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Hurricane Opal (1995) experienced a rapid, unexpected intensification in the Gulf of Mexico that coincided with its encounter with a warm core ring (WCR). The relative positions of Opal and the WCR and the timing of the intensification indicate strong air-sea interactions between the tropical cyclone and the ocean. To study the mutual response of Opal and the Gulf of Mexico, a coupled model is used consisting of a nonhydrostatic atmospheric component of the Naval Research Laboratory's Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS), and the hydrostatic Geophysical Fluid Dynamics Laboratory's Modular Ocean Model version 2 (MOM 2).The coupling between the ocean and the atmosphere components of the model are accomplished by conservation of heat, salt, momentum, as well as the sensible and latent heat fluxes at the air-sea interface. The atmospheric model has two nests with spatial resolutions of 0.6Њ and 0.2Њ. The ocean model has a uniform resolution of 0.2Њ. The oceanic model domain covers the Gulf of Mexico basin and coincides with a fine-mesh atmospheric domain of the COAMPS. The initial condition for the atmospheric component of COAMPS is the archived Navy Operational Global Atmospheric Prediction System operational global analysis, enhanced with observations. The initial ocean condition for the oceanic component is obtained from a 2-yr MOM 2 simulation with climatological forcing and fixed mass inflow into the Gulf. The initial state in the Gulf of Mexico consists of a realistic Loop Current and a shed WCR.The 72-h simulation of the coupled system starting from 1200 UTC 2 October 1995 reproduces the observed storm intensity with a minimum sea level pressure (MSLP) of 918 hPa, occurring at 1800 UTC 4 October, a 6-h delay compared to the observation. The rapid intensification to the maximum intensity and the subsequent weakening are not as dramatic as the observed. The simulated track is located slightly to the east of the observed track, placing it directly over the simulated WCR, where the sea surface temperature (SST) cooling is approximately 0.5ЊC, consistent with buoy measurements acquired within the WCR. This cooling is significantly less over the WCR than over the common Gulf water due to the deeper and warmer layers in the WCR. Windinduced currents of 150 cm s Ϫ1 are similar to those in earlier idealized simulations, and the forced current field in Opal's wake is characterized by near-inertial oscillations superimposed on the anticyclonic circulation around the WCR.Several numerical experiments are conducted to isolate the effects of the WCR and the ocean-atmosphere coupling. The major findings of these numerical experiments are summarized as follows. 1) Opal intensifies an additional 17 hPa between the times when Opal's center enters and exits the outer edge of the WCR. Without the WCR, Opal only intensifies another 7 hPa in the same period. 2) The maximum surface sensible and latent heat flux amounts to 2842 W m Ϫ2 . This occurs when Opal's surface circulation brings northwesterly...

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Free surface effects induced by an idealized hurricane based on observed air-sea variables in Hurricane Frederic are revisited to examine the barotropic and baroclinic response. Over five inertial periods comparisons between a one-layer and a 17-level model indicate a difference of 6-8 cm s Ϫ1 in the depth-averaged current and sea level oscillations of 4-5 cm. In a one-layer simulation, the surface slope geostrophically balances the depthaveraged current, whereas the 17-level model simulations indicate a near-inertially oscillating current of 7-8 cm s Ϫ1 found by removing the depth-averaged flow from the geostrophic currents induced by the surface slope. Surface undulations are driven by the depth-averaged nonlinear terms in the density equation, that is, [u ], x [ y ], and [w z ]. Based on fits of the 17 levels of demodulated horizontal velocities at 1.03f (f the Coriolis parameter) to the eigenfunctions, maximum amplitudes of the barotropic and first baroclinic modes are 7 and 58 cm s Ϫ1 , respectively. The barotropic mode amplitude is consistent with the current found by removing the depth-averaged flow from the geostrophic current that contributes 2%-3% to the energy in the near-inertial wave pass band. Vertical velocity eigenfunctions at the surface indicate that the barotropic mode is at least 50 to 80 times larger than the baroclinic mode. Surface displacements by the barotropic mode have amplitudes of Ϯ4 cm, explaining 90% to 95% of the height variations. The first baroclinic mode contributes about 0.2-0.4 cm to the free surface displacements. The weak barotropic near-inertial current provides a physical mechanism for the eventual breakup of the sea surface depression induced by the hurricane's wind stress and surface Ekman divergence.