Gilbert.fli (17449708 bytes)
Gilbert.avi (16618330 bytes)
During Gilbert's passage over the Gulf of Mexico, the ocean response was extensively measured using current and temperature observations acquired from the deployment of airborne expendable current profiles and airborne expendable bathythermographs from the NOAA WP- 3D-s in the western Gulf of Mexico from 14-19 September 1988 (Shay et al. 1992) The observations indicated the presence of a warm-core ring northeast of the Gilbert track formed from a cut-off meander of the Loop Current. The presence of a warm-core eddy may potentially have a significant impact on the dynamics of air-sea interaction in hurricanes and therefore should be taken into account for realistic simulations. For the present experiment, the observed warm-core ring was assimilated after completion of the third step of the ocean initialization described above using the initialization procedure presented in Ginis et al. (1996). The resulting prestorm sea surface temperature and surface currents in the ocean model are shown in Fig.1. The Loop Current is well represented in the model but its structure is not sharpened enough due to the coarse resolution of the climatological fields used for the model initialization. Note that prestorm SSTs of 28.4° to 28.7°C were fairly uniform over the Gulf and Caribbean Sea before Gilbert's arrival.
The track forecast made by the model was exceptionally accurate Fig.2, with a 48h forecast error of only 40 km. Gilbert produced a cold wake at the sea surface over a broad area of 100-200 km wide. It has a pronounced rightward bias in the SST cooling with respect to the storm track. This rightward asymmetry is well established from previous observations and numerical studies (see the review article of Ginis, 1995), and is correlated with the asymmetry in the mixed layer currents and the mixed layer deepening underneath the storm. The maximum SST cooling in this experiment exceeded 4°C at some places as the SSTs decreased to a minimum of 24.9°C. The cold wake pattern demonstrates very different ocean responses along the Gilbert track. The SST cooling is rather insignificant over the shallow continental shelf surrounding the Yucatan Peninsula. Maximum cooling occurred over the continental slope and the deep water areas in the middle of Gulf where the mixed layer is shallower and the thermocline stratification is steeper. Despite its extreme intensity in the north- western Caribbean, Gilbert generated relatively small SST decrease there (maximum of 1.7°C) mainly due to the significantly deeper initial upper mixed layer (see Figure)
The magnitude and the pattern of SST cooling produced by the coupled model was generally in very good agreement with the observations (Shay et al. 1992). The comparisons between the model and observed temperatures indicate very good correlation ( Fig.3 ), especially in the region of strong wind-driven currents near the storm track (Ginis at al. 1996).
At the start of the forecast, the hurricane was near maximum intensity
with a minimum sea level pressure of 892 hPa and maximum low-level
winds of nearly 72 m/s. The observed storm was about 35 hPa deeper
than the model storm (
Fig. 4.), since the model resolution was
insufficient to reproduce the storm's extreme intensity. Once the
storm made landfall on the Yucatan Peninsula and began weakening, the
observed and model storm filled to 949 hPa and 959 hPa, respectively.
After moving into the Gulf and prior to the final landfall on the
eastern Mexican coast 48h later, Hurricane Gilbert underwent only
very slight reintensification (4 hPa) to 946 hPa. In contrast, in the
forecast without ocean coupling, the storm began to gradually
reintensify over the warm open Gulf waters, deepening 25 hPa to 934
hPa during the next two days. With the coupling included the
reintensification was much smaller and closer to the observed rate,
with about a 10 hPa deepening. Although the storm in the coupled
model remained somewhat weaker than observed during this period, the
general tendency of the minimum sea-level pressure was much better
represented compared to the uncoupled model. This suggests that the
excessive SST cooling which was enhanced by Gilbert's enormous
circulation, was likely one of the majors factor that prevented
significant reintensification after the hurricane moved across the
very warm waters of the Gulf of Mexico.
To Tropical Cyclone-Ocean Interactions
