Oceanic eddy-induced meridional heat transport (EHT) is an important process in the Southern Ocean heat budget, the variability of which significantly modulates global meridional overturning circulation (MOC) and Antarctic sea-ice extent. Although it is recognized that mesoscale eddies with scales of ~40–300 km greatly contribute to the EHT, the role of submesoscale eddies with scales of ~1–40 km remains unclear. Here, using two state-of-the-art high-resolution simulations (resolutions of 1/48° and 1/24°), we find that submesoscale eddies significantly enhance the total poleward EHT in the Southern Ocean with an enhancement percentage reaching 19–48% in the Antarctic Circumpolar Current band. By comparing the eddy energy budgets between the two simulations, we detect that the primary role of submesoscale eddies is to strengthen mesoscale eddies (and thus their heat transport capability) through inverse energy cascade rather than directly through submesoscale heat fluxes. Due to the submesoscale-mediated enhancement of mesoscale eddies in the 1/48° simulation, the clockwise upper cell and anti-clockwise lower cell of the residual-mean MOC in the Southern Ocean are weakened and strengthened, respectively. This finding identifies a potential route to improve the mesoscale parameterization in climate models for more accurate simulations of the MOC and sea ice variability in the Southern Ocean.

Sea surface height (SSH) is composed of a barotropic component associated with the mass of the water column and a baroclinic component arising from variations of water density. The baroclinic component of SSH is termed steric height (SH) and is related to the commonly used dynamic height by a factor of g (gravitational acceleration). In practice, the SH can be either computed using temperature/salinity (T/S) profiles or obtained through removing the atmospheric pressure loading and ocean bottom pressure from the observed SSH. Because variation of the SH can result from multiscale dynamic processes such as baroclinic Rossby and Kelvin waves, mesoscale to submesoscale fronts and eddies, as well as different types of internal gravity waves (IGWs), a quantitative knowledge of how these processes contribute to the SH is a prerequisite for exploring ocean dynamics from the SSH data (e.g.