FORCED STAGE RESPONSE TO A MOVING HURRICANE

The upper ocean's response to three hurricanes [Norbert( 1984), Joseph ine(l984) and Gloria(1985)] is examined using held observations and a numerical ocean model. Our goal is to describe the physical processes that determine the structure and amplitude of hurricane-driven upper-o cean currents. All three of these Northern Hemisphere hurricanes produ ced a rightward-biased response of the mixed-layer current and transpo rt. This asymmetry arises because the wind stress vector rotates clock wise on the right side of the track and remains nearly parallel with t he inertially rotating mixed-layer current during most of the hurrican e passage. The maximum observed mixed-layer current varied from 0.8 m s(-1) in response to Josephine, which was a large but comparatively we ak hurricane, to 1.7 m s(-1) in response to Gloria, which was very lar ge and also intense. These cases have been simulated with a three-dime nsional numerical model that includes a treatment of wind-driven verti cal mixing within the primitive equations. The simulations give a fair ly good representation of the horizontal pattern and amplitude of the mixed-layer current, accounting for over 80% of the variance of the ob served current. Model skill varies considerably with the amplitude of the mixed-layer Current, being much higher for stronger currents than it is for weaker currents. This and other evidence suggest that a majo r contributor to the difference between the observed and simulated cur rents may be a noise component of the observed current that arises fro m measurement and analysis error and from prehurricane currents. The N orbert case was distinguished by a large Burger number, similar to 1/2 , which is a measure of pressure coupling between the forced stage mix ed-layer currents and the relaxation stage thermocline currents. The o bservations and the simulation show upwelling of up to 25 m and strong thermocline-depth currents up to 0.3 m s(-1) under the rear half of N orbert. Thermocline currents have a very simple vertical structure, a monotonic decay with increasing depth, and nearly constant direction, Their horizontal structure is more complex but appears to be due to an acceleration toward a low pressure anomaly associated with the first upwelling peak about 100 km behind the eye of Norbert.