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John L. Wilkin

Abstract

A modeling study of summer ocean circulation on the inner shelf south of Cape Cod, Massachusetts, has been conducted. The influences of winds, air–sea heat fluxes, tides, and shelfwide circulation are all incorporated. The model reproduces recognized features of the regional summer circulation: warm temperatures and weak eastward flow in Nantucket Sound, cool tidally mixed waters and an associated anticyclonic flow encircling the Nantucket Shoals, and strong stratification south of Martha’s Vineyard. Comparisons with satellite and in situ observations show the model simulates the major features of the temperature patterns that develop during summer 2002. The evolution of the summer heat budget is characterized by three regimes: Nantucket Sound heats rapidly in June and then maintains warm temperatures with little net air–sea heat flux; tidal mixing on the Nantucket Shoals maintains perpetually cool ocean temperatures despite significant air–sea heating; and midshelf south of Martha’s Vineyard the surface waters warm steadily through July and August because of sustained air–sea heating with only modest cooling resulting from the mean circulation. In the environs of the Martha’s Vineyard Coastal Observatory tidal eddy heat flux emanating from Nantucket Sound produces a bowl of warm water trapped against the coast and significant local variability in the net role of advection in the heat budget. A suite of idealized simulations with forcing dynamics restricted, in turn, to only one of winds, tides, or shelfwide inflows shows that tidal dynamics dominate the regional circulation.

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John L. Wilkin and David C. Chapman

Abstract

An analytical solution is presented for the scattering of a free shelf wave incident upon a discontinuity in shelf width in a barotropic ocean. The discussion of solutions relying on backscattered free-waves with large wavenumbers which may not exist in a realistically stratified ocean is avoided by considering only the range of parameters over which energy transmission is nearly 100%. There is a substantial transfer of energy to modes other than that of the incident wave. The mode most readily excited is that which has the cross-shelf structure most closely coinciding with that of the incident wave. The resultant presence of multiple modes produces a strong alongshelf modulation in flow intensity and phase progression downstream of the scattering region which may affect the interpretation of shelf wave observations. A nondispersive long shelf wave pulse is shown to scatter into a train of pulses of differing mode number, each propagating at its own flee wave speed.

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John L. Wilkin and David C. Chapman

Abstract

No abstract available.

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John L. Wilkin and David C. Chapman

Abstract

The scattering of freely-propapting coastal-trapped waves (CTWs) by large variations in coastline and topography is studied using a numerical model which accomodates arbitrary density stratification, bathymetry and coastline. Particular attention is paid to the role of stratification which in moderate amounts can eliminate backscattered free-waves which occur. theoretically, in a barotropic ocean.

Numerical simulations using widening and narrowing shelf topographies show that the strength of the forward scattering into transmitted CTW modes is proportional to a topographic warp factor which estimates the severity of the topographic irregularities. The forward-scattering is further amplified by density stratification. Within the scattering region itself, the strengths of the scattered-wave-induced currents exhibit substantial variation over short spatial scales. There is generally a marked intensification of the flow within the scattering region, and rapid variations in phase. On narrowing shelves, the influence of the scattering can extend upstream into the region of uniform topography even when no freely-propagating backscattered waves exist.

A simulation is conducted of CTW scattering at a site on the East Coast of Australia where observations suggest the presence of scattered freely-propagating CTWs. The success of the model simulation in reproducing, features of observations supports the notion that realistic shelf geometries can matter significant levels of CTW energy, and that the watered waves can have an appreciable signal in current-meter observations made on the continental shelf. This suggests that along irregular coastlines, it is important to account for the possibility that CTW scattering may be occurring if oceanographic observations are to be interpreted correctly.

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Byoung-Ju Choi and John L. Wilkin

Abstract

The dispersal of the Hudson River plume in response to idealized wind forcing is studied using a three-dimensional model. The model domain includes the Hudson River and its estuary, with a realistic coastline and bottom topography of the New York Bight. Steady low river discharge typical of mean conditions and a high-discharge event representative of the spring freshet are considered. Without wind forcing the plume forms a southward coastally trapped current at low river discharge and a large recirculating bulge of low-salinity water during a high-discharge event. Winds affect the freshwater export through the mouth of the estuary, which is the trajectory the plume takes upon entering the waters of the Mid-Atlantic Bight inner shelf, and the rate at which freshwater drains downstream. The dispersal trajectory is also influenced by the particular geography of the coastline in the apex of the New York Bight. Northward wind causes offshore displacement of a previously formed coastally trapped plume and drives a new plume along the Long Island coast. Southward wind induces a strong coastal jet that efficiently drains freshwater to the south. Eastward wind aids freshwater export from the estuary and favors the accumulation of freshwater in the recirculating bulge outside the mouth of Raritan Bay. Westward wind delays freshwater export from Raritan Bay. The momentum balance of the modeled plume shows that buoyancy and wind forces largely determine the pattern of horizontal freshwater dispersal, including the spreading of freshwater over ambient, more saline water and the bulge formation.

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John L. Wilkin, James V. Mansbridge, and J. Stuart Godfrey

Abstract

Meridional heat transport in the North Pacific Ocean in a seasonally forced high-resolution global ocean general circulation model is compared to observations. At 24°N, annual mean heat transport in the model of 0.37×1011W is half the most recent direct estimate of 0.76±0.3×1015W from hydrographic data. The model value is low because the model ocean loses too little heat in the region of the Kuroshio Current Extension. The water ventilated in this region returns southward across 24°N at depth between 200 m and 500 m approximately 2°−4°C too warm. If the model surface temperature were relaxed to a temperature adjusted for the influence of persistent atmospheric cooling in this region, rather than relaxed to climatological sea surface temperature, the model heat transport would improve.

Assumptions inherent in estimating meridional heat transport from hydrographic sections are tested by examining the model. Rather than the abyssal circulation being steady, the model's deep western boundary currents vary seasonally to balance the seasonal cycle of Ekman transport, producing a larger seasonal variation in heat transport than is generally supposed for direct heat flux calculations. But the variability is such that there is no net contribution to the mean beat transport through a seasonal correlation between winds and surface temperature. The use of surface temperature observed during a single hydrographic section can seasonally bias an estimate of the wind-driven component of the beat transport, so a modification is proposed to the procedure by which compensation is made for seasonal variability in direct beat transport calculations. The most recent direct estimate was based on a springtime section, for which the model beat transport would be underestimated by about 0.05×1015W.

Interannual timescale correlations in the transport and temperature of the Kuroshio Current contribute a net southward transport of some 0.07×1015W. The role or simulated mesoscale eddies is minor.

Given the comparable order of the southward interannual heat transport and the northward seasonal bias, this present study does not suggest any significant revision to the latest direct heat transport estimate for 24°N in the Padfic.

Other features of the model general circulation are noted, including a Kuroshio Current transport that is stronger than observed and the persistence of a branch of the Kuroshio that does not separate at 35°N but continues close to the coast forming unrealistically deep mixed layers through intense surface cooling.

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Gregory P. Gerbi, Robert J. Chant, and John L. Wilkin

Abstract

This study examines the dynamics of a buoyant river plume in upwelling-favorable winds, concentrating on the time after separation from the coast. A set of idealized numerical simulations is used to examine the effects of breaking surface gravity waves on plume structure and cross-shore dynamics. Inclusion of a wave-breaking parameterization in the two-equation turbulence submodel causes the plume to be thicker and narrower, and to propagate offshore more slowly, than a plume in a simulation with no wave breaking. In simulations that include wave breaking, the plume has much smaller vertical gradients of salinity and velocity than in the simulation without breaking. This leads to decreased importance of shear dispersion in the plumes with wave breaking. Much of the widening rate of the plume is explained by divergent Ekman velocities at the off- and onshore edges. Some aspects of plume evolution in all cases are predicted well by a simple theory based on a critical Richardson number and an infinitely deep ocean. However, because the initial plume in these simulations is in contact with the sea floor in the inner shelf, some details are poorly predicted, especially around the time that the plume separates from the coast.

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Weifeng G. Zhang, John L. Wilkin, and Robert J. Chant

Abstract

This study investigates the dispersal of the Hudson River outflow across the New York Bight and the adjacent inner- through midshelf region. Regional Ocean Modeling System (ROMS) simulations were used to examine the mean momentum dynamics; the freshwater dispersal pathways relevant to local biogeochemical processes; and the contribution from wind, remotely forced along-shelf current, tides, and the topographic control of the Hudson River shelf valley. The modeled surface currents showed many similarities to the surface currents measured by high-frequency radar [the Coastal Ocean Dynamics Applications Radar (CODAR)]. Analysis shows that geostrophic balance and Ekman transport dominate the mean surface momentum balance, with most of the geostrophic flow resulting from the large-scale shelf circulation and the rest being locally generated. Subsurface circulation is driven principally by the remotely forced along-shelf current, with the exception of a riverward water intrusion in the Hudson River shelf valley. The following three pathways by which freshwater is dispersed across the shelf were identified: (i) along the New Jersey coast, (ii) along the Long Island coast, and (iii) by a midshelf offshore pathway. Time series of the depth-integrated freshwater transport show strong seasonality in dispersal patterns: the New Jersey pathway dominates the winter–spring seasons when winds are downwelling favorable, while the midshelf pathway dominates summer months when winds are upwelling favorable. A series of reduced physics simulations identifies that wind is the major force for the spreading of freshwater to the mid- and outer shelf, that remotely forced along-shelf currents significantly influence the ultimate fate of the freshwater, and that the Hudson River shelf valley has a modest dynamic effect on the freshwater spreading.

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Weifeng G. Zhang, John L. Wilkin, and Oscar M. E. Schofield

Abstract

The time scales on which river inflows disperse in the coastal ocean are relevant to a host of biogeochemical and environmental processes. These time scales are examined in a modeling study of the Hudson River plume on its entry to the New York Bight (NYB). Constituent-oriented age and residence-time theory is applied to compute two time scales: mean age, which is calculated from the ratio of two model tracers, and residence time, which is calculated using the adjoint of the tracer conservation equation.

Spatial and temporal variability associated with river discharge and wind is investigated. High river discharge lowers surface water age and shortens residence time in the apex of the NYB. Easterly winds increase surface water age and extend the duration waters along the Long Island coast remain in the NYB apex. Southerly winds increase age along the New Jersey coast but drive a decrease in age of offshore surface waters and prolong the time that surface waters close to the New Jersey coast stay in the NYB apex. Residence time along the Long Island coast is high in spring and summer because of the retention of water north of the Hudson shelf valley.

Patterns of modeled surface water age and an age proxy computed from the ratio of satellite-measured irradiance in two channels show qualitative agreement. A least squares fit gives a statistically significant empirical relationship between the band ratio and modeled mean age for NYB waters.

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Weifeng G. Zhang, Glen G. Gawarkiewicz, Dennis J. McGillicuddy Jr., and John L. Wilkin

Abstract

A two-dimensional cross-shelf model of the New England continental shelf and slope is used to investigate the mean cross-shelf and vertical circulation at the shelf break and their seasonal variation. The model temperature and salinity fields are nudged toward climatology. Annual and seasonal mean wind stresses are applied on the surface in separate equilibrium simulations. The along-shelf pressure gradient force associated with the along-shelf sea level tilt is tuned to match the modeled and observed depth-averaged along-shelf velocity. Steady-state model solutions show strong seasonal variation in along-shelf and cross-shelf velocity, with the strongest along-shelf jet and interior onshore flow in winter, consistent with observations. Along-shelf sea level tilt associated with the tuned along-shelf pressure gradient increases shoreward because of decreasing water depth. The along-shelf sea level tilt varies seasonally with the wind and is the strongest in winter and weakest in summer. A persistent upwelling is generated at the shelf break with a maximum strength of 2 m day−1 at 50-m depth in winter. The modeled shelfbreak upwelling differs from the traditional view in that most of the upwelled water is from the upper continental slope instead of from the shelf in the form of a detached bottom boundary layer.

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