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Peter Hamilton

Abstract

Direct current measurements using moored arrays have been made below 1000 m in the eastern, central and western Gulf of Mexico basin. The major low frequency velocity fluctuations in the lower 1000 to 2000 m of the water column in the three regions have the characteristics of topographic Rossby waves (TRWs). Spectral peaks are observed at periods of about 25 days and 40 to 100 days. Motions are highly coherent with depth. Variances increase toward the bottom despite the very weak stratification of the deep waters of the Gulf. Wave-lengths are about 150–250 km and phase propagation is offshore with energy propagation westward.

A group velocity of about 9 km day−1 could be directly estimated from significantly coherent signals between eastern and western arrays. This value is consistent with estimates derived from the dispersion relation and is higher than the westward translation speed of 3 to 6 km day−1 of the large anticyclonic eddies shed from the Loop Current. It appears that a major source of TRWs are the fluctuations of the Loop Current and there is evidence that the deep water fluctuations become progressively more decoupled from upper layer currents as the TRWs and warm eddies propagate into the western basin.

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Peter Hamilton

Abstract

Observations of deep currents, from a closely spaced array deployed at the base of the Sigsbee Escarpment, south of the Mississippi Delta, are dominated by energetic, short-period (∼10 days) topographic Rossby waves. Over a 2-yr interval, distinct trains of waves occurred, with differing characteristic periods, wavelengths, and horizontal energy distribution, usually initiated with a burst of large-amplitude current fluctuations that slowly decay over the next 2–3 months. Two of the wave trains were associated with the shedding and westward passage of major Loop Current anticyclones; however, one event occurred when upper-layer currents were quiescent. This latter wave train showed that bottom-trapped topographic wave motions can be traced to within 300 m of the surface in a 2000-m water column. Ray tracing showed that a likely source region of the short-period waves was the west side of an extended Loop Current. The data and ray paths suggest that, through refraction and reflection, the steep escarpment keeps the energetic waves confined to the deep water. These mechanisms help to explain why short-period fluctuations are not observed farther west in the central and western gulf basin.

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Peter Hamilton and Antoine Badan

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Subsurface jets, defined as having velocity maxima >40 cm s−1 at depths between 100 and 350 m, and being surrounded by much weaker near-surface currents, have been observed over the northwestern Gulf of Mexico continental slope. The observations were from an array of 14 moorings equipped with upward-looking 75-kHz ADCPs deployed at 450–500 m. A total of 10 jet events were observed in 18 ADCP years of velocity profile data, where these events were clearly not the result of downward-propagating inertial internal waves. The jets had durations from about 1 to 8 days and were usually associated with interactions between similarly sized cyclones and anticyclones over the slope or with the interaction of an eddy with upper-slope topography. The jets are associated with potential vorticity anomalies and their inferred length scales indicate that the dynamics depart from simple geostrophic balances. Observed anomalous density gradients present during the jets seem to involve the tilting of the vertical axis of the center of rotation of one or more of the interacting eddies.

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Barbara M. Hickey and Peter Hamilton

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The primary goal of this paper is to prescribe the practical limitations of the two-dimensional, baroclinic, time-dependent model of Hamilton and Rattray (1978) for the Oregon-Washington continental shelf. A unique time series of density sections at approximately 3-day intervals over a 5-week period makes the test possible. The Columbia River effluent was modeled by a surface freshwater flux that allowed the plume to become detached from the coast during periods of northerly winds. Energy levels and phases of fluctuations in longshore currents and sea level could be effectively predicted with a vertically averaged version of the two-dimensional model whose essential longshore dynamics are a balance between local acceleration (multiplied by shelf depth), surface friction and bottom friction. With respect to simulation of the density field, the two-dimensional model was effective in predicting offshore displacement of surface isopycnals and vertical displacement of isopycnals below the surface layer for periods as long as 15 days. Significant non-local effects were observed twice in 30 days. Some improvement in agreement for deeper isopycnals was obtained when barotropic non-local effects were included by driving the model with h/∂t, where h is the shelf depth and the depth-mean current. This function, which almost exactly reproduces the barotropic longshore current, is based on an observed time-scale separation between wind stress and current.

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Peter Hamilton and Maurice Rattray Jr.

Abstract

A numerical model of the upwelling circulation on a continental shelf is presented which employs f-plane dynamics. is continuously stratified, and assumes that all quantities are uniform alongshore with a local mass balance in the plane perpendicular to the coast at the seaward boundary. The model is an extension of the study by Allen (1973a) to include nonlinear effects, by the use of the complete conservation of density equation, variable shelf topography, and Richardson-number-dependent vertical eddy coefficients. A number of spinup experiments for a wind stress impulsively applied at t = 0 are discussed, to show that the width and strength of the coastal jet are dependent on the magnitude and form of the horizontal and vertical eddy coefficients as well as on details of the advective velocity field. Geostrophic shear in the longshore flow outside the coastal jet region, which may result in a poleward undercurrent, is only slightly altered by an upwelling event. Sloping shelf geometry intensifies the flow in the bottom Ekman layer and may produce secondary cross-shelf circulations in the interior, depending on the choices made for the eddy coefficients. A study of the spindown shows that persistence of the onshore flow in the bottom Ekman layer would lead to a general upslope of the density surfaces despite fluctuations in the wind.

The model is used to illustrate the current and density fields for a wind event off northwest Africa. Results are compared with hydrographic sections and Current meter data. The correspondence between the model and data is reasonable considering the simplifications made.

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Dong-Ping Wang, Lie-Yauw Oey, Tal Ezer, and Peter Hamilton

Abstract

This study evaluates a data-assimilated model simulation of near-surface circulation in DeSoto Canyon (DSC), Gulf of Mexico, with emphasis on analyzing moored current-meter observations and comparing them with satellite data and model results. The study period is for two years from April 1997 to April 1999. The model results are from a high-resolution Gulf of Mexico model forced by analyzed wind and surface heat flux. Two types of data are used to deduce near-surface circulation: moored current meters at 13 locations in the DSC, and satellite sea level anomaly. The moored currents are mapped through multivariate objective analysis to produce surface currents and surface geopotentials, against which satellite- and model-derived sea surface heights and geostrophic currents are compared. Coupled patterns between the observations, model results, and satellite data are obtained using the singular value decomposition (SVD) analysis. There are two dominant modes: a “single-eddy” mode, in which currents are concentrated at the foot of the canyon, and an “eddy-pair” mode, in which one eddy is at the foot of the canyon and the other, a counterrotating eddy, is over the head of canyon. Mode 1 appears to be associated with the mesoscale eddy traveling around the Loop Current crest and trough, and mode 2 is associated with the intrusion of Loop Current crest and trough over the west Florida shelf. The observed and model currents are in good agreement about the means and variances. The model currents also appear to be well constrained by the steep topography. However, the model velocity field contains only the first mode. The satellite-derived velocity field, on the other hand, contains both the first and second modes; though, the satellite field does not adequately resolve the velocity structures over the slope.

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Peter Hamilton, G. S. Fargion, and D. C. Biggs

Abstract

The paths of anticyclonic Loop Current eddies in the western Gulf of Mexico have been investigated using ARGOS-tracked drifters accompanied by hydrographic surveys. The analysis used orbit parameters derived from a least square fit of a translating ellipse kinematic model and showed that paths from four quite different eddies had a number of similar features. They are a general increase in rotational period over time, clockwise rotation of ellipse axes that slows with time and often becomes stationary in the far western Gulf, swirl velocities that decay quite slowly, and a tendency of the eddies to have low divergence. In three cases, 20- to 30-day oscillations of the orbit parameters were observed. Translation velocities of the orbits showed the characteristic stalls and sprints that have been previously observed. In two cases, stalls and deviations from solid body rotation could be attributed to the presence of vigorous lower continental slope cyclones situated to the northwest of the eddies in question. Comparison of relative vorticity, calculated from orbits and hydrography, showed reasonable agreement though deviations from solid body rotation were found in all cases. Vorticity also remained fairly constant for the 3–6-month periods investigated. Major perturbations were tentatively attributed to absorption of weaker, older Loop Current anticyclones in the western Gulf.

Statistics on eddy characteristics, derived from drifters, were compiled for 10 eddies between 1985 and 1995. The paths were separated into two, east and west of 94°W, corresponding to the deep western basin and under the influence of the steep western slope, respectively. The paths occupied a broad band of 2°–3° latitude in width in the center of the basin with a mean west-southwest trend. There were no apparent preferred paths either in the main basin or near the western slope where eddies were equally likely to move northward or southward along the boundary. Eddy paths also showed frequent occurrences of 20- to 30-day anticyclonic perturbations similar to that found from the individual analyses.

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Peter Hamilton, Jimmy C. Larsen, Kevin D. Leaman, Thomas N. Lee, and Evans Waddell

Abstract

Transports were calculated for four sections of the Florida Current from Key West to Jupiter, Florida, using a moored current-meter array and voltages from cross-channel telephone cables at the western and northern ends of the Straits of Florida. In addition, moored arrays were used to estimate transport through the Northwest Providence, Santaren, and Old Bahama Channels that connect the Florida Current to the southwestern part of the North Atlantic Ocean. Transport measurements were obtained for an 11-month period from December 1990 to November 1991. Mean transports of ∼25 Sv (1 Sv ≡ 106 m3 s−1) for the flow across the western ends of the straits, which agree quite well with recent estimates of 23.8 ± 1 Sv entering the Gulf of Mexico through the Yucatan Channel, were obtained from both the Key West to Havana cable and the moored array. This estimate is about 5 Sv less than the generally accepted transport through the northern end of the straits at 27°N. This difference was partially accounted for by inflows through the side channels with more transport from the Old Bahama than the Northwest Providence Channel. The variability in the southern part of the straits was larger than at 27°N and included large diversions of the Florida Current south of the Cay Sal Bank and into the Santaren Channel that were caused by large meanders of the flow. The variability of transport in the side channels contributed to the variability of the Florida Current and reduces the correlations of the transports at the ends of the straits. Therefore, the well-measured transport at 27°N is not an accurate indicator of the transport of the Loop Current out of the Gulf of Mexico.

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David M. Goodrich, William C. Boicourt, Peter Hamilton, and Donald W. Pritchard

Abstract

Multiyear continuous observations of velocity and salinity in the Chesapeake Bay indicate that wind-induced destratification occurs frequently from early fall through midspring over large areas of the estuary. Storm-driven breakdown of summer stratification was observed to occur near the autumnal equinox in two separate years. Surface cooling plays an important, though secondary, role in the fall destratification by reducing the vertical temperature gradient in the days prior to the mixing event. Large internal velocity shear precedes mixing events, suggesting a mechanism involving the generation of dynamic instability across the pycnocline. Destratification is shown to fundamentally alter the response of the velocity field to subsequent wind forcing; in stratified conditions, response is depth-dependent, while after mixing a depth-independent response is observed.

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Peter Hamilton, Amy Bower, Heather Furey, Robert Leben, and Paula Pérez-Brunius

Abstract

A set of float trajectories, deployed at 1500- and 2500-m depths throughout the deep Gulf of Mexico from 2011 to 2015, are analyzed for mesoscale processes under the Loop Current (LC). In the eastern basin, December 2012–June 2014 had >40 floats per month, which was of sufficient density to allow capturing detailed flow patterns of deep eddies and topographic Rossby waves (TRWs), while two LC eddies formed and separated. A northward advance of the LC front compresses the lower water column and generates an anticyclone. For an extended LC, baroclinic instability eddies (of both signs) develop under the southward-propagating large-scale meanders of the upper-layer jet, resulting in a transfer of eddy kinetic energy (EKE) to the lower layer. The increase in lower-layer EKE occurs only over a few months during meander activity and LC eddy detachment events, a relatively short interval compared with the LC intrusion cycle. Deep EKE of these eddies is dispersed to the west and northwest through radiating TRWs, of which examples were found to the west of the LC. Because of this radiation of EKE, the lower layer of the eastern basin becomes relatively quiescent, particularly in the northeastern basin, when the LC is retracted and a LC eddy has departed. A mean west-to-east, anticyclone–cyclone dipole flow under a mean LC was directly comparable to similar results from a previous moored LC array and also showed connections to an anticlockwise boundary current in the southeastern basin.

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