Robert E. Wilson

Observations of Low-Frequency Variability in Great South Bay and Relations to Atmospheric Forcing

Abstract Sea level and current data collected around Great South Bay, New York during December 1979 are examined in conjunction with atmospheric data for evidence of wind-forced, low frequency variability in the Bay and on the adjacent shelf. The subtidal sea level along the coast was found to be highly coherent from Sandy Hook to Montauk Point, with a single empirical mode accounted for more than 97% of the total variance. These coherent fluctuations were forced primarily by longshore winds (along 250–070°T) through the coastal Ekman effect. The sea level within the Bay exhibited large and spatially coherent subtidal fluctuations as a result of a strong coupling with the adjacent shelf. The characteristic volume exchange associated with this Bay-shelf coupling was an active simultaneous inflow or outflow through both ends of the Bay (with fluctuations in excess of 20 cm s−1) in response to the rise or fall of coastal sea level induced by longshore winds.

Coastal Countercurrent and Mesoscale Eddy Formation by Tidal Rectification near an Oceanic Cape

Abstract Cape St. James is an extensive triangular-shaped promontory located in a tidally energetic region at the southern tip of the Queen Charlotte Islands approximately 150 km off the mainland coast or British Columbia. Several years of oceanographic data collected in vicinity of the cape reveal a regional circulation characterized by a strong (0.50 m s−1) coastal current along the western continental margin and respective clockwise and counterclockwise rotating mesoscale baroclinic eddies to the west and south of the cape. The coastal current flows counter to the prevailing winds while the anticyclonic eddy to the west of the cape is a particularly intense feature that appears consistently in AVHRR imagery of the region. The structure of the mean flow, combined with the marked O(0.1 0 m s−1) low-frequency current variability at fortnightly and monthly tidal periods plus significant coherence at fortnightly periods between low-frequency currents and demodulated tidal flow, suggests that rectification o...

Modeling Influence of Stratification on Lateral Circulation in a Stratified Estuary

Abstract The dynamics of lateral circulation in the Passaic River estuary is examined in this modeling study. The pattern of lateral circulation varies significantly over a tidal cycle as a result of the temporal variation of stratification induced by tidal straining. During highly stratified ebb tides, the lateral circulation exhibits a vertical two-cell structure. Strong stratification suppresses vertical mixing in the deep channel, whereas the shoal above the halocline remains relatively well mixed. As a result, in the upper layer, the lateral asymmetry of vertical mixing produces denser water on the shoal and fresher water over the thalweg. This density gradient drives a circulation with surface currents directed toward the shoal, and the currents at the base of the pycnocline are directed toward the thalweg. In the lower layer, the lateral circulation tends to reduce the tilting of isopycnals and gradually diminishes at the end of the ebb tide. A lateral baroclinic pressure gradient is a dominant dri...

Increased Tidal Ranges Coinciding with Jamaica Bay Development Contribute to Marsh Flooding

Abstract Sea level rise has been identified as a possible factor contributing to marsh loss in Jamaica Bay, New York. However, Jamaica Bay has also experienced increases in tidal ranges throughout as a consequence of natural and engineering modifications that occurred in the bay during the first half of the twentieth century. The increases in the elevations of high tides are of the same order as the increase in regional sea level that occurred since the early 1900s. Marsh inundation in the bay is thus greater than noted to date and greater than that in adjacent bays where tidal ranges have been more stable. Changes in tidal hydrodynamics may be one more factor to consider in the list of potential causes of marsh loss in Jamaica Bay.

Effects of sill bathymetry, oscillating barotropic forcing and vertical mixing on estuary/ocean exchange

Intratidal and low-pass filtered fluctuations of density stratification in eastern Long Island Sound are apparently produced by both hydraulic effects and tidally induced vertical mixing. In order to evaluate the effects of bathymetry, oscillating barotropic forcing and vertical mixing on density stratification and on the exchange between estuarine and oceanic waters, a series of numerical experiments over a simplified sill bathymetry are performed. Hydraulic effects over a sill decrease exchange by 25% relative to flat bottom. There is a fundamental change in the nature of the flow from weak to strong barotropic forcing: during weak tidal forcing, sill exchange can be subject to hydraulic control and the intratidal stratification is determined by internal wave motion on the pycnocline; during strong forcing, the density field is determined by the gradient advection of the predominant barotropic tide, the hydraulic control is broken, and exchange is determined by the tidal prism. Vertical mixing tends to decrease exchange and to break the hydraulic control.

A PERSPECTIVE ON BOTTOM WATER TEMPERATURE ANOMALIES IN LONG ISLAND SOUND DURING THE 1999 LOBSTER MORTALITY EVENT

Abstract Analyses of time series data for bottom or near bottom temperatures for 50 stations distributed throughout Long Island Sound reveal distinctive features of the bottom water temperature history during the lobster mortality event of 1999. These include: temperatures that exceeded 23.5°C in shallow, well-mixed areas; markedly higher temperatures, in general, in those areas with water column depth <20 m; basin-averaged bottom temperatures that were the highest for the decade during the months of July and August; and a rapid increase in bottom temperatures in late August caused by the vertical mixing of warm surface waters during a strong wind event. Results indicate that anomalies in the local surface heat flux made an important contribution to bottom temperature anomalies.

The Physical Oceanography of Long Island Sound

Coastal ocean ecosystems are strongly influenced by circulation, tides, waves, and the rates of mixing of the water. Many shoreline communities are increasingly threatened by the same phenomena, most notably through flooding and coastal erosion. In this review we summarize the observations that have been acquired in LIS to describe and explain the magnitude and variability of these physical processes. We also comment on the status of our theoretical understanding of the links between them and some of their consequences. Analysis of available buoy and coastal wind observations suggests that the shear stress due to wind over LIS is under-predicted by a factor of between 2 and 3 if shore station winds are used to make the estimates. This difference is significant to both water quality and wave forecasting. We describe the magnitude of seasonal variations in wind and waves and use long-term records from coastal stations to show that there are decadal-scale variations in both wind speed and directions. Available wave data from two buoys suggest that the wave field is consistent with that predicted by fetch- limited wind forcing. Semi-diurnal tidal sea level variations and vertically averaged currents are well described by theoretical models, however, recent observations show high amplitude over tides in the western LIS that remain to be explained. The vertical structure of the tidal currents is much more complex and a closer examination of model predictions, particularly of the across Sound velocity components, should be conducted. The interaction of the vertical structure of tidal currents and the salinity and temperature distributions may lead to significant heat and salt transport vertically and horizontally. Observations of the mean density, temperature, and salt distributions and the mean circulation in LIS are qualitatively consistent with several models and we summarize the recent work. A more critical evaluation is now appropriate. We also discuss evidence of long-term trends. The role of shorter time-scale meteorological forcing and the bathymetry of the Sound on the structure and variability of the circulation is summarized using observations and simulation. Long-term observations of both hypoxia duration and hypoxia areal extent in western and west central Long Island Sound are analyzed to determine the directional response to wind forcing. We show that a substantial fraction of the inter-annual variability in area and duration can be explained by the directional statistics of wind. Using simulation, we demonstrate that the geometry of the basin and across isobath flow may be significant. The response of the Sound to severe storms is outlined and the technical developments in simulation and observation that are necessary to the improvement of model predictions are suggested.

The Icelandic Low as a Predictor of the Gulf Stream North Wall Position

AbstractThe Gulf Stream’s north wall east of Cape Hatteras marks the abrupt change in velocity and water properties between the slope sea to the north and the Gulf Stream itself. An index of the north wall position constructed by Taylor and Stephens, called Gulf Stream north wall (GSNW), is analyzed in terms of interannual changes in the Icelandic low (IL) pressure anomaly and longitudinal displacement. Sea surface temperature (SST) composites suggest that when IL pressure is anomalously low, there are lower temperatures in the Labrador Sea and south of the Grand Banks. Two years later, warm SST anomalies are seen over the Northern Recirculation Gyre and a northward shift in the GSNW occurs. Similar changes in SSTs occur during winters in which the IL is anomalously west, resulting in a northward displacement of the GSNW 3 years later. Although time lags of 2 and 3 years between the IL and the GSNW are used in the calculations, it is shown that lags with respect to each atmospheric variable are statistica...

Evidence for Directional Wind Response in Controlling Inter-annual Variations in Duration and Areal Extent of Summertime Hypoxia in Western Long Island Sound

Observations of summertime hypoxia duration and hypoxia areal extent in western Long Island Sound from 1991 to 2010 are correlated with local wind forcing. Results show a strong dependence on wind direction consistent with straining due to axial winds. The analysis of current data from moored ADCPs in the western Sound shows that the dominant mode of response is that of axial currents to axial winds. Estimates for the Wedderburn number (W) are relatively low ranging from 0.75 to 2.5 for prevailing winds, putting the western Sound in a regime more dominated by wind straining than by wind mixing. Estimates for the Kelvin number (Ke) range from approximately 1 to 6 in this diverging channel suggesting the importance of rotation; in the wide part of the basin, current observations show that significant vertically sheared lateral currents develop consistent with the tilting of planetary vortex lines by wind-driven axial current shear. Lateral straining, however, offsets longitudinal straining in the wide part of the basin supporting the hypothesis that simple longitudinal straining associated with axial winds exerts the most important control on the development summertime hypoxia in the constricted part of the western Sound.

Geologic Structure and Hydrodynamics of Egmont Channel: An Anomalous Inlet at the Mouth of Tampa Bay, Florida

Abstract High-resolution bathymetry surveys of Egmont Channel were conducted in 1999 and 2001 using a Kongsberg Simrad EM 3000 multibeam bathymetric system. These data were supplemented with other bathymetry data, seismic profiles, underwater scuba observations, and current velocity data, in order to investigate the geologic and hydrodynamic characteristics of Egmont Channel. Bounded to the north by a linear steep scarp (∼38°) and by a more gradual slope (>10°) to the south Egmont Channel is an asymmetric tidal inlet and the main shipping channel for Tampa Bay, Florida. The cross sectional area (17,964 m2) and the tidal prism (6×108 m3) for Egmont Channel were derived in this study. Currents measured at Egmont Deep and the Sunshine Skyway Bridge (∼11 km away) with Acoustic Doppler Current Profilers, have a high correlation (97%) indicating the current velocities at Sunshine Skyway Bridge can be used as a proxy for current velocities at Egmont Deep. Seismic profile data indicate that both the mouth of Tampa Bay and the bay proper contain many stratigraphically controlled depressions. Egmont Deep is located at one of these depressions. Bathymetry and seismic data indicate that the main ebb jet for Egmont Channel is deflected northward by a local stratigraphic high located at the north end of Egmont Key. The repeated high-resolution multibeam bathymetric surveys document sediment bedform migration. The bottom characteristics of the deep fluctuate due to the erosion and deposition of gravelwaves. Analysis of seismic data and SCUBA observations suggest that the most likely origin for Egmont Deep is a combination of erosion-resistant limestone strata interspersed with pockets of dissolution which is overlain by an irregular bed of mobile sediments. The strong tidal current scour maintains the depth of the feature and assures that any sediment that becomes incorporated in the deep is short-lived.