Impact of mesoscale oceanic features on tropical cyclone intensity

Wind-induced mixing of the upper ocean by a tropical cyclone can cool the sea surface via entrainment of cooler water into the oceanic mixed layer (OML) from below. Therefore, the future intensity of a given tropical cyclone depends not only on the initial temperature of the sea surface below the storm, but also on the magnitude of the wind-induced sea surface cooling in the region providing heat energy to the storm. The magnitude of the sea surface cooling depends largely on the depth of the OML, which can be significantly deeper (shallower) in warm (cold) mesoscale ocean features than in the surrounding oceanic environment. Here, two versions of the Geophysical Fluid Dynamics Laboratory/University of Rhode Island coupled hurricane prediction system (hereafter GFDL model) are used to assess the impact of mesoscale oceanic features on the intensity of a tropical cyclone.

The first GFDL model version is idealized with prescribed initial atmospheric and oceanic conditions. In the atmospheric component, a three-dimensional, axisymmetric tropical cyclone vortex is embedded within a horizontally-homogeneous environmental wind field. In the oceanic component, a three dimensional warm or cold-core ring (WCR/CCR) is embedded within a horizontally-homogeneous environmental ocean temperature field by modifying the subsurface temperature. At the air-sea interface, the SST field is initialized as homogeneous everywhere, thereby approximating the actual SST in, for example, the Gulf of Mexico during the height of hurricane season. Using this model version, a variety of sensitivity cases are used to determine the impact of a WCR or a CCR on the intensity of an idealized tropical cyclone.

The second GFDL model version is the most recent one used operationally at NOAA's National Centers for Environmental Prediction to forecast hurricanes in the Atlantic basin. With this model version, Hurricane Katrina (2005) is simulated using a control case and a modified case. In the control case, the Loop Current and a partially-attached WCR are initialized in the ocean component of the model in their actual late-August 2005 locations based on near real-time satellite altimetry. In the modified case, no WCR is initialized, and the Loop Current is initialized with minimal intrusion into the Gulf of Mexico, thereby representing a less favorable (but still physically realistic) oceanic condition for Hurricane Katrina's intensification in the Gulf. Comparison of these cases reveals the impact of the Loop Current and WCR's position on the intensity of Hurricane Katrina prior to landfall in Louisiana.