Current and Wind-driven Upwelling Events Inshore of the Agulhas Current

We investigate upwelling events inshore of the Agulhas Current at 34^◦S. These events exchange shelf and slope waters, potentially enhancing primary productivity along the shelf and advecting larvae offshore. Hydrographic observations of a wind-driven upwelling event and a current-driven upwelling event show that they can advect slope waters more than 130 m upward onto the continental shelf resulting in a 9^◦C cooling. We use satellite data to assess the frequency, strength and forcing mechanisms of cold events during an 11-year period. We define cold events as days when the shelf-averaged sea surface temperature anomaly falls below the 95th percentile. We identify 47 events with an average length of 3.5 days and sea surface temperature anomaly of -2.4^◦C. We explore 4 forcing mechanisms: alongshore wind speed, wind stress curl, Agulhas Current core distance and Agulhas Current core speed. We find that stronger forcing leads to longer cold events and, with the exception of Agulhas Current core distance, also colder events. Current-driven cold events occur 3 times more often than wind-driven cold events, but the latter are slightly stronger. 75% of cold events associated with strong wind stress curl are also associated with Agulhas meanders, suggesting that frontal curvature is driving local wind stress curl anomalies. The combination of these forcings leads to the strongest cold events. Cold event frequency exhibits strong seasonality, with most occurring in austral summer and fall. We believe this is due to the seasonality of the alongshore wind directionality, which is upwelling favorable from November to April.