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Current Meter Measurements
One of the best set of observations of hurricanes and tropical storms over the northeastern Gulf of Mexico occurred between June and October 1979. During this time, Halper and Schroeder (1990) were monitoring current meters deployed at sites on the Texas Louisiana, Alabama and West Florida Shelves. During the deployment period of these sites, five tropical disturbances (Hurricane Bob, Tropical Storm Claudette, Tropical storm Elena, Hurricane David, Hurricane Frederic, and Hurricane Henri) affected the area.
Hurricane Bob, which passed 620 km to the west of the Florida Middle Ground mooring, provided winds as high as 25 knts. At the mooring off the Alabama coast. When the area was under the influence of the hurricane, the currents in the bottom layer were flowing in opposition to the wind stress.
Tropical Storm Claudette, however, passed directly over the mooring located off of the Texas/Louisiana coast. Remotely, the storm caused a rotation of the surface current vectors from south to north at the Florida Middle Grounds. With the winds blowing a steady north-northwest heading, the maximum currents reached ~40 cm/s and after the passage of the storm, inertial oscillations of ~20 cm/s to ~25 cm/s lasted for 5 days. Closer to the bottom, the flow shifted from a weak flow to the south to a stronger flow to the northwest. The maximum current speeds after this shift in direction reached as high as 61 cm/s.
The most dramatic, recorded responce to Claudette occurred off the Texas/Louisiana coast. The mooring measured currents at 60 m and 96m in 100 m of water. With maximum sustained winds of 45 knts., the 28 hour lowpass currents, at the upper mooring, went from weak 5-10 cm/s magnitudes, rotating from a westerly to a southerly direction. They then made a rapid shift in direction to the north-northwest and an increase in speed to 25-30 cm/s. The unfiltered currents reached a maximum of 55-60 cm/s to the northwest. The filtered series illustrated the generation of intense oscillations at the inertial/diurnal period after the storm passage. These oscillations were accompanied by a 2° C drop in temperature lasting one day. The near-bottom lowpass flow was to the south prior to the storm. This shifted to the north-northeast and the magnitudes increased 5 cm/s to 15 cm/s. This drop in temperature was also observed and Halper and Schroeder attribute it to upwelling of cold water over the shelf break near the bottom. Inertial oscillations were also observed at this depth.
Tropical Storm Elena and Hurricane David passed through the area at roughly the same time. Elena had little effect, but the effects of David were felt on the Florida Middle Grounds as the hurricane travelled up the east coast of Florida. The 28 hour lowpassed data from the upper current meter, at 9 m in 31 m of water, showed an abrupt shift to the south as David moved up the Atlantic coast. The maximum unfiltered current was 55 cm/s with a maximum of 40 cm/s in the filtered data. The flow was primarily to the south until Hurricane Frederic passed over the mooring less than a week later.
Hurricane Frederic produced estimated maximum sustained winds of 115 knts. The center of the storm passed directly over the Alabama mooring destroying the mooring there. Lowpassed data from the mooring at the Florida Middle Grounds revealed a direction shift from south to the north as the storm entered the Gulf of Mexico east of the Yucatan Channel. The strongest flow was along the isobaths to the north-northwest and had maximum speeds of about 85-90 cm/s. At the bottom meter, the flow was slightly to the left of that measured closer to the surface with maximum currents of about 25-30 cm/s. Inertial oscillations were formed with magnitudes of 25 cm/s and lasted for 5 days. Prior to the arrival of Frederic, the thermocline was near the bottom with a depth range of 23-25 m in 31 m of water. After the storm passage, the water temperature near the bottom had increased 2-3° C and a thermocline did not develop for the rest of that summer.
Hurricane Henri showed no appreciable responces at any of the stations. There were inertial oscillations setup at the Texas/Louisiana station which were comparable to the other oscillations previously discussed. The previous mixing of the water column by Hurricane Frederic produced a barotropic response to Henri.
All of these storms setup inertial oscillations and the results form the Florida Middle Ground mooring suggest that the water is set into motion as the storms enter the gulf basin before the winds from the storm present themselves locally. These storms also forced sea level setup/setdown at the coast which drove the flow response on the shelf as a result of the sea level gradient and the subsequent relaxation of that setup.
In 1994, Tropical Storm Alberto moved into the Gulf of Mexico in late June. Its passage, on July 2, was marked by 30 knt. winds at the USF mooring located at 28° N, 84° W. The 36 hour lowpassed currents flowed at about 30 cm/s to the north-northeast and then rotated toward the east over the next two days. Inertial oscillations were also observed after the passage of this storm.
The 1995 hurricane season was the most active one recorded. There were 21 named storms, and 5 crossed the northeastern Gulf of Mexico. Allison was the first of the season, and was small in scale. The two strongest hurricanes that traversed this area were Erin and Opal. Erin formed over the Bahamas and quickly reached Category 1 strength before reducing to a tropical storm as it crossed the Florida peninsula. After reaching the warm waters of the Gulf of Mexico, it spun back up to hurricane strength and was more organized when it moved inland over Pensacola, Florida. Opal, however, formed over the Yucatan Peninsula of Mexico and moved northward, intensifying from a mild Category 1 hurricane to a Category 4 over the course of 12 hours. It weakened to a Category 3 as it travelled over cooler waters in the northeastern Gulf of Mexico before reaching landfall near Pensacola, Florida.
Datahave been collected from a University of South Florida meteorological buoy located at 27° 41.4' N, 83° 13.2' W for the period spanning July 25 through August 8 1995. This allows us to study the passage of Hurricane Erin and its effects on the eastern Gulf of Mexico.
Preliminary analyses of the these data suggest that the west Florida shelf experienced about a 0.5° C reduction in sea-surface temperature. The hurricane passage was marked by a 15 mbar drop in barometric pressure and wind speeds around 12 m/s at the buoy. When the in situ data are compared with NOAA 14 AVHRR images they seem to be consistent, and this cooling trend is apparent over the whole northeastern Gulf of Mexico. The sea-surface temperature increased about 1° C within a couple of weeks.
Although the area presented seemed to cool homogeneously, it did not heat up similarly. The shallows in the Big Bend area showed a sea-surface temperature that was almost a whole degree warmer than the deeper waters offshore. This may be explained by the increased stratification due to the freshwater flux into the area via turbid river runoff and the rainfall delivered by the storm. This stratification will effectively shorten the length of the water column that is heated by the solar radiation. Alternatively, this preferential heating may also be explained by studying the bottom albedo in the Big Bend area. The higher temperatures seem to be confined to the shallows where sea grass is prevalent. These grasses have a darker albedo, thus allowing for the water column to be more efficiently heated. Further studies on the heat budget should be undertaken to better understand the physical processes in this area.