Shelfbreak Circulation Research Articles

Evolution of the Freshwater Coastal Current at the Southern Tip of Greenland

AbstractShipboard hydrographic and velocity measurements collected in summer 2014 are used to study the evolution of the freshwater coastal current in southern Greenland as it encounters Cape Farewell. The velocity structure reveals that the coastal current maintains its identity as it flows around the cape and bifurcates such that most of the flow is diverted to the outer west Greenland shelf, while a small portion remains on the inner shelf. Taking into account this inner branch, the volume transport of the coastal current is conserved, but the freshwater transport decreases on the west side of Cape Farewell. A significant amount of freshwater appears to be transported off the shelf where the outer branch flows adjacent to the shelfbreak circulation. It is argued that the offshore transposition of the coastal current is caused by the flow following the isobaths as they bend offshore because of the widening of the shelf on the west side of Cape Farewell. An analysis of the potential vorticity shows that the subsequent seaward flux of freshwater can be enhanced by instabilities of the current. This set of circumstances provides a pathway for the freshest water originating from the Arctic, as well as runoff from the Greenland ice sheet, to be fluxed into the interior Labrador Sea where it could influence convection in the basin.

Underwater sound propagation variability over the Chukchi Sea continental slope

In the Canada Basin and Chukchi Sea regions, a vertical sound duct can be formed between the Pacific Summer Water Layer on the top and the Atlantic Water Layer on the bottom, providing an acoustic pathway connecting the deep basin and the shallow shelf over the Chukchi Sea continental slope. Previous studies have shown that the shelfbreak circulation (specifically upwelling), the sub-mesoscale eddies spun off the shelfbreak jet, and the ice coverage are the three major causes of the temporal and spatial variability of the Pacific Summer Water Layer in the region. In this paper, numerical simulations utilizing the Parabolic-Equation (PE) method are conducted to investigate the sound propagation variability over the Chukchi Sea shelfbreak and slope, along with an idealized ocean circulation model providing the fundamental basis of water-column fluctuations. The sound pressure sensitivity kernel derived from the acoustic propagator in the PE method is also used to provide physical insights into the sound pro...

Numerical analysis of underwater sound propagation over the Chukchi Sea shelfbreak

During the 2015 Canada Basin Acoustic Propagation Experiment (CANAPE) expedition on the southern edge of Canada basin (northern Chukchi Sea shelfbreak), shipboard suspended underwater sound sources were deployed to transmit acoustic signals to hydrophone arrays moored on the deep basin as well as the shallow shelf. In this paper, numerical simulations utilizing the Parabolic Equation method are conducted to provide physical insights into the variability of signals propagating over the shelfbreak and slope and recorded on a vertical array on the Chukchi Sea shelf. The numerical models simulate sound propagating over the slope via the Pacific Halocline duct, which is a water-borne vertical sound duct formed between the layers of Pacific summer water and Atlantic water. The source to receiver distance is about 130 km, and realistic variability is introduced in the numerical models. The previous studies reported in the literature have concluded that the shelfbreak circulation, specifically upwelling, and the sub-mesoscale eddies spun off the shelfbreak jet are the two major causes of the temporal and spatial variability of the Pacific Summer Water Layer over the slope in the region. The numerical simulation study also emphasize on the scattering and attenuation effects caused by ice cover and roughness. A preliminary data-model comparison is made and discussed in this paper. [Work supported by the Office of Naval Research.]

Numerical Investigation of the Middle Atlantic Bight Shelfbreak Frontal Circulation Using a High-Resolution Ocean Hindcast Model

Abstract A nested high-resolution ocean model is used to hindcast the Middle Atlantic Bight (MAB) shelfbreak circulation from December 2003 to June 2008. The model is driven by tidal harmonics, realistic atmospheric forcing, and dynamically consistent initial and open boundary conditions obtained from a large-scale circulation model. Simulated shelfbreak sea levels and tracer fields compare favorably with satellite observations and available in situ hydrographic climatology, demonstrating the utility of this nested ocean model for resolving the MAB shelfbreak circulation. The resulting time and space continuous hindcast solutions between January 2004 and December 2007 are used to describe the mean structures and temporal variations of the shelfbreak front and jet, the bottom boundary layer detachment, and the migration of the shelfbreak front. It is found that the shelfbreak jet and boundary convergence reach their maximum intensities in the spring, at which time the foot of the front also migrates to its farthest offshore position. Vorticity analyses reveal that the magnitude ratio of the mean relative vorticity between the seaward and the shoreward portions of the shelfbreak front is about 2:1. The shelfbreak ageostrophic circulation is largely controlled by the viscosity in the boundary layers and by the nonlinear advection in the flow interior. Simulated three-dimensional velocity and tracer fields are used to estimate the transport and heat and salt fluxes across the 200-m isobath. Within the model domain, the total cross-shelf water transport, the total eddy heat flux, and the total eddy salt flux are 0.035 ± 0.26 Sv (1 Sv ≡ 106 m3 s−1), 1.0 × 103 ± 4 × 104 W m−2, and 6.7 × 10−5 ± 7.0 × 10−4 kg m−2 s−1. The empirical orthogonal function (EOF) analysis on the 4-yr shelfbreak circulation hindcast solutions identifies two dominant modes. The first EOF mode accounts for 61% variance, confirming that the shelfbreak jet is a persistent year-round circulation feature. The second mode accounts for 13% variance, representing the baroclinic eddy passages across the shelf break.

The coastal-trapped wave paddle and open boundary conditions

The problem of the specification and interpretation of backward open boundary conditions (b.c.’s) is examined in the context of regional numerical models of wind-forced shelf circulation. (Backward here is opposite to the direction of coastal-trapped wave (CTW) propagation.) In particular, we examine the problem where the effect of the wind stress vanishes at a geographical origin. This origin ( y = 0) is the point at which CTWs are generated and its inclusion or otherwise, can have an important effect on the shelf circulation. Due to computational costs however, the origin may be chosen to lie well outside the site ( y = Y) of the backward open b.c. of the regional model. In order to simulate the wind-forcing over the omitted region [0, Y], previous studies have used coastal sea level data to drive a first mode CTW paddle at the backward boundary. Using linear theory, we show that if only one mode is present, then the sum of the (local) wind and paddle forced solutions obtained for y > Y is exact. That is, the total solution is identical to that obtained if the origin were included. Where higher modes are present, a first mode CTW paddle may lead to errors in the shelf break circulation and upwelling. However, using results for the CODE region, we show that these errors decrease with frequency and that relative to the first mode, may be 20% or less. A first mode paddle might be used in storm surge and models of coastal circulation. These results are used to discuss the nature and design of regional models of wind-forced shelf circulation.

A note on cross-shelf exchange in the northern Gulf of Alaska

The continental shelf of the Gulf of Alaska (GOA) is a complex system characterized by large freshwater runoff and strong winds. The GOA supports one of the world's richest ecosystems, including numerous species of fishes, marine mammals and sea birds. The mechanisms that provide nutrients to support this ecosystem are not well understood. The rivers and streams that provide freshwater to the shelf are low in nitrate, and the regional winds favor downwelling. High concentrations of nitrate are available in the deep basin of the GOA, but these must be introduced to the shelf in order to support the high productivity. We present evidence for cross-shelf exchange due to three different mechanisms. Episodes of downwelling relaxation result in a flux of saline, nutrient-rich water onto the shelf at depth. Eddies, formed in the northeastern GOA, propagate along the shelf-break influencing cross-shelf exchange by carrying shelf-origin water from the formation region into the basin and by interacting with the shelf-break circulation. Bathymetric steering in the many canyons that incise the GOA shelf results in flow into the canyons where strong tidal mixing results in cross-isobath movement of water properties.

Shelfbreak Circulation and Thermohaline Structure in the Northern South China Sea—Contrasting Spring Conditions in 2000 and 2001

Two intensive, high-resolution hydrographic surveys during April 2000 and May 2001 are used to characterize the thermohaline and current structure at the shelfbreak in the South China Sea. In 2000, a strong anticyclonic circulation was present in the northern portion of the South China Sea with strong onshore currents east of Dongsha Island. The flow became polarized along isobaths as it encountered shallow water, with northeastward flows of over 0.9 m/s along steep topography. The flow was driven by strong density contrasts between waters of the outer shelf and upper slope. Shelf water was both cooler and more fresh than the water offshore, which had salinities close to that of Kuroshio water. In contrast, the mean flow in the northern South China Sea was predominantly cyclonic in 2001. Flow over the slope was to the southwest at up to 0.2 m/s. The water mass properties of the outer shelf and upper slope were similar, so that there were not the strong cross-shelf density gradients present as in 2000. A potential difference between the water mass structure of the two years was the difference in cooling during the preceding winters. In December, 1999, unusually strong cooling may have resulted in cooler shelf waters relative to the following year. The ASIAEX study area may be a particularly sensitive region to both seasonal and interannual variability, as it is near a bifurcation point associated with the Kuroshio Intrusion into the South China Sea.

Current response at the shelf break to transient wind forcing

An analysis of current meter and hydrographic data collected from an array of moorings situated at the shelf break off Nova Scotia is presented. In particular, the focus is on the ocean response to moderate (10 to 20m s−1) transient winds of two or more days duration blowing parallel to the local bathymetry. Under these conditions, upwelling occurs at the shelf break from depths of 400 m and with peak vertical velocities of about 2 mm s−1. The upwelling appears to be confined to within 10 km of the slope. Acceleration, Coriolis, and pressure‐gradient terms are important for the along‐ and cross‐slope momentum balances. Local topographic variations contribute to the anomalously large bottom currents on the shoreward side of the shelf break. Both coastal sea level and the shelf break circulation respond coherently to synoptic wind forcing, but quantitative agreement with two‐layer shelf models is poor.

Shelf break circulation in the northern Gulf of Alaska

Current observations from a mooring on the continental shelf near the shelf break in the Gulf of Alaska, with supporting hydrographic and meteorological data, are discussed for the period 1976 to March 1977. The described features suggest strong influence by the cyclonic Alaska Gyre for the periods April–June 1976 and October 1976 to March 1977. From July–September 1976 there is evidence of current veering and rotation. It is hypothesized that these current fluctuations are eddies which are important in mixing processes across the shelf.

Shelf-Break Circulation, Fronts, and Physical Oceanography: East and West Coast Perspectives: ABSTRACT

Physical oceanographic processes present at the continental shelf-slope interfaces of both the east and west coasts of the United States are presented, compared, and contrasted. The Southeast Atlantic Bight shelf-break sector is principally dominated by the Gulf Stream and its associated cyclonic front. At the high-frequency end of the current spectrum, the semi-diurnal tide and inertial currents are dominant. Subinertial frequency motions are dominated by: Gulf Stream frontal meanders, warm core filaments, and cold core eddy ridges; wind forcing; thermohaline forces; and the effects of topography. The Mid-Atlantic Bight and Gulf of Maine outer shelf are shown to contain many of the dynamic elements of the southeastern shelf-slope, but the relative importances of various andom surface and offshore low-frequency driving forces change. West coast shelf-break processes differ principally from their east coast counterparts due to the absence of a Western Boundary Current. Principal circulation elements include various forms of continental shelf waves, wind-driven currents, tides, the California Current System and thermohaline effects. Both subtidal and supertidal frequency events are shown to be capable of initiating sediment motion and of suspending sediments, but lower frequency events are shown to be responsible for the bulk of sediment migration on the outer shelf and slope environs. Gulf Stream frontal phenomena and winter-time atmospheric storms figure prominently in providing physical mechanisms for sediment movement. The interplay of bottom topography with the physics of the outer continental margin is presented. Bottom features such as shoals, bumps, ridges, and canyons are shown to be regions of sediment erosion, deposition, and draping. Furthermore these features are shown to be causally related to upwelling and downwelling phenomena and to the deflection and scattering of waves and currents. End_of_Article - Last_Page 971------------