摘要:

During the last 10 years, a good deal of research has been conducted in the Gulf of Mexico (GOM). Study topics have included the Loop Current, coastal processes, eddies, tides, and the effects of tropical storms. Scientific contributions have been made by the offshore industry, academia, environmental agencies, and naval research groups. A great deal of information has quietly accumulated concerning the gulf, most of which is applicable to other regions in the world.

ISSN:0148-0227    

DOI:10.1029/92JC00024

年代:1992

数据来源: WILEY

摘要:

Argos drift buoy trajectory data showed that a region of anticyclonic circulation about 100 km in diameter was present over the upper continental slope of the NW corner of the Gulf of Mexico in September–October, 1988. Guided by these data, Texas A&M University scientists joined by colleagues from Mexico's Direccion General de Oceanografia Naval surveyed the area from October 17–22 on R/VGyrecruise 88G‐05 with a dense grid of conductivity‐temperature‐depth and expendable bathythermograph stations. The presense of a subsurface salinity maximum greater than 36.5 psu within the upper 150 m of this anticyclone indicated that it had originated as a warm‐core eddy of the Loop Current; however, a maximum of only 36.54 psu at σt= 25.5 in contrast to as much as 36.88 psu at this density surface in a “fresh” ring indicated that this feature had spent many months in the western gulf since its separation from the Loop Current. Biologically, the warm‐core ring was oligotrophic: its surface waters were generally depleted in nitrate to depths of more than 100 m, and chlorophyll standing stocks (<20 mg m−2), primary productivity (<0.4 mg C m−3h−1), and Zooplankton biomass (only 4 mL 100 m−3day and 6 mL 100 m−3night in the upper 200 m) were all extremely low. Deployments of floating sediment traps within the feature on two consecutive days intercepted fluxes of only 10–20 mg dry weight m−2d−1at 50 m and at 100 m. By comparison, at ring periphery where there was measurable nitrate at 100 m (0.2–1 μg‐at. L−1), chlorophyll standing stocks and primary production in the su

ISSN:0148-0227    

DOI:10.1029/90JC02020

年代:1992

数据来源: WILEY

摘要:

A Loop Current anticyclonic ring ∼330 km in diameter and extending to a depth of>1500 m was observed to collide in January of 1984 against the continental shelf slope of the western Gulf of Mexico between 21.5° and 23°N. The collision occurred precisely at the time we conducted our Argos 84−1 hydrographic cruise in the western gulf (26°00′ to 19°20′N) aboard the R/VJusto Sierra. The Caribbean Subtropical Underwater (SUW) was used as a tracer to identify the Loop Current anticyclonic ring within the western gulf. The collision was identified from temperature and salinity distributions and from the dynamic topography distribution relative to 500 m. The ring's collision zone was identified by the presence of a horizontal baroclinic flow divergence, to the east of Tamiahua, that divides the surface circulation into northward and southward baroclinic currents parallel to the western gulf's continental shelf break, with speeds of 85 and 32 cm s−1, respectively. Horizontal divergence and vertical convergence (ring asymmetries) resulted at the focus of the anticyclonic ring's collision and originated the alongshore self advection and northward translation of the colliding anticyclone. Upon colliding the anticyclonic ring shed approximately one third of its volume (∼2 × 104km3), mass, and transferred angular momentum to the south flanking water mass, thus generating a cyclonic ring to the south of the collision zone. The observed alongshelf southward current results from mass conservation and volume continuity requirements associated with the anticyclonic ring's volume shedding and most probably constitutes the colliding ring's potential vorticity conservation mechanism. The weakening of the anticyclonic ring's relative vorticity due to the collision is most likely made up by gain of vorticity from lateral shear in the northward and southward current jets parallel to the continental shelf break. The core of both the anticyclonic and cyclonic rings had typical SUW salinity (>36.5‰) and temperature (∼22.5°C) values. The rings were separated by a 5 × 104km2divergence zone occupied by Gulf Common Water (GCW). The SUW was absent within the collision zone to the east of Tampico (22.3°N, 97.8°W). The GCW within this divergence zone resulted from the convective mixing and dilution of the SUW with less saline (36.1 ≤S≤ 36.3‰) water from the uppermost layer of the thermocline. Hence the collision of Loop Current anticyclones against the western continental shelf slope of the gulf constitutes a primary mechanism by which 30 Sv of SUW are converted to GCW in the Gulf of Mexico. On the other hand, the coastal and continental shelf water temperature and salinity distributions that resulted from the ring's collision indicate that the offshore GCW mass intrudes the continental shelf to the east of Tamiahua and is diluted by low‐salinity coastal water within the western continental shelf. This GCW mass intrusion most probably constitutes a principal and efficient exchange mechanism between the western gulf's conti

ISSN:0148-0227    

DOI:10.1029/91JC00486

年代:1992

数据来源: WILEY

摘要:

The large eddy that broke off from the Loop Current in July 1985 was the most extensively studied eddy ever to appear in the Gulf of Mexico. Other investigators have described its early evolution based on Lagrangian drifters and its later evolution using moored current meters in the western gulf. This paper provides additional insight on the early evolution of the eddy using results from air dropped expendable bathythermographs and air dropped expendable current profilers in early May, a hydrographic ship survey in mid July, and a detailed ship survey in August using expendable bathythermographs and a current profiler. The May survey established a center of circulation at about 26°N but showed that the eddy had not separated from the Loop Current. A maximum velocity of 171 cm/s was observed near the northern edge of the feature. The evidence suggests that a large elongated eddy then separated from the Loop Current and later split into two smaller eddies. The July hydrographic cruise showed a clear separation of the large eddy from the Loop Current to the southeast. Two weeks later, the August survey showed an asymmetric eddy, with the maximum surface current 178 cm/s south of the center of circulation and 132 cm/s to the north. A western eddy named Ghost Eddy then separated from an eastern eddy named Fast Eddy. Using the current profiles and data from the drifters, we constructed a simple kinematic feature model for eddies in the Gulf of Mexico

ISSN:0148-0227    

DOI:10.1029/91JC02905

年代:1992

数据来源: WILEY

摘要:

Current meters, an inverted echo sounder, hydrography, drifters, and satellite imagery are used to characterize relatively small (100‐ to 150‐km diameter) cold cyclonic eddies in the central basin and on the lower Louisiana slope of the Gulf of Mexico. These cyclones are shown to be long‐lived (6 months or more), have limited movements when compared with Loop Current anticyclones, and be fairly vigorous, with upper layer currents of 30–50 cm s−1. They usually have only small temperature differences with surrounding water masses in the upper 50 to 100 m of the water column and are therefore not readily apparent in satellite thermal imagery. The largest isotherm displacements occur over depths of 200–800 m. In one case, a cyclone was traced from the deep basin near 92°W to the northern slope as a major anticyclone propagated southwestward through the gulf. The presence of cyclones and anticyclones on the Louisiana slope is consistent with observed current meter measurements in the upper half of the water column that have long (several months) periods and approximately equal variances in the cross‐slope and along‐

ISSN:0148-0227    

DOI:10.1029/91JC01496

年代:1992

数据来源: WILEY

摘要:

Using altimetry data obtained from the Geosat Geodetic Mission (April 1985 to October 1986), low‐frequency sea surface height (SSH) variations are investigated in the Gulf of Mexico. SSH time series are formed using the method of Fu and Chelton and are used to calculate surface geostrophic current vectors. Spatial patterns of SSH and current vector variations enable the tracking of two major rings shed from the Loop Current. The rings drifted southwestward across the gulf and into the western boundary region at an average speed of about 3.4 cm/s. The buildup of the Loop Current was monitored, as well as the appearance of an eddy of uncertain origin in the southwestern gulf. Verification of the Geosat results are provided with surface drifters, AVHRR imagery, and hydrograph

ISSN:0148-0227    

DOI:10.1029/90JC02400

年代:1992

数据来源: WILEY

摘要:

Data indicate that the climatological vorticity of the lower atmosphere of the northwestern Gulf of Mexico is relatively strong, with monthly means exceeding a Rossby number of 3 from October through March, with a maximum of 4.6 in December. It has been hypothesized that wintertime cyclogenesis over the northwestern gulf is enhanced by this lower level atmospheric vorticity field when it occurs simultaneously with appropriate upper level venting. Observations from November 1982 to mid‐February 1983 were studied in which seven significant cyclones were generated in the northwestern gulf. It was found that all seven storms occurred when the vorticity correlate of horizontal air temperature difference was ∼3–5°C above the climatological mean difference. Although these results corroborate the postulate that such lower level vorticity is related to cyclogenesis, the data from mid‐February through March 1983 were less supportive. During those months, only two of the six significant storms developed when the horizontal air temperature difference was above the climatological mean. Surprisingly, the location of the most unstable air within the atmospheric boundary layer is not found in the northwestern Gulf of Mexico but instead along the shelf break. Relatively strong free convection in the atmospheric boundary layer is found over the Loop Current and the deep western gulf as expected. The unstable regions of the deep gulf can enhance the potential for frontogenesis and cyclogenesis, but it is likely that the linear character of the strong shelf break free convection does not enhance cyclogenesis. It is shown that a maximum in the frequency of tropical storms within the Gulf of Mexico exists some 275 km south of the Mississippi delta at 27°N, 90°W. This maximum is a result of only those storms which originate within the gulf. Two plausible effects of the Loop Current and its rings on tropical storms are discussed. One is that these ocean features are large and consolidated heat and moisture sources from which a nearby, slowly moving atmospheric disturbance can extract energy. The second is that of the cyclonic vorticity that can be generated in the lower atmosphere by such oceanographi

ISSN:0148-0227    

DOI:10.1029/91JC01447

年代:1992

数据来源: WILEY

摘要:

This study consists of numerical model simulations of hurricane‐induced surge and currents on the Texas‐Louisiana shelf. The numerical experiment includes the simulation of multiple hurricane tracks with landfalling points along the Texas‐Louisiana coast. A parallel storm that traverses the entire Texas‐Louisiana coastline is also modeled to assess the difference in shelf response between landfalling and parallel storms. The grid extends from the Texas‐Mexico border to the Gulf Coast of Florida, with the ocean open boundary seaward of the shelf break. Along‐shelf and cross‐shelf surge and current variability are assessed as a function of shoreline geometry and bottom topography. A complementary one‐dimensional mixed layer model is used to evaluate the vertical structure of the currents and the maximum depth of hurricane influence. The choice of landfalling storm tracks is based principally on historical storm tracks for the Texas‐Louisiana area. Storm intensity, i.e., central pressure depression, for all storms is set equal to the 10‐year recurrence interval central pressure. Strong cyclonic flow is induced by the hurricane wind field within a distance from the storm's center equal to 3 to 5 times the storm's radius of maximum winds. Far‐field hurricane winds generate a northward moving coastal jet along the west coast of Florida, particularly for the landfalling storms. The far‐field continental shelf currents are topographically steered by the shoreline and bathymetry. There is strong evidence of a cyclonic‐anticyclonic eddy pair being generated in the wake of the parallel storm traveling eastward over the continental shelf. An intercomparison of the one‐ and two‐dimensional models shows that for depths less than 50 m, the combined use of the surge and mixed layer models permits a description of the vertical and horizontal hurricane‐induced current patterns. The surge height computations for a given storm strength point to the importance of several factors that determine surge levels as a function of offshore distance. These factors are bathymetry, topography, angle of approach of the storm, distance to landfall location, and shelf width. The most dramatic increase in surge levels occurs in depths less than 10 m, where the surge profile mirrors the changes in the bottom slope. In contrast, the sensitivity of surge dynamics to shelf width is neither strong nor consistent. A narrow spread of maximum surge values is observed. Each transect is examined individually to explain local behavior and the reason for the modest variation in surge values among dissimilar geographical areas. No critical dependence on the exact storm angle exists for storm angles within the range of angles associated with maximum surge generation (within 20° counterclockwise from the shore‐normal direction). Since the landfalling storms used in this study fall into this category, no significant dependence of surge height on

ISSN:0148-0227    

DOI:10.1029/90JC02191

年代:1992

数据来源: WILEY

摘要:

Almost a decade after the Geochemical Ocean Sections Study Indian Expedition, the new deep3He data from the INDIGO program give a further insight into the distribution of this tracer in the Indian Ocean. This distribution exhibits some major features related on one hand to a hydrothermal3He input in the Gulf of Aden and on the Mid‐Indian Ocean Ridge, and on the other to the origin of the water masses and to the characteristics of the deep circulation. The main pattern is a significant north‐south3He gradient, with deep waters of the southern ocean showing δ3He values around 8–9% due to the influence of the Atlantic deep waters poor in3He and relatively high values in the northern and central regions (15% to 18% between 2000 m and 3000 m depth) originating from the hydrothermal activity. In the easternmost part of the basin, the3He values exhibit a significant increase at shallower depths (around 1000 m) probably due to the Pacific water flow through the Indonesian sills, whereas the data in the Indian sector of the Antarctic ocean show a maximum of the order of 10%, south of the Polar Front, interpreted as showing the presence of the Pacific deep waters in the Antarctic Circumpolar Current. These different aspects are summarized by mapping the horizontal distribution of the δ3He maxima all over the Indian basin. This map points out some characteristics of the deep circulation but also stresses the need for further measurements in order to clarify the description of this tracer in several ke

ISSN:0148-0227    

DOI:10.1029/91JC02062

年代:1992

数据来源: WILEY

摘要:

The two most promising methods of estimating precipitable waterWover global oceans rely on spaceborne microwave radiometers and atmospheric general circulation models. Global fields ofWfrom the special sensor microwave imager (SSMI) were compared with those from the European Center for Medium Range Weather Forecasts (ECMWF) model. They agree over most ocean areas; both data sets capture the two annual cycles examined and the interannual anomalies during an El Niño‐Southern Oscillation episode. However, they show significant differences in the dry air masses over the eastern tropical‐subtropical oceans, particularly in the southern hemisphere. In these regions, comparisons with radiosonde data indicate that overestimation by the ECMWF model accounts for a large part of the differences. As a check on theWdifferences, surface level specific humidityQderived fromW, using a statistical relation, were compared withQfrom the ECMWF model. The differences inQwere found to be consistent with the differences inW, and indirectly validating theQ‐Wrelation. In bothWandQ, SSMI was able to discern clearly the equatorial extension of the tongues of dry air in the eastern tropical ocean, while both ECMWF and climatological fields have reduced spatial gradients and weaker intensity. One of the reasons for the ECMWF moisture overestimation is suggested to the poor resolution of Tiros‐N operational vertical sounder data which were assimilated into the ECM

ISSN:0148-0227    

DOI:10.1029/91JC02615

年代:1992

数据来源: WILEY