The interaction between Hurricane Opal (1995) and a warm core ring in the Gulf of Mexico

Hurricane Opal (1995) experienced a rapid, unexpected intensification in th e Gulf of Mexico that coincided with its encounter with a warm core ring (W CR). The relative positions of Opal and the WCR and the timing of the inten sification indicate strong air-sea interactions between the tropical cyclon e and the ocean. To stud; the mutual response of Opal and the Gulf of Mexic o, a coupled model is used consisting of a nonhydrostatic atmospheric compo nent 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 a re accomplished by conservation of heat, salt, momentum. as well as the sen sible and latent heat fluxes at the air-sea interface. The atmospheric mode l has two nests with spatial resolutions of 0.6 degrees and 0.2 degrees. Th e ocean model has a uniform resolution of 0.2 degrees. The oceanic model do main covers the Gulf of Mexico basin and coincides with a fine-mesh atmosph eric domain of the COAMPS. The initial condition for the atmospheric compon ent of COAMPS is the archived Navy Operational Global Atmospheric Predictio n System operational global analysis, enhanced with observations. The initi al 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 Curre nt 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 press ure (MSLP) of 918 hPa, occurring at 1800 UTC 4 October, a 6-h delay compare d to the observation. The rapid intensification to the maximum intensity an d the subsequent weakening are not as dramatic as the observed. The simulat ed track is located slightly to the east of the observed track, placing it directly over the simulated WCR, where the sea surface temperature (SST) co oling is approx -imately 0.5 degrees C, consistent with buoy measurements a cquired within the WCR. This cooling is significantly less over the WCR tha n over the common Gulf water due to the deeper and warmer layers in the WCR . Wind-induced currents of 150 cm s(-1) are similar to those in earlier ide alized simulations, and the forced current field in Opal's wake is characte rized by near-inertial oscillations superimposed on the anticyclonic circul ation around the WCR Several numerical experiments are conducted to isolate the effects of the W CR 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 cent er enters and exits the outer edge of the WCR. Without the WCR, Opal only i ntensifies another 7 hPa in the same period. 2) The maximum surface sensible and latent heat flux amounts to 2842 W m(-2 ). This occurs when Opal's surface circulation brings northwesterly flow ov er the SST gradient in the northwestern quadrant of the WCR. 3) Opal extracts 40% of the available heat capacity (temperature greater th an 26 degrees C) from the WCR. 4)While the WCR enhances the tropical cyclone and ocean coupling as indicat ed by strong interfacial fluxes, it reduces the negative feedback. The nega tive feedback of the induced SST cooling to Hurricane Opal is 5 hPa. This s mall feedback is due to the relatively large heat content of the WCR, and t he negative feedback is stronger in the absence of the WCR, producing a dif ference of 8 hPa in the MSLP of Opal.