The impact of synoptic meteorology on observed surface heat fluxes over the Southern Ocean

Abstract. A 14-year climatology of the bulk sensible and latent heat fluxes (SHF and LHF) made from the Southern Ocean Flux Station (SOFS) is analyzed with respect to the synoptic meteorology and mesoscale cellular convection (MCC). A K-means clustering algorithm identified five synoptic regimes: High Pressure/Ridging (HPR), Tasman Blocking High (TBH), Zonal, Frontal, and Cold Air Advection (CAA). Among these, CAA showed the strongest air-sea coupling, with mean SHF of -40.4 W/m² and LHF of -131.0 W/m², which are 3.5 and 2 times greater than the overall mean, respectively. This striking increase in fluxes during CAA is associated with a high marine cold-air outbreak index (M-index) and weak inversion coupled with cold and dry air transport towards SOFS by the strong south-westerly wind. The SOFS measurements are also employed to evaluate ERA5 fluxes, finding that ERA5 accurately represents the observed bulk SHF and LHF, with a mean bias of 1.6 W/m² for SHF and -6.2 W/m² for LHF, along with significant correlation coefficients of r=0.9 and 0.92, respectively. Turning to open and closed MCC, relatively weak differences in the fluxes are observed between these two states, suggesting that the SHF and LHF are not the primary drivers in the transition between open and closed MCC. In open MCC, SHF and LHF show a strong correlation with the M-index, while closed MCC is associated with a stable atmosphere with a strong inversion, where the M-index relationship with surface fluxes is weak.