Forced Stage Response to a Moving Hurricane

Abstract

The upper ocean's response to three hurricanes [Norbert (1984), Josephine (1984) and Gloria (1985)] is examined using field observations and a numerical ocean model. Our goal is to describe the physical processes that determine the structure and amplitude of hurricane-driven upper-ocean currents. All three of these Northern Hemisphere hurricanes produced a rightward-biased response of the mixed-layer current and transport. This asymmetry arises because the wind stress vector rotates clockwise on the right side of the track and remains nearly parallel with the inertially rotating mixed-layer current during most of the hurricane passage. The maximum observed mixed-layer current varied from 0.8 m s−1 in response to Josephine, which was a large but comparatively weak hurricane, to 1.7 m s−1 in response to Gloria, which was very large and also intense. These cases have been simulated with a three-dimensional numerical model that includes a treatment of wind-driven vertical mixing within the primitive equations. The simulations give a fairly good representation of the horizontal pattern and amplitude of the mixed-layer current, accounting for over 80% of the variance of the observed 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 major contributor to the difference between the observed and simulated currents may be a noise component of the observed current that arises from measurement and analysis error and from prehurricane currents. The Norbert case was distinguished by a large Burger number, ∼1/2, which is a measure of pressure coupling between the forced stage mixed-layer currents and the relaxation stage thermocline currents. The observations 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 Norbert. 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.