Peter Hamilton

Circulation of slopewater

Abstract The recently conducted Mid-Atlantic Slope and Rise (MASAR) experiment yielded much new information on the structure and behavior of slopewater, the water mass occupying the upper “Slope Sea”, a narrow band of ocean between the Gulf Stream and the continental shelf from Cape Hatteras to the Grand Banks. The results of this experiment, combined with earlier evidence, have been used to construct a new empirical schema of slopewater circulation. Key features are: (1) inflow of Coastal Labrador Sea Water (CLSW) across the Grand Banks at the rate of 4 × 10 6 m 3 s −1 , and isopycnal advection from the Gulf Stream thermocline at the rate of 6 × 10 6 m 3 s −1 , the total draining eastward; (2) a closed cyclonic gyre in the western Slope Sea, transporting approximately 3 × 10 6 m 3 s −1 along the New Jersey coast southward; and (3) seasonal formation of a pycnostad by convective overturn and its flushing in approximately 6 months. Some of the CLSW inflow retroflects and turns eastward following entry into the Slope Sea, but a significant fraction flows westward along the coast and rounds the western gyre before draining eastward. The circulation just described reaches to an approximate depth of 500 m. Deeper layers move through the Slope Sea southwestward. The layers of the Gulf Stream thermocline in contact with slopewater along isopycnals encompass the Antarctic Intermediate Water (AAIW) core, as well as the nutrient maximum and oxygen minimum layers. Shoreward advection of these layers and seasonal overturn to 200 m establish conditions favoring productivity in the upper slopewater, as well as on the shelf.

Historical changes in the Columbia River Estuary

Historical changes in the hydrology, sedimentology, and physical oceanography of the Columbia River Estuary have been evaluated with a combination of statistical, cartographic, and numerical-modelling techniques. Comparison of data digitized from US Coast and Geodetic Survey bathymetric surveys conducted in the periods 1867–1875, 1926–1937, and 1949–1958 reveals that large changes in the morphology of the estuary have been caused by navigational improvements (jetties, dredged channels, and pile dikes) and by the diking and filling of much of the wetland area. Lesser changes are attributable to natural shoaling and erosion. There has been roughly a 15% decrease in tidal prism and a net accumulation of about 68 × 106m3 of sediment in the estuary. Large volumes of sediment have been eroded from the entrance region and deposited on the continental shelf and in the balance of the estuary, contributing to formation of new land. The bathymetric data indicate that, ignoring erosion at the entrance, 370 to 485 × 106m3 of sediment has been deposited in the estuary since 1868 at an average rate of about 0.5 cm y−1, roughly 5 times the rate at which sea level has fallen locally since the turn of the century. Riverflow data indicate that the seasonal flow cycle of the Columbia River has been significantly altered by regulation and diversion of water for irrigation. The greatest changes have occurred in the last thirty years. Flow variability over periods greater than a month has been significantly damped and the net discharge has been slightly reduced. These changes in riverflow are too recent to be reflected in the available in the available bathymetric data. Results from a laterally averaged, multiple-channel, two-dimensional numerical flow model (described in Hamilton, 1990) suggest that the changes in morphology and riverflow have reduced mixing, increased stratification, altered the response to fortnightly (neap-spring) changes in tidal forcing, and decreased the salinity intrusion length and the transport of salt into the estuary. The overall effects of human intervention in the physical processes of the Columbia River Estuary (i.e. decrease in freshwater inflow, tidal prism, and mixing; increase in flushing time and fine sediment deposition, and net accumulation of sediment) are qualitatively similar to those observed in less energetic and more obviously altered estuarine systems. A concurrent reduction in wetland habitats has resulted in an estimated 82% reduction in emergent plant production and a 15% reduction in benthic macroalgae production, a combined production loss of 51,675 metric tons of organic carbon per year. This has been at least partially compensated by a large increase in the supply of riverine detritus derived from freshwater phytoplankton primary production. Comparison of modern and estimated preregulation organic carbon budgets for the estuary indicates a shift from a food web based on comparatively refractory macrodetritus derived from emergent vegetation to one involving more labile microdetritus derived from allochthonous phytoplankton. The shift has been driven by human-induced changes to the physical environment of the estuary. While this is a relatively comprehensive study of historical physical changes, it is incomplete in that the sediment budget is still uncertain. More precise quantification of the modern estuarine sediment budget will require both a better understanding of the fluvial input and dredging export terms and a sediment tranport model designed to explain historical changes in the sediment budget. Oceanographic studies to better determine the mechanisms leading to the formation of the turbidity maximum are also needed. The combination of cartography and modelling used in this study should be applicable in other systems where large changes in morphology have occurred in historical time.

Deep Currents in the Gulf of Mexico

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...

Transport, potential vorticity, and current/temperature structure across Northwest Providence and Santaren Channels and the Florida Current off Cay Sal Bank

Currents and temperatures were measured using Pegasus current profilers across Northwest Providence and Santaren Channels and across the Florida Current off Cay Sal Bank during four cruises from November 1990 to September 1991. On average, Northwest Providence (1.2 Sv) and Santaren (1.8 Sv) contribute about 3 Sv to the total Florida Current transport farther north (e.g., 27°N). Partitioning of transport into temperature layers shows that about one-half of this transport is of “18°C” water (17°C–19.5°C); this can account for all of the “excess” 18°C water observed in previous experiments. This excess is thought to be injected into the 18°C layer in its region of formation in the northwestern North Atlantic Ocean. Due to its large thickness, potential vorticities in this layer in its area of formation are very low. In our data, lowest potential vorticities in this layer are found on the northern end of Northwest Providence Channel and are comparable to those observed on the eastern side of the Florida Current at 27°N. On average a low-potential-vorticity 18°C layer was not found in the Florida Current off Cay Sal Bank.

Wind-Induced Destratification in Chesapeake Bay

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.

A Numerical Model of the Depth-Dependent, Wind-Driven Upwelling Circulation on a Continental shelf

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 intensif...

Loop Current Eddy Paths in the Western Gulf of Mexico

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...

Observations of high speed deep currents in the northern Gulf of Mexico

Recent measurements in water depths of 2000 m in the vicinity of the Sigsbee Escarpment, south of New Orleans, show the presence of recurring high speed current episodes, some of which had velocities greater than 85 cm/s only 10 m above the local bottom. In the Gulf of Mexico, currents above about 1000 m are not locally coupled with the relatively depth-independent currents in the deeper layers. Topographic Rossby waves, with periods of approximately 10 to 14 days, appear to control the dynamics of the lower water-column.

Lower continental slope cyclonic eddies in the central Gulf of Mexico

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-slope directions.

Near-Surface Currents in DeSoto Canyon (1997-99): Comparison of Current Meters, Satellite Observation, and Model Simulation

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.