Annales Geophysicae

ISSN: 0992-7689 (printed version)ISSN: 1432-0576 (electronic version)

Abstract Volume 13 Issue 10 (1995) pp 1027-1038

Observations of the upper ocean response to storm forcing in the SouthAtlantic Roaring Forties

Robert Marsh

James Rennell Centre for Ocean Circulation, Gamma House, Chilworth Research Centre, Chilworth, Southampton SO1 7NS,UK

Received: 12 October 1994/Revised: 6 February 1995/Accepted: 17 February 1995

Abstract. In the austral summer of 1992-1993 the passage of a storm system drove a strong upper ocean response at 45°S inthe mid-South Atlantic. Good in situ observations were obtained. CTD casts revealed that the mixed layer deepened by \sim40m over 4 days. Wind stirring dominated over buoyancy flux-driven mixing during the onset of high winds. Doppler shear currentsfurther reveal this to be intimately related to inertial dynamics. The penetration depth of inertial currents, which are confined to themixed layer, increases with time after a wind event, matched by a downward propagation of low values of the Richardsonnumber. This suggests that inertial current shear is instrumental in producing turbulence at the base of the mixed layer. Evolutionof inertial transport is simulated using a time series of ship-observed wind stress. Simulated transport is only 30-50% of theobserved transport, suggesting that much of the observed inertial motion was forced by an earlier (possibly remote) storm. Closeproximity of the subtropical front further complicates the upper ocean response to the storm. A simple heat balance for the upper100 m reveals that surface cooling and mixing (during the storm) can account for only a small fraction of an apparent \sim1 °Cmixed layer cooling.

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