Physical ocean-atmosphere variability over the shelf of South Africa from reanalysis products

Abstract

Inter-relationships between physical ocean-atmosphere processes over the eastern Agulhas Bank are explored using data assimilation by the Hybrid Coordinate Ocean Model (HYCOM), which provides daily fields and time series off the south coast of South Africa (33.75–34.5S, 24.0–26.5E) in the period 2009–2017. Our objectives are: i) to describe and understand the sharp gradients over the shelf, ii) to quantify the temporal controls and lagged uptake of weather events and, iii) to analyze the spatial effects of capes on mesoscale oceanographic structure. Sea surface temperature (SST) and zonal winds (U-wind) were cross-correlated with salinity, currents, waves and atmospheric variables such as heat budget components, air pressure (SLP), air temperature. Correlations were made over daily and annual time scales to identify the leading ocean-atmosphere interactions and the strength of processes.At daily time scales weak correlations were found between SST and a variety of ocean-atmosphere parameters, suggesting that multiple processes affect the thermodynamic condition. SST showed most correlation with radiation and heat budget components (r = −0.35, 0.39), SLP (r = −0.30) and meridional currents (r = −0.26). In contrast, the simultaneous correlation of SST with U-wind was weak (r = −0.08). We attribute this to: i) delayed response, ii) opposing effects of coastal upwelling and summer heating, and iii) competing shelf-edge processes.Over the mean annual cycle, correlations between SST, U-winds, salinity, radiative fluxes and V-currents were strong, indicating how kinematic and thermodynamic controls conspire to induce seasonality. SST was well correlated (r < −0.82) with SLP, long-wave radiation, salinity and wave height, but weakly correlated with currents. U-wind was correlated significantly with evaporation, radiative fluxes, curl (wind shear), meridional currents and wave period.Case studies of transient cold and warm events south of South Africa are described to reveal the large-scale atmospheric forcing of local SST. In addition, comparisons between the HYCOM reanalysis time series and independent insitu data are provided as a measure of confidence in global data assimilation systems, which open new opportunities for mesoscale oceanographic research in coastal zones.