Investigating the dominant processes of air-sea turbulent heat flux variability in the Cape Basin using Saildrone and ERA5 data

The Cape Basin, located in the South Atlantic outside Africa's southwest coast, is host to one of the highest energetic domains globally. The Agulhas Current transports warm, salty Indian Ocean water into the Cape Basin through meanders, so-called Agulhas leakage, where it is exposed to colder air masses and in the direct path of mid-latitude storm tracks. The persistent mesoscale Agulhas eddies in the Cape Basin are suggested to have a direct effect on local wind patterns by creating local air temperature gradients through strong air-sea heat fluxes. To determine the dominant drivers of this air-sea heat exchange, this study investigates the fluctuations of the bulk air-sea flux parameters - namely wind speed, air-sea temperature and humidity gradients - on the variability of turbulent heat fluxes in the region. A 3-month in-situ-Saildrone expedition augmented with ERA5 reanalysis is used to provide both a local and broad-scale overview of how the different bulk parameters affect the turbulent heat fluxes in the Cape Basin and Agulhas region. By validating ERA5 with the Saildrone data, we show using ERA5 that there is a strong correlation between wind speed and turbulent heat flux fluctuations in the Agulhas leakage. Variability of air density, transfer coefficients of heat and moisture, specific heat capacity and latent heat of vaporization is negligible in the region of this study. Turbulent heat fluxes vary considerable over small distances and comparing a higher resolution (5 km) OSTIA SST data to the ERA5 SST (~25 km) shows ERA5 is not resolving smaller SST fronts due to an insufficient spatial resolution, which leads to a weaker Saildrone-ERA5 turbulent heat flux correlation. As a result, turbulent heat loss events can be misrepresented in the ERA5 reanalysis by up to 240 Wm-2. We show that Agulhas eddies are important for setting strong air-sea temperature and humidity gradients, and combined with elevated winds from passing cyclones, this leads to a strong impact on the turbulent heat flux fluctuations inside Agulhas eddies. Using both in-situ and reanalysis data, we improve the knowledge of the relationship between turbulent heat fluxes and the impact from mesoscale anticyclonic eddies in the Cape Basin.