NASA - National Aeronautics and Space Administration

Recent 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. Plain Language Summary Recent research has highlighted the importance of balanced motions in the ocean with horizontal dimensions less than 30 km, as these motions play a crucial role in carrying heat upward with frequencies greater than 1 day −1 in the surface mixed layer (ML). These motions are influenced by atmospheric forcing, which impacts the heat flux. This study investigates how high-frequency vertical heat fluxes (VHFs) respond to atmospheric cooling by using numerical simulations to generate structures with horizontal dimensions less than 30 km in two scenarios: one with active surface cooling and one without. The simulations reveal that surface diurnal cooling amplifies VHFs with frequencies higher than 1 day −1. This amplification occurs through the enhancement of the generation of structures driven by convective motions in the ML. Through comparison with a realistic numerical simulation that generates horizontal motions less than 30 km and tidal motions, the study finds that diurnal cooling partially explains the vertical fluxes with frequencies greater than 1 day −1 estimated in the realistic simulations. This research sheds light on the complex mechanisms involved in vertical heat transport in the ocean and highlights the roles of balanced motions with horizontal dimensions of less than 30 km and atmospheric forcing.