OBSERVATIONS OF THE UPPER OCEAN RESPONSE TO STORM FORCING IN THE SOUTH-ATLANTIC ROARING-FORTIES

In the austral summer of 1992-1993 the passage of a storm system drove a strong upper ocean response at 45 degrees S in the mid-South Atlant ic. Good in situ observations were obtained. CTD casts revealed that t he mixed layer deepened by similar to 40 m over 4 days. Wind stirring dominated over buoyancy flux-driven mixing during the onset of high wi nds. Doppler shear currents further reveal this to be intimately relat ed to inertial dynamics. The penetration depth of inertial currents, w hich are confined to the mixed layer, increases with time after a wind event, matched by a downward propagation of low values of the Richard son number. This suggests that inertial current shear is instrumental in producing turbulence at the base of the mixed layer. Evolution of i nertial transport is simulated using a time series of ship-observed wi nd stress. Simulated transport is only 30-50% of the observed transpor t, suggesting that much of the observed inertial motion was forced by an earlier (possibly remote) storm. Close proximity of the subtropical front further complicates the upper ocean response to the storm. A si mple heat balance for the upper 100 m reveals that surface cooling and mixing (during the storm) can account for only a small fraction of an apparent similar to 1 degrees C mixed layer cooling.