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Robert L. Molinari

Abstract

The effects of surface wind patterns and Loop Current position on surface distributions of latent and sensible heat fluxes in the eastern Gulf of Mexico are demonstrated. Mean monthly fields of thew fluxes computed from data collected during February 1975 and February 1976 are computed. The wind fields of February 1975 and 1976 are decomposed into two different modes, a north-wind mode associated with winter outbreaks of dry cold continental air masses and a trade-wind mode associated with advection from the south of warm moist maritime air. The distributions of sensible and latent heat fluxes are different for each mode with both beat heat fluxes considerably larger over the northern Gulf, in particular, during times of the northerlies. However, during these two months, trade-wind days are more numerous and the mean monthly flux patterns reflect this pre-ponderance. A simple model of the effect of extreme Loop Current configurations and the associated sea surface temperature distributions on air parcels traversing the Gulf below the inversion layer is presented. Total changes in air parcel temperature and specific humidity are shown to depend on the configuration of the Loop Current. Parcels which traverse the Gulf and cross the U.S. coastline between Louisina and Florida during the time of a deep northern Loop intrusion have 1.3°C higher temperatures and 1.0 g kg−1 greater specific humidities than parcels which crow the Gulf during a shallow Loop intrusion.

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Robert L. Molinari

The Global Ocean Observing System Center (GOOSC) at the National Oceanic and Atmospheric Administration's (NOAA) Atlantic Oceanographic and Meteorological Laboratory operates two global observing networks, a drifting buoy array, and a Voluntary Observing Ship network. The arrays provide in real time surface atmospheric and subsurface oceanographic data needed by NOAA weather and climate forecasters. The data are used in delayed mode to verify model simulations of the ocean and atmosphere, to provide in situ calibration/validation data for remote sensing observations, and to increase understanding of the dynamics of the ocean and atmosphere. The operational and research lessons learned in the operation of the GOOSC are reviewed. Operationally, it was learned that, because of costs, international participation is required to maintain global networks; data management methodology is a critical component of operations; and integrated observing systems using multiple platforms provide more accurate products. Scientifically, it was learned, for example, that accurate characterizations of the salinity field must be available in model simulations. As more data become available it is found that scales of important phenomena such as equatorial upwelling are smaller, and high-frequency signals can impact on the mean structure of the upper ocean. These findings must be considered when designing effective sampling strategies.

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Robert L. Molinari

Abstract

Temperature, salinity and Lagrangian current data collected during the summer of 1971 in the western Caribbean Sea are employed to evaluate the ageostrophic components of the flow in the formation region of the Yucatan Current. The ratio of tangential and centripetal accelerations to Coriolis acceleration for data averaged over 24 h periods remain less than 10% except in two areas. An anticyclonic turn, centered at 19°30′N, 86°W, has the largest centripetal accelerations, and in the region of Cozumel Island significant tangential accelerations occur. The large-scale accelerations and additional evidence support the hypothesis that inertial effects dominate in the formation of the Yucatan Current.

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Robert L. Molinari

Abstract

The curl of the annual mean wind stress is proposed as the forcing mechanism for the anticyclonic gyre observed in the Cayman Sea. A simple wind-driven model is presented to illustrate how a steady-state gyre in the Cayman Sea and another gyre in the western Gulf of Mexico can be spun-up by the wind. The model results also indicate that the exchange of mass between the two basins can be enhanced by the wind field. Temporal changes in the upper layer temperature structure of the Cayman Sea gyre are consistent, qualitatively, with changes predicted by a simple wind-forcing model. The same model does not appear valid in the western Gulf of Mexico.

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Robert L. Molinari

Abstract

An observational and numerical study of the circulation in the Cayman Sea is presented. Data taken in three different years suggest a common February to May circulation pattern. A well-developed current crosses 85W south of the Cayman Ridge. An anticyclonic eddy in the central basin appears to be a common feature of this season's circulation. Finally, the data from these cruises consistently portray significant accelerations occurring in the vicinity of Cozumel Island where the flow merges with the Yucatan Current. A different pattern is inferred from data collected in July and August. The north component of the flow over the western edge of the Cayman Ridge appears to determine the type of flow regime observed.

The numerical model is based upon predictive equations for the vorticities in a two-layer ocean on a beta-plane, and includes topographic, advective and friction effects. The model is driven by lateral input boundary conditions derived from an April–May 1968 observational study. The baroclinic western boundary current of the numerical model develops in response to eastern input boundary conditions, while the barotropic current is constrained to intensify and flow along the continental slopes.

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Ants Leetmaa and Robert L. Molinari

Abstract

A section along 110°W in the eastern Pacific from about 6°N to 6°S was occupied in March and June of 1981. Measurements consisted of absolute velocity profiles and CTD cuts. The large-scale structure of the subsurface zonal flow remained relatively invariant between these cruises. The Equatorial Undercurrent and North and South Equatorial Undercurrents appear as strong eastward flows, separated by westward currents. Away from the equator, comparison of currents estimated geostrophically with the direct observations indicate that the two techniques are in agreement within estimated errors except close to the surface. In the vicinity of the equator the geostrophic technique in general fails and the directly measured currents must be used. During March, within 3° of the equator from the surface to 700 m, the flow was more eastward by about 0.15 m s−1; than in June. In March, the flow and temperature fields were relatively symmetric about the equator. By June, strong asymmetries had developed. In the top 100 m, eastward flow extended from the Undercurrent to about 3°S. A strong, shallow westward flow was situated over and to the north of the Undercurrent. A shallow southward flow developed from 4°N to 2°S. Order-of-magnitude estimates suggest that this can advect westward momentum onto the equator in the top 50 m and modify the Undercurrent. Asymmetry also developed in the near-surface thermal field. In June, upwelling was primarily located south of the equator. This resulted in a cold band lying south of the equator at the core of which the flow was predominantly eastward. A strong meridional temperature gradient at the equator separated the colder water from warmer water to the north. Thee asymmetries develop presumably in response to the seasonal increase from March to June of the winds. Computations of zonal transports in various σt-classes in the near-surface layers suggest that the bulk of the Undercurrent water does not return west on the same density surfaces, but does so in the surface layers.

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Shari L. Vaughan and Robert L. Molinari

Abstract

A ten-year time series (1984–1993) of repeat hydrographic sections from offshore Abaco Island, the Bahamas (26.5°N), is used to define the mean and time dependent characteristics of the deep western boundary current (DWBC). The DWBC flow is divided into four vertical layers based on chlorofluorocarbon (CFC) concentration and formation regions (upper layer: CFC core, θ ∼ 3.9°–5.0°C; second layer: classical Labrador Sea Water, θ ∼ 3.2°–3.9°C; third layer: CFC minimum, θ ∼ 2.4°–3.2°C; deepest layer: CFC core, θ ∼ 1.85°–2.4°C). Time series analysis of mean layer properties and their anomalies showed that the temperature and salinity of each layer did not increase or decrease monotonically with time. Variations in temperature and salinity were characterized by 2–3-yr period oscillations. Variability between years is illustrated by subtracting repeat sections of temperature and salinity along levels of both constant pressure and constant potential density. To determine an original water mass modification that could be responsible for the observed variability in the section differences, an analytical method, which uses both types of differencing schemes, was applied to the DWBC data. Variability in the upper layer between 1987 and 1993 was shown to originate primarily from an increased salinity of the source waters for this layer. Variability in the second layer was shown to arise from a combination of cooling and salinification. Variability in the two deepest layers seemed to be almost entirely due to vertical movement of the isopycnals. Increases in potential temperature and salinity observed in a sublayer of the second layer defined by σ 1.5 ∼ 34.68–34.74 (classical Labrador Sea Water) from 1991 to 1993 was shown to be mainly the result of cooling. It is suggested that this cooling may have originally occurred in the central Labrador Sea during the period of active deep water renewal in the early 1970s.

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Kevin D. Leaman and Robert L. Molinari

Abstract

The effect of local topography in modifying the structure and variability of the Florida Current is examined using shipboard acoustic Doppler and PEGASUS acoustic current profiler data. PEGASUS absolute velocity data were obtained during 16 cruises in the Florida Current at 27°N as part of the Subtropical Atlantic Climate Studies (STACS) program. The ensemble average of all PEGASUS velocity data shows that the effect of the constriction imposed on the mean Florida Current by Little Bahama Bank can be detected up to 30 km into the Straits of Florida. A simple model is proposed to explain how this effect can produce the subsurface maximum of northward flow commonly observed in the eastern Straits.

PEGASUS and acoustic Doppler data obtained during the March 1984 STACS cruise are ~used to describe the temporal and spatial variability of the flow. It is shown that intermittent southward flow can exist in a band 10–15 km wide off Little Bahama Bank; one such event was detected during this cruise. The PEGASUS data suggest that these events are associated with meandering of the Florida Current. These results may explain earlier observations in satellite synthetic aperture radar images of small-scale vortices moving southward across the mouth of Northwest Providence Channel.

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John F. Festa and Robert L. Molinari

Abstract

A volunteer observing ship (VOS)-expendable bathythermograph (XBT) network has been proposed for the Atlantic Ocean to satisfy World Ocean Circulation Experiment (WOCE) objectives in the upper water column. These objectives include measuring changes in upper-layer temperature. An evaluation of the proposed WOCE XBT network to resolve variability in sea surface temperature (SST), temperature distribution at 150 m (T150), and average temperature of the upper 400 m (T400L) between 25°S and 35°N is performed. A sampling design study based on an optimum interpolation (OI) of the historical XBT dataset is used to construct uncertainty distributions for various XBT networks. The OI technique requires statistical representations of the variability (in the form of structure functions) of the three variables that are derived from the historical database. The structure functions and various sampling grids are used to construct uncertainty maps. Two seasons are used in the analysis of SST. In both seasons, uncertainties in mapped SST values for the proposed WOCE grid range from 0.3° to 0.4°C in regions of adequate data coverage. Errors are larger along the boundary. Uncertainties in the T150 fields are larger (0.5°−0.7°C) because of the smaller scales of spatial variability at depth. Errors in T400L range from 0.3° to 0.4°C. The effect of alternative observing strategies on the error maps are shown. Finally, error maps derived from the XBT network as it exists today (i.e., incomplete) are given. The maps indicate that monthly resolution is not available from the incomplete network.

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Robert L. Molinari and Dennis A. Mayer

Abstract

Current-meter observations obtained at two sites on the continental slope of the eastern Gulf of Mexico, at nominal positions of 29°N, 88°W (the Mobile site) and 27.5°N, 85.5°W (the Tampa site) are presented. Data were collected at three levels at Mobile (90,190 and 980 m) from July 1977 through August 1978 and at four levels at Tampa (150, 250, 550 and 950 m) from June 1978 through June 1979. At 90 and 190 m, the flow at Mobile was on the average to the east. Sustained periods of flow to the west were observed during the summer 1977 and spring 1978. During the periods of eastward flow, the wind was generally out of the north and during the periods of westward flow, the wind was out of the east. The flow at the top meter at Tampa was on the average to the west, in the same direction as the average wind. At both sites, the motions are perturbed by events associated with the Loop Current. These events make it difficult to define any seasonal variability in the upper layers. The flow at the bottom meters is strongly aligned with the bottom topography and lacks a strong seasonal signal. Little barotropic tidal energy was observed at either site. At both sites, maximum diurnal energy occurred near the local inertial frequency at the upper levels. These motions are probably induced by either cold-front passages or other atmospheric events. At the bottom meters, maximum diurnal-band energy occurred near the K1-tidal constituent. These motions are strongly time-dependent and they may be related to internal tides.

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