Abstract
The modulation of near-surface currents off Durban (29.9S, 31E) is studied using monthly and daily ocean and atmosphere reanalysis products. Poleward flow on the shelf-edge exhibits long-lived 2–3 yr and 50–150 day fluctuations, whereas cyclonic circulations inshore exhibit short-lived ∼6 day pulsing by coastal winds. Monthly meridional currents near Durban 1979–2015 relate to lower air pressure over the SW Indian Ocean (∼37S, 40E) that slows the offshore anticyclonic gyre and shelf-edge Agulhas Current. Daily meridional currents near Durban 2009–2015 relate to a pattern of offshore positive / inshore negative sea surface temperatures that propagate poleward at ∼7 km/day.The upper ocean structure and circulation off KwaZulu-Natal is analyzed using daily 8 km resolution fields over the area 28–31.5S, 30–33E. The shelf oceanography of a cyclonic shear event on 25–26 October 2010 corresponds with a transient low pressure cell, 12 m/s southwest winds and 5 m swells. A coastal wind jet near Durban deepens the mixed layer >50 m during a cold air outbreak with latent heat flux >250 W/m2.While the regional atmosphere modulates longshore winds and inshore currents at high frequency, the offshore anticyclonic gyre pulses the shelf-edge Agulhas Current at low frequency in association with poleward moving eddies. As these advance down the coast, water draws off the shelf contributing to upwelling and productivity.
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