Jury, M.R., 2017. Multiple drivers of intense coastal upwelling at Cape Town.

Cape Town, South Africa is well known for its intense coastal upwelling during austral summer. This study provides new insights using daily 4–9 km resolution satellite and ocean reanalysis datasets in the period 2009–2015. A key purpose of this work is to highlight the value of these finely detailed products in coastal research. The most intense case of 23–24 January 2011 revealed a cold plume <10°C extending offshore into a filament. SE winds accelerated cyclonically around Cape Town, and the enhanced wind vorticity lifted <10°C water over the shelf. Yet a statistical analysis found that sea surface temperature and equatorward winds are correlated at <−0.10. A significant outcome could only be achieved in summer (December–February) due to opposing thermal and kinematic influences. The intense upwelling of 23–24 January 2011 was supported by the approach of a midlatitude upper anticyclone and coupled shelf wave. Regression analysis over 2009–2015 showed that a warmer tropical SE Atlantic contributes to a trough over Namibia that spawns coastal lows. They advance southward toward Cape Town at 6–12 day intervals, pulsing the upwelling and turning the equatorward winds offshore and cyclonic. The strong pressure gradient, shallow marine layer, and steep mountains induce a Venturi effect and low level wind jet over the shelf edge. Leeward of Cape Town, a wind shadow and poleward currents contribute to phytoplankton blooms that support a rich ecosystem. In summary, the multiple factors of intense coastal upwelling at Cape Town include: (1) cyclonic wind shear over the shelf due to shallow equatorward flow obstructed by coastal mountains, (2) passing atmospheric coastal low and oceanic shelf wave from the large scale weather, and (3) passing oceanic anticyclonic eddy offshore shed by the Agulhas Current.