The Important Role of High-Frequency Processes for the Entrainment Heat Flux in the Southern Ocean

Abstract The entrainment process plays a vital role in the vertical exchange between the mixed layer and the subsurface water, but the importance of entrainment heat flux (EHF) in the mixed layer heat budget remains controversial in the Southern Ocean, which may be attributed to the coarse resolution of the available dataset and lack of objectivity in the way the EHF is estimated. Here, we develop an objective bulk method to calculate the EHF for the gridded dataset and subsequently compare this new EHF resolved by daily and monthly data. The results highlight the dominant role of the high-frequency variability in the seasonal cycle of the EHF. The spatial distribution of EHF components shows significant warming from the lateral induction south of Australia in the southeastern Indian Ocean and subantarctic regions during winter. The detrainment-induced heat flux is about half of the entrainment-induced in magnitude, however, mainly owing to the discontinuity in the time evolution of the mixed layer depth when the surface freshwater starts to control the stratification and forms the new mixed layer. It is noteworthy that winter entrainment warming can compensate for the surface heat loss in the high-latitude Southern Ocean, which cannot be captured by either the monthly data or the traditional pointwise estimates in previous studies. In particular, the surplus EHF associated with the temperature inversion barrier layer causes a strong net bottom-up warming effect on the sea surface temperature, with a magnitude of up to 190 W m −2 .