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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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Dennis A. Mayer and Jimmy C. Larsen

Abstract

A linear relationship between tidal height (sea level of tidal frequencies) and tidal transport near 27°N in the Straits of Florida is confirmed. Transport estimates from this relationship for the O1 and M2 constituents are compared with those computed from cable voltages across the Florida Current. These estimates are independent in that the weighted tidal height model (tidal-height transport relationship) was developed using collective sets of current meter and velocity profiler data obtained at different times of the year and in different locations. The cable voltages, however, were calibrated using a quasi-synoptic sectional integration of depth-averaged profiler data. Further, a means is suggested by which changes in the cable calibration can be detected.

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George R. Halliwell Jr. and Dennis A. Mayer

Abstract

Frequency response properties of North Atlantic (5°–57°N) sea surface temperature anomaly (Tsa) variability with periods of several months to 20 years are characterized using the Comprehensive Ocean-Atmosphere Data Set (COADS). Significant direct forcing of Tsa variability by the anomalous wind field (primarily through the resulting anomalous surface turbulent heat flux) is observed in the westerly wind and trade wind belts. To characterize properties of the large-scale climatic Tsa response to this forcing over the entire frequency band resolved, it is necessary to consider the dual role of anomalous surface heat flux as both the dominant local forcing mechanism and the dominant damping mechanism, the latter through a negative linear feedback (Newtonian relaxation). At frequencies where wind forcing is important, good agreement exists between the frequency response function estimated from data and the same function theoretically predicted by a simple stochastic forcing model where the locally forced response is damped by a negative linear feedback with a decay time scale of 3 mo. To make this comparison, the total anomalous surface heat flux represented by the standard bulk formula was decomposed into two components, one primarily representing the local wind forcing and the other primarily representing negative feedback damping. In the westerlies, wind forcing is effective over periods from several months to 8 yr, primarily 2–4 yr, and is ineffective at periods of 8–20 yr. These fluctuations are primarily forced in the western part of the basin then propagate to the east and northeast across the Atlantic at a characteristic speed of 6 km day−1. When time series of winter-only Tsa are analyzed, however, wind forcing of winter to winter Tsa variability remains significant at decadal and longer periods. In the trades, wind forcing is effective over periods from 8 mo to 13.3 yr, primarily 2-3 yr and 7–13.3 yr, and significant seasonal differences are not observed.

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Dennis A. Mayer and Robert H. Weisberg

Abstract

Using COADS data spanning 1947–1988, we describe the regional nature of the Atlantic Ocean wind-driven circulation between 30°8 and 60°N and its annual and interannual variability. The Sverdrup streamfunction defines the circulation gyres. Our focus is on three central gyres: the Northern Hemisphere anticyclonic subtropical gyre, the cyclonic tropical gyre just north of the equator, and the clockwise equatorial gyre straddling the equator. This rendition of the Sverdrup streamfunction, computed with constant drag coefficient and air density, compares favorably with that from other climatologies. In the Straits of Florida, analyses suggest that differences between the annual cycle in Sverdrup transport and observations may be due to regional winds farther north. In the tropical gyre, the Sverdrup circulation argues against a continuous western boundary current transporting water from the equatorial region into the Caribbean in boreal winter, bringing to question the mechanisms for the known interhemisphere and intergyre exchanges of heat and mass. A conceptual model is proposed involving two stages. First, the western boundary current closing the clockwise equatorial gyre is instrumental in storing heat and mass between the North Equatorial Countercurrent ridge and the North Equatorial Current trough in boreal summer. Transport farther north, across the tropical gyre and into the subtropical gyre, in boreal winter is then accomplished by Ekman transport, as the seasonal change in wind-stress torque deepens the thermocline, thus allowing for vortex stretching and northward Sverdrup transport over the region of warmest waters. Once in the subtropical gyre, the Ekman transport continues to be northward despite the fact that the Sverdrup transport reverses to be southward. Annual and interannual variability is addressed by examining the spectrum of curl and its regional distribution. Outside the tropics and the Sargasso Sea, interannual exceeds annual variability by at least a factor of 1.5. A pentadal analysis in the subtropical gyre indicates that wind-stress curl was not a major factor in the density structure differences reported between 1955–1959 and 1970–1974; hence, these require other explanations.

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Dennis A. Mayer, Kevin D. Leaman, and Thomas N. Lee

Abstract

A linear relationship exists between sea level and the north component of the depth-averaged tidal velocity in the Straits of Florida. This relationship is used as a one-dimensional model to predict barotropic tidal currents across the Straits near 27°N. Predictions are independent of the choice of a sea-level reference site between Key West and Patrick Air Force Base. The model, when compared with three sets of depth-averaged velocity obtained from current profilers, can account for at least 70% of the variance in the diurnal and semidiurnal tidal bands. The predicted diurnal tidal current is dominant and can account for more than 80% of the predicted tidal energy. Twice a year the one-dimensional model yields a maximum amplitude of 12 cm s−1 ± 3.5 cm s−1 (rms). This corresponds to a tidal transport of 5.1 × 106 m3 s−1 ± 1.5 × 106 m3 s−1 (5.1 Sv).

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Dennis A. Mayer, Harold O. Mofjeld, and Kevin D. Leaman

Abstract

On 10 August 1976 Hurricane Belle passed rapidly over the highly stratified shelf of the New York Bight. Records from Aanderaa current-meter moorings show that the response to the hurricane depended strongly on bathymetry. At deeper stations (∼70 m depth), intense, first-mode, internal near-inertial oscillations were generated at frequencies ∼1% less than the local inertial frequency. At shallower stations (∼50 m depth), only weak, heavily damped second-mode oscillations were observed in the current records, with no corresponding inertial signals in temperature. In the Hudson Shelf Valley, inertial motion occurred only near the surface. This was probably due to topographic effects. The divergence and curl of the wind stress contributed equally to the forcing. The response at the deeper stations is consistent with Geisler's (1970) theory for the open ocean in which a hurricane leaves a wake of internal-inertial oscillations if it travels faster than the internal phase speed and if its horizontal scale is comparable to the internal Rossby radius. The observed frequency shifts (subinertial motion) and observed relative vorticity are consistent with Mooer's (1975a) theory that relative and planetary vorticities combine to give an effective inertial frequency. Here it is suggested that lack of strong inertial motion at the shallower stations is due to a lack of resonance and the likelihood that frictional effects are more important in shallower water, resulting in a more heavily damped response.

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Dennis A. Mayer, Jyotika I. Virmani, and Robert H. Weisberg

Abstract

Current observations are compared from upward- and downward-looking acoustic Doppler current profilers (ADCPs) deployed on the West Florida Shelf (WFS). Despite regional differences, statistical analyses show good agreement between all sets of observations throughout the water column except in the upper few meters where all downward-looking ADCPs exhibit small, but significant, reduction in rms speed values. Evidence suggests that this reduction is mooring related. It is possible that the presence of near-surface bubbles caused by wave activity could bias the near-surface observations. Otherwise, either the upward- or downward-looking mooring systems produce equivalent observations with differences due to spatial variations.

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