Alexander E. Yankovsky

The cyclonic turning and propagation of buoyant coastal discharge along the shelf

Buoyant coastal discharge typically forms a current flowing along the coast in the direction of Kelvin wave propagation (hereinafter referred to as the downstream direction). In this paper the opposite, upstream penetration of buoyancy-driven current is studied using numerical modeling. Previous models of coastal buoyancy-driven currents repeatedly predicted the upstream spreading while in the field this feature was not commonly observed. The mechanism responsible for the propagation of buoyant flow along the coast upstream from its source is identified as follows. In many cases, the boundary conditions applied for buoyant discharge oversimplify the actual dynamics at the mouth blocking landward flow in the lower layer. This generates a strong cyclonic vorticity disturbance with corresponding upstream turning of buoyant flow at the source. This process initiates the upstream spreading of buoyant flow. Alternative boundary condition maintaining constant net transport but allowing baroclinic adjustment of buoyant inflow is formulated and shown to reduce the generation of cyclonic vorticity at the mouth. The upstream propagation is further enhanced by the vertical mixing. The buoyant water forms an anticyclonic bulge at the river or estuary mouth. While spreading around the center of this anticyclone, the fresher water gradually becomes saltier due to vertical mixing/diffusion. As a result, the pool of lightest water does not coincide with the center of the anticyclone (in the sense of integral streamfunction) but tends to occupy the upstream and inshore segment of the bulge where the buoyant water comes first. This sets a new center for the anticyclonic turning at the surface and promotes the upstream shift of the anticyclonic bulge. This process sustains continuous growth of the buoyant plume upstream. It is shown that the upstream ambient current does not produce a similar effect. Instead, buoyant flow periodically sheds anticyclones advected upstream with the mean current. Under certain conditions, upstream spreading is also possible in nature. For example, Beardsley et al. (1985) reported substantial upstream penetration of the Changjiang River discharge in the East China Sea during the period of high runoff.

Inner shelf circulation patterns and nearshore flow reversal under downwelling and stratified conditions off a curved coastline

[1] The role of a curved coastline and associated bathymetry in the development of downwelling circulation in a stratified inner shelf is examined through a number of numerical experiments. Different scenarios include constant versus variable wind-forcing and variations in bottom friction. The three-dimensional response of the shelf within the domain (embayment enclosed by capes) is associated with the generation of a velocity/ pycnocline disturbance at the upstream cape and its subsequent downstream advection. This disturbance is more pronounced under variable wind conditions. Its downstream advection through the bay exhibits different patterns depending on the competition between inertia and bottom friction near the cape. When inertia dominates, the disturbance separates from the cape and travels downwind with an enhanced downstream flow offshore and a countercurrent inshore. The separation occurs at a low Rossby number (Ro � 0.15), which is attributed to the positive curvature of the coastline forming the cape. When friction dominates, the advection path is constrained along the coastline, resulting in an alongshore temperature gradient and a transient thermal front running almost perpendicular to the coast/isobaths. Simulations with spatially variable bottom friction, with higher friction toward the coast, result in the generation of eddy-like features. The numerical results are in agreement with both observations and surface temperature imagery from Long Bay, South Carolina, an embayment enclosed by two capes, and emphasize the role that coastline and associated shelf morphology can play in enhancing cross-shelf transport and exchange.

Anticyclonic eddies trapped on the continental shelf by topographic irregularities

Nonlinear effects produced during the scattering of a barotropic shelf wave (BSW) by a spatially varying mean current are studied using a primitive equation numerical model. Both the BSW phase and the mean current propagate in the same (positive) direction along shelf/slope topography which is uniform everywhere except for a localized topographic irregularity, e.g., a submarine canyon. The mean current is specified at the upstream boundary and adjusts to the topography, closely following isobaths through the model domain. The incident BSW signal is then introduced at the upstream boundary either as a harmonic wave or as a pulse of finite duration. The BSW signal scatters its energy into other available wave modes when it encounters the topographic irregularity. The scattered wave field is dominated by evanescent modes which are trapped at the topographic irregularity and appear as intense mesoscale flows between the coast and the mean current. Nonlinear dynamics transform these large-amplitude evanescent modes into persistent eddy-like features on the shelf. The nonlinear interaction is much stronger when the current on the shelf associated with the BSW is opposite to the mean current direction (i.e., negative), so anticyclonic eddies are preferentially generated at the topographic irregularity. For a harmonic BSW, an anticyclonic eddy periodically appears when the negative current phase passes and disappears when the positive current phase passes. A BSW pulse with negative velocity at the coast produces a strong anticyclonic eddy which persists, after the pulse has passed, for a time period substantially longer than the pulse duration. A pulse with positive velocity at the coast does not generate any persistent features on the shelf. The anticyclonic eddies produce mass exchange between the shelf and the mean current and could contribute significantly to cross-shelf exchange on continental shelves.

The impact of ambient stratification on freshwater transport in a river plume

The purpose of this study is to delineate the influence of stratified ambient water on river plume structure and freshwater transport in the plume. The Regional Ocean Modeling System (ROMS) is applied to conduct a parameter study, where the inflow salinity anomaly, ambient stratification and the bottom slope are varied systematically. Ambient water is thermally stratified. Temperature is set uniform in the top 15 m layer and then decreases linearly, while in the non-stratified case temperature remains constant at its surface value. The results show that in all non-stratified cases an anticyclonic bulge at the mouth and a parallel coastal current further downstream form. On the other hand, under stratified conditions, an inflow with a lower salinity anomaly causes the formation of a frontal disturbance at an earlier stage downstream of the plume bulge, with more eddies developing later in time along the density front. These eddies grow rapidly in both offshore and downstream directions and move with the downstream current. In extreme cases, they spread 100 km offshore by the end of the model run (60 days), which is triple the offshore extension in non-stratified cases. Under the same fresh water input, more eddies develop either when the inflow salinity anomaly is smaller or when the stratification is stronger. Eddies form later, grow at a slower rate and are less developed offshore with a gentler slope. All the eddy forming plumes are bottom advected plumes. Under stratified conditions the density front detaches from the bottom at a shallower depth but spreads further offshore at the surface. Reconstructed alongshore velocities based on geostrophic balance prove that the different distributions of isopycnals cause differences in alongshore velocity fields. Higher salinity anomaly inflows form surface advected plumes which perform similarly to non-stratified cases, without eddy formation. Freshwater fluxes are calculated in both downstream and offshore directions to quantify the difference between stratified and non-stratified cases under low salinity anomaly conditions. The rapid growth of eddies traps a large amount of freshwater which substantially (up to 35%) reduces the downstream freshwater flux compared to the non-stratified case. As eddies pass through the fixed transect, the downstream freshwater flux fluctuates up to 30% of a corresponding value in the non-stratified case. The offshore freshwater flux estimated 30 km from the coast is several times higher due to the presence of frontal eddies when compared with corresponding non-stratified case. Energy transfer diagnostics indicate that frontal eddies are likely to be produced through the barotropic instability of the buoyancy-driven current.

Long-Wave Response of the West Florida Shelf to the Landfall of Hurricane Wilma, October 2005

Abstract Direct observations of the storm surge and the subsequent long-wave response induced by Hurricane Wilmas landfall on the West Coast of Florida on 24 October 2005 are presented. The data set consists of weeklong time series of storm surge and barometric pressure measured by the U.S. Geological Survey (USGS) Florida Integrated Science Center. The survey area spanned more than 100 km alongshore from the landfall site northward. The USGS data were augmented with measurements at the National Oceanic and Atmospheric Administration tide gauge and Coastal Marine Automated Network (CMAN) stations. At the time of Wilmas arrival, the storm surge was minimal to the north of landfall site but was significant in the southern sector. Subsequently, it evolved into an edge wave pulse propagating northward as Wilma moved inland and the surge was no longer sustained by the wind forcing. The height of the wave pulse exceeded 1.5 m in detided sea level data. However, its magnitude was somewhat obscured in direct surge measurements because the wave pulse propagated during the low tide. The duration of this wave pulse was approximately 6 hours. The propagation speed of the wave front was ∼25 m s−1, while the pulse crest traveled at a lower speed of ∼10 m s−1, which indicates the dispersion effects. A relatively low phase speed suggests that the wave energy was trapped nearshore, in the water depth of 10–20 m. The wave pulse was followed by a train of weaker wavelike undulations, also propagating northward. The edge wave pulse seemed to be attenuated by a complex topography in the vicinity of Sanibel Island–Pine Island.