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Alexander E. Yankovsky

Abstract

A comprehensive dataset obtained in summer of 1996 on the New Jersey shelf off Atlantic City is analyzed to determine the pathway for cold water during a period of sustained upwelling. The data include shipboard CTD/ADCP surveys, time series from three across-shelf lines of moorings, and remote sensing. An upwelling event that occurred from 30 June through 10 July 1996 is studied. The event comprised three stages: first, the cold water was upwelled through the bottom layer; second, the onshore flow concentrated in the pycnocline; and third, mesoscale eddylike features developed. During the first upwelling-favorable wind pulse, which lasted approximately 2 days, a northward alongshelf current was generated and the pycnocline outcropped at the surface approximately 5 km off the coast. The northward current occupied the whole water column, and cold water was brought to the coast through the near-bottom layer, in the manner of Ekman dynamics. After that, an alongshelf pressure gradient was set, which was associated with the bending coastline approximately 100 km to the north at the corner formed by the intersection of the New Jersey coast and the southern coast of Long Island. The onshore flow was now maintained through the pycnocline, with maximum velocities at 10–12-m depth. At the same time, the temperature anomaly transport (pathway for colder water) was centered at 14–17-m depth, corresponding to the lower part of the pycnocline. This onshore transport was primarily balanced by alongshelf pressure gradient; the acceleration of alongshelf current was less important. Approximately 6 days after the onset of upwelling, mesoscale currents began to dominate the study area, establishing a three-dimensional flow field with spatially localized (in the alongshelf direction) onshore currents.

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Alexander E. Yankovsky
and
David C. Chapman

Abstract

A simple theory that predicts the vertical structure and offshore spreading of a localized buoyant inflow onto a continental shelf is formulated. The theory is based on two competing mechanisms that move the buoyant fluid offshore: 1) the radial spread of the lighter water over the ambient water, being deflected by the Coriolis force and producing an anticyclonic cyclostrophic plume, and 2) offshore transport of buoyant water in the frictional bottom boundary layer that moves the entire plume offshore while maintaining contact with the bottom. The surface expression of the cyclostrophic plume moves offshore a distance
y s gh0 υ  2 i gh0 υ  2 i 1/2f,
where g′ is reduced gravity based on the inflow density anomaly, h 0 is the inflow depth, υ i is the inflow velocity, and f is the Coriolis parameter. The plume remains attached to the bottom to a depth given by
h b i h0fg1/2
where L is the inflow width. Both scales are based solely on parameters of the buoyant inflow at its source.

There are three possible scenarios. 1) If the predicted h b is shallower than the inflow depth, then the bottom boundary layer does not transport buoyancy offshore, and a purely surface-advected plume forms, which extends offshore a minimum of more than four Rossby radii. 2) If the h b isobath is farther offshore than y s , then transport in the bottom boundary layer dominates and a purely bottom-advected plume forms, which is trapped along the h b isobath. 3) If the h b isobath is deeper than the inflow depth but shoreward of y s , then an intermediate plume forms in which the plume detaches from the bottom at h b and spreads offshore at the surface to y s .

The theory is tested using a primitive equation numerical model. All three plume types are reproduced with scales that agree well with the theory. The theory is compared to a number of observational examples. In all cases, the prediction of plume type is correct, and the length scales are consistent with the theory.

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Alexander E. Yankovsky
and
Richard W. Garvine

Abstract

Subinertial dynamics on the inner New Jersey shelf is examined using time series of the forcing agents (atmospheric pressure, wind stress, and Hudson River streamflow), adjusted sea level (ASL) along the southern part of the Mid-Atlantic Bight, and mooring data collected during the summer of 1996.

High-frequency (period 1–3 days) transient wind-driven events were evident both in ASL and alongshelf current data. ASL events propagated southward with remarkably high speed (∼10 m s−1) in the manner of free coastally trapped waves (CTW). However, these transients were forced by the wind events within the study domain: both ASL and alongshelf current fluctuations were coherent with the local alongshore wind stress. ASL amplitude substantially increased downshelf (southward). These transient flows propagated from the corner in the coastline formed by the southern Long Island and northern New Jersey coasts. This bend of the coastline created a discontinuity in the alongshore wind stress component that caused the generation of CTW pulses at this location.

During the period of observations, enhanced buoyant flows arrived at the site of the moorings. They were associated with increased Hudson River discharge. These buoyant flows and transient wind-driven events strongly interacted: transient wind-driven currents were dramatically amplified in the buoyant water while the buoyant water was spread offshore. Amplified transient currents were not associated with the enhanced vertical shear.

Lower-frequency wind forcing generated upwelling events with typical duration of 8–10 days. During the upwelling, temperature dropped through the whole water column, but the stratification remained significant (5°–6°C in 8–10 m of water). Even though upwelling-favorable winds dominated, record-mean currents in the upper layer were weak (2–5 cm s−1) due to the close competition between wind and buoyancy forcing.

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Alexander E. Yankovsky
and
George Voulgaris

Abstract

This study presents observations of a buoyant plume off Winyah Bay, South Carolina, which was formed under conditions of high freshwater discharge and upwelling-favorable wind forcing. Analysis of observations demonstrates that the response of the anticyclonic bulge formed by tidally modulated estuarine outflow to the light upwelling-favorable wind is more complex than the previously studied far-field response. The latter can be described by a slab-like model with mixing concentrating at the offshore edge of a buoyant layer. The observed plume depth increased from ~3 m near the mouth to 6 m at the offshore edge, with plume depth changing in a steplike fashion rather than continuously. CTD profiles near these steps revealed overturning indicative of vigorous mixing. Estimates of the gradient Richardson number confirmed the likelihood of mixing/entrainment not only at the offshore edge of the plume but also in the proximity of the observed steps. We hypothesize that these steps represent tidal fronts that undergo geostrophic adjustment and are advected offshore by the superimposed Ekman drift. Scaling analysis suggests that mixing and entrainment at the observed interior fronts can be enhanced by superposition of geostrophic and wind-induced shear.

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Ziming Ke
and
Alexander E. Yankovsky

Abstract

A full set of long waves trapped in the coastal ocean over a variable topography includes a zero (fundamental) mode propagating with the coast on its right (left) in the Northern (Southern) Hemisphere. This zero mode resembles a Kelvin wave at lower frequencies and an edge wave (Stokes mode) at higher frequencies. At the intermediate frequencies this mode becomes a hybrid Kelvin–edge wave (HKEW), as both rotational effects and the variable depth become important. Furthermore, the group velocity of this hybrid mode becomes very small or even zero depending on shelf width. It is found that in midlatitudes a zero group velocity occurs at semidiurnal (tidal) frequencies over wide (∼300 km), gently sloping shelves. This notion motivated numerical experiments using the Regional Ocean Modeling System in which the incident HKEW with a semidiurnal period propagates over a wide shelf and encounters a narrowing shelf so that the group velocity becomes zero at some alongshore location. The numerical experiments have demonstrated that the wave energy increases upstream of this location as a result of the energy flux convergence while farther downstream the wave amplitude is substantially reduced. Instead of propagating alongshore, the wave energy radiates offshore in the form of Poincaré modes. Thus, it is concluded that the shelf areas where the group velocity of the HKEW becomes zero are characterized by an increased tidal amplitude and (consequently) high tidal energy dissipation, and by offshore wave energy radiation. This behavior is qualitatively consistent with the dynamics of semidiurnal tides on wide shelves narrowing in the direction of tidal wave propagation, including the Patagonia shelf and the South China Sea.

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Alexander E. Yankovsky
and
Tianyi Zhang

Abstract

In boundary areas of the World Ocean, a semidiurnal tide propagates in the form of a Kelvin wave mode trapped by the coastline. Over wide continental shelves, the semidiurnal tide is no longer a pure Kelvin wave but attains features of a zero-mode edge wave. As a result, the wave structure and the alongshore energy flux concentrate over the continental shelf and slope topography and become very sensitive to the variations of shelf geometry. When a semidiurnal Kelvin wave encounters alongshore changes of the shelf width, its energy scatters into other wave modes, including internal waves. A particularly strong scattering occurs on wide shelves, where Kelvin wave structure undergoes significant modifications over short alongshore distances. These dynamics are studied using the Regional Ocean Modeling System (ROMS). This study found that when the alongshore energy flux in the Kelvin wave mode converges on the shelf, the offshore wave radiation occurs through barotropic waves, while for the divergent alongshore energy flux, internal waves are generated. Under favorable conditions, more than 10% of the incident barotropic Kelvin wave energy flux can be scattered into internal waves. For the surface-intensified stratification mostly the first internal mode is generated, while for the uniform with depth stratification, multiple internal modes are present in the form of an internal wave beam. A nondimensional internal wave scattering parameter is derived based on the theoretical properties of a Kelvin wave mode, bottom topography, and stratification.

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Alexander E. Yankovsky
,
Richard W. Garvine
, and
Andreas Münchow

Abstract

Shipboard hydrographic and acoustic Doppler current profiler surveys conducted in August 1996 on the New Jersey inner shelf revealed a buoyant intrusion advancing southward along the coast. This buoyant intrusion originated from the Hudson estuary more than 100 km upshelf and appeared as a bulge of less saline water with a sharp across-shelf frontal zone at its leading edge. During this time, the study area was also forced by a brief upwelling-favorable wind event opposing the direction of buoyant flow propagation.

The interaction of buoyancy and wind forcing generated a spatially variable velocity field. In particular, across-shelf currents were comparable to their alongshelf counterparts. Variability in the alongshelf direction occurred on the scales of the order of the baroclinic Rossby radius. Intensive across-shelf currents reached speeds of 20– 40 cm s−1 and appeared as spatially localized mesoscale flows with a width of O(10 km). They were generated at the leading edge of the buoyant intrusion and persisted over the period of observations, slowly propagating southward along with the buoyant flow. They were essentially baroclinic with strong vertical shear and were further amplified by the wind forcing.

The upwelling-favorable wind event also generated cyclonic circulation within the buoyant intrusion, which has not been observed before. Interaction of the opposing wind and buoyancy forcings deformed the pycnocline into a dome. This dome was effectively isolated from wind-induced turbulent mixing by overlying buoyant water. The adjustment of the velocity field to this density disturbance occurred geostrophically, even though the water depth was only 20–30 m and friction was important. Relative vorticity associated with this cyclonic flow was at least 0.3f.

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