Southern Ocean latent heat flux variability driven by oceanic meso- and submesoscale motions

TL;DR

This study uses high-resolution simulations to show that ocean mesoscale and submesoscale motions significantly influence latent heat flux variability over the Southern Ocean, affecting air-sea interactions and atmospheric responses.

Contribution

It quantifies the scale-dependent impact of oceanic meso- and submesoscale motions on latent heat flux variability in the Southern Ocean using a kilometer-scale coupled simulation.

Findings

Ocean mesoscale and submesoscale variability accounts for up to 80% of LHF variance in eddy-rich regions.

Fine ocean scales are largely unresolved in reanalysis, leading to underestimation of atmospheric responses.

Strong SST fronts induce secondary circulations extending into the mid-troposphere.

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 ($\leq$ 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.