. Wind blowing from cool to warm SST produces an imbalance that generates a positive turbulent heat flux anomaly on the warm side of the front, enhancing ocean heat loss1 . This thermal imbalance deepens and destabilizes the boundary layer, enhancing turbulent mixing, decreasing wind shear and increasing surface wind stress. In the warm-to-cool case, the effects are reversed. Steeper SST gradients and stronger winds further accentuate the air-sea thermodynamic disequilibrium. In addition, recent observational and numerical studies show that submesoscale fronts of SST enhance vertical mixing, diabatic processes, and promote cloud formation and precipitations (Kaouah et al. 2025;Yang et al. 2024;Vivant and Lapeyre 2025;Vivant et al. 2025;Strobach et al. 2022). Yet identifying the drivers of LHF variability remains challenging due to the complex interactions between the ocean and the atmosphere occurring over a broad range of spatial and temporal scales (from hours to months and kilometers to thousands of kilometers, Fig. 1).
The Southern Ocean (SO) offers an ideal natural laboratory to address this question as it features a wide range of oceanic dynamical regimes, from large-scale currents to mesoscale eddies and submesoscale filaments, to intense wind events. The SO comprises areas of elevated eddy kinetic energy (EKE) (Thompson and Naveira Garabato 2014;Beech et al. 2022;Meijer et al. 2022), as well as regions of weak EKE (Rintoul 2018). Regions of high EKE feature abundant mesoscale eddies and submesoscale fronts, concentrated along the ACC and near its confluences with the Agulhas and Brazil Currents (Rosso et al. 2014;Siegelman 2020;Siegelman et al. 2020;Taylor and Thompson 2023;Marshall and Speer 2012). The SO also experiences some of the strongest near-surface winds on Earth (Sampe and Xie 2007), which modulate air-sea heat fluxes. Despite the region’s pivotal role in ocean-atmosphere heat exchanges (Morrison et al. 2022;Gruber et al. 2019;Frölicher et al. 2015;Williams et al. 2023), latent heat fluxes in the SO remain poorly constrained, largely due to the scarcity of observations imposed by harsh remote conditions that restrict in situ measurements and persistent cloud cover that limits optical satellite
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