Environmental controls on marine productivity near Cape St Francis, South Africa

Abstract. This study considers ocean-atmosphere influences on marine productivity over the shelf near Cape St Francis, South Africa. Multi-day estimates of chlorophyll fluorescence in the period 2006–2017 with an area: 34.5–33.75S, 24–26.5E, provide the basis for evaluation using data from high resolution reanalysis. Correlations with the mean annual cycle of chlorophyll fluorescence were significant for salinity, linking marine productivity and the coastal hydrology. A strengthened Agulhas Current induces cyclonic shear that lifts water at the shelf edge. Composite high chlorophyll fluorescence events were dominated by a large-scale mid-latitude atmospheric ridge of high pressure. The resultant easterly winds caused offshore transport and the upwelling of cool nutrient-rich water, in multi-day events at the beginning and end of austral summer. Environmental controls on inter-annual fluctuations of the commercial fishery were also explored. Southwestward currents and diminished heat fluxes favoured squid catch, while anchovy and sardine were linked with upper northerly wind, consistent with large-scale weather patterns that underpin coastal upwelling and river discharge. Productivity lags a few days behind cyclonic wind and current shear and the upstream coastal hydrology, which shares a common atmospheric driver.