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 analysed 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, the CAA regime exhibited the most pronounced air-sea coupling, with a mean SHF of −40.4 W m−2 and LHF of −131.0 W m−2, which are 3.5 and 2 times greater than the overall mean, respectively. For the Zonal, Frontal, and CAA regimes, a strong correlation between the surface fluxes and the M-index (surface − 850 hPa potential temperature difference) is observed, with an R2 of 0.58 for the SHF and M-index relationship in the Zonal regime and an R2 of 0.48 for the LHF and M-index in the CAA regime when the estimated inversion strength (EIS) is less than zero. Furthermore, the flux transfer rate demonstrates a two-fold increase with the M-index when the capping inversion weakens and collapses (EIS shifts from > 0 to < 0). The relationship between surface fluxes and the Mindex is weak in the HPR and TBH regimes, which are characterised by stronger inversions at SOFS. Turning to open and closed MCC, relatively smaller differences in the fluxes are observed between these two cloud states at SOFS, indicating that SHF and LHF are not the primary drivers in the transition between these cloud types. However, the EIS and M-index exhibit considerable differences between the cloud types, which may be more significant for the morphology of open and closed MCCs, rather than the surface flux release. The SOFS measurements were employed to evaluate ERA5 fluxes, revealing that ERA5 accurately represents the observed bulk SHF and LHF with significant correlation coefficients of r = 0.9 (p < 0.01) and 0.92 (p < 0.01), respectively. A mean bias of 1.6 W m−2 is noted for SHF and −6.2 W m−2 for LHF in ERA5. The bias in SHF is attributed to the underestimation of wind speed (10 m) in ERA5, whereas the dry bias of specific humidity (2 m) leads to the overestimation of LHF.