Hurricane intensity change modulated by air-sea interaction effects based on unique airborne measurements during the 1998–99 hurricane seasons

Unique measurements were made in Hurricanes Bonnie and Danielle in 1998 and Hurricanes Bret, Dennis and Floyd in 1999 which illustrate the ocean's modulation of hurricane intensity change in several interesting ways: 1) reduced intensification via storm-induced cooling, 2) reduced intensification via the cold wake of a previous storm and 3) strengthening via enhanced ocean heat content from pre-existing warm eddies. In Bonnie 98, the first Scanning Radar Atlimeter (SRA) measurements throughout the inner core revealed peak significant waveheights in the right-front quadrant, while peak wave steepness was observed in the right-rear quadrant. Details of these measurements are discussed in separate companion papers. Wave spectra showed swell generated in the eyewall region moves outward from the center at about a 40 deg angle to the surface wind, and further a bimodel spectrum characterizes the right-rear quadrant. A second swell component exists, generated by the front-quadrant eyewall winds, which moves at 90 deg to the primary swell component. AXBT measurements throughout the storm show that strong SST cooling existed in Bonnie, especially in the right-rear quadrant, where wave steepness was greatest and wave spectra most complex. Surface winds were measured by the NOAA Stepped Frequency Microwave Radiometer (SFMR). The storm-induced cooling appeared to limit intensification of this storm. AXBT observations in Danielle 98 before during and after storm passage showed that intensification of this storm appeared to be limited by the prior cold wake produced by Bonnie. Steady intensification commenced after just after Danielle crossed the band of cooling caused by Bonnie and continued as Danielle re-crossed Bonnie's track during a turn to the north. Satellite AVHRR images showed that Bonnie's 'cold wake' consisted of cool patches about 300 km apart, consistent with inertial forcing. Danielle passed between two of the cold pools after during the recurvature, resulting in an unimpeded strengthening.

AXBT measurrements before and during Bret 99 as well as TOPEX/POSEIDON and ERS2 satellite altimeter data revealed the presence of two warm core eddies in the western Gulf in the region traversed by the storm as it intensified rapidly to a CAT4 storm. These eddies had formed a year earlier from the Gulf Loop Current and were weakening as they interacted with the west Gulf shelf. These observations show that even moderately deep, warm mixed layers can contribute to sufficiently large ocean heat contents which act to impede storm-induced cooling and any reduction in surface air-sea fluxes.

AXBT measurements in Dennis 99, and AVHRR images after storm passage, showed that the extent and magnitude of cooling in the right quadrant of the storm overwhelmed enhanced fluxes from the relatively narrow Gulf Stream east of the center and inhibited rapid strengthening as it moved parallel to the eastern boundary of the Gulf Stream. Floats designed to oscillate up and down across the mixed layer were deployed ahead of the storm and made detailed turbulence measurements just to the right of the eyewall as well as left of center, near the Gulf Stream.

AXBT measurements, TOPEX data and AVHRR images from Floyd 99 showed that subsurface cooling caused previously by Dennis rapidly cooled the ocean over Floyd's inner core and acted as a break on intensification.