Impact of Atmospheric Cooling on the High‐Frequency Submesoscale Vertical Heat Flux

Abstract

AbstractRecent simulations suggest that submesoscale motions with scales smaller than 30 km and frequencies greater than 1 day−1 drive upward vertical heat transport. These simulations have prompted us to revisit the mechanisms that explain high‐frequency (HF) vertical heat fluxes (VHFs) within the surface mixed layer (ML). Here, an idealized numerical simulation of a re‐entrant channel flow with an unbalanced submesoscale thermal front is used to analyze the impact of surface cooling on HF VHFs. Two types of simulations are analyzed: forced and unforced. The VHFs cospectrum analysis shows that surface diurnal cooling increases VHFs, reaching frequencies larger than 1 day−1. However, the fastest‐growing length scale of ML instabilities limits the extension of positive VHFs toward fine scales. Symmetric and gravitational instabilities are the main conduits producing ageostrophic HF and small‐scale structures, which in turn enhance upward VHFs across the diurnal frequency. A comparison between forced‐idealized simulations with the K‐profile parameterization scheme and a realistic regional simulation in the frequency‐wavenumber space, reveals that the two simulation types reproduce similar VHFs near the diurnal frequency. However, the realistic simulation displays higher VHFs than the forced‐idealized simulation. This study emphasizes that surface diurnal cooling significantly impacts HF VHFs. However, this impact is not sufficient to reach the HF VHFs estimated in realistic submesoscale‐permitting and tidal‐resolving simulations.