摘要:

A numerical model is developed to simulate currents generated by hurricanes on the outer continental shelf and slope. Emphasis is on the mixed‐layer response within a few hours of storm passage; however, some attention is given to the lower layer and shelf wave responses. The model is based on a layered, explicit, finite difference formulation using the nonlinear primitive equations including conservation of heat. The problem of topography intersecting the model layer is resolved by introducing artificial steps of the order of 100 m where the layer intersects the slope. Model comparisons are presented for three Gulf of Mexico hurricanes using a 0.2° grid. For two of the storms, the model reproduces better than 80% of the observed velocity variance with correlation coefficients of greater than 0.8 for the mixed layer. Discrepancies in the comparisons are traced to unresolved local topography and nonstorm forcing such as warm‐core rings. Further model simulations reveal that (1) substantial shelf waves were generated with phase speeds of 4 to 10 m s−1, (2) the response is primarily baroclinic even in water as shallow as 200 m, (3) an entrainment law which scales with the velocity difference between the mixed layer and upper thermocline yields markedly better comparisons than one which scales with the wind stress, and (4) deviations from a straight‐line storm path can significantly alter the

ISSN:0148-0227    

DOI:10.1029/JC094iC09p12513

年代:1989

数据来源: WILEY

摘要:

A numerical model described and verified in part 1 of this two‐part series (Cooper and Thompson, this issue) is applied to study the sensitivity of hurricane‐generated currents on the outer shelf and slope. Numerical experiments are performed in a simple basin with a straight shelf. The sensitivity of the response to changes in storm parameters, direction of storm approach, and topography is quantified. Response is measured in terms of the mixed‐layer velocity and depth at sites along the storm track. Results reveal the most important factors are (in decreasing order) wind speed, storm translation speed, direction of storm approach, asymmetry in the wind field, entrainment parameterization, and advection at slower storm translation speeds. Response is largely insensitive (less than 10%) to radius of maximum wind, shelf and slope configuration, bottom friction, atmospheric pressure gradients, and further reductions in the model grid size. For a storm approaching cross shelf, the response is primarily baroclinic (greater than 90%) and only weakly dependent (less than 10%) on the water depth at the

ISSN:0148-0227    

DOI:10.1029/JC094iC09p12540

年代:1989

数据来源: WILEY

摘要:

A broad (about 40 km wide) cold filament observed off Point Arena in advanced very high resolution radiometer images at the end of June 1986 evolved into a narrow (10–20 km wide) cold filament embedded in a large‐scale (about 200 km alongshore) cool anomaly by mid July. The cool anomaly was part of a much larger‐scale anomaly, stretching from north of Cape Mendocino to south of San Francisco, in which several cold filaments were embedded. The filament off Point Arena, surveyed by ship from July 7 to 19, 1986, extended offshore around the edge of a cyclonic center. The horizontal temperature gradient was sharpest (up to 1.5°C/2.2 km) across the southern edge of the filament, and a salinity front was located within the cold filament; temperature and salinity fronts were not coincident. The surface salinity was high (33.2 psu (practical salinity unit)) in the cyclonic center and decreased across the cold filament to about 32.6 psu. The entire cool anomaly was denser than surrounding water, and there was a sharp density front (1 sigma‐tunit/10 km) at its northern edge. At the inner (cyclonic) edge of the filament the temperature and salinity variations were density compensating. The cold filament was advected offshore by a jet which was symmetric about its axis, located about 10 km north of the cold filament core. The surface manifestation of the cold filament was highly variable in space and time: the location of the filament core changed by about 5 km in 14 hours along one transect; the velocity field was less variable. Several narrow (10 km wide) water mass anomalies were evident, including a salinity subduction zone in the northern segment of the cold filament. Thermohaline intrusions 50–100 m thick may have been caused by complex frontal interactions upstream of the anomaly and offshore in a region of jet confluence. From a second survey (July 27 to August 5, 1986) the offshore jet had reoriented to alongshore, possibly related to an observed increase in the poleward flow over the continental slope. Prior to the second survey there were changes in the nearshore wind stress and the large‐scale wind

ISSN:0148-0227    

DOI:10.1029/JC094iC09p12555

年代:1989

数据来源: WILEY

摘要:

A long‐term moored array was maintained over the northern California continental shelf from April 1981 to April 1983 as part of the Coastal Ocean Dynamics Experiment to determine the seasonal differences in the current and water temperature characteristics. The array consisted of two midshelf moorings and one upper slope mooring from April 1981 through July 1982 and one midshelf mooring from August 1982 to April 1983. The two summers were characterized by southeastward mean wind stresses (upwelling favorable), cold water, isotherms sloping upward toward the coast, and a sheared equatorward mean shelf current. The intervening fall and winter were characterized by a weak mean wind stress, relatively warm shelf water, level isotherms, and a more barotropic poleward mean shelf current. In contrast to the differences in the characteristics of the means many of the characteristics of the current and temperature variability are the same for the summer and winter. The vertical structure of the largest current empirical orthogonal functions (EOFs) for each of the shelf moorings, which account for more than 75% of the subtidal variance, do not vary with season. The current is vertically sheared, oriented 15°–20°clockwise relative to local isobaths near the surface and along isobaths near the bottom. Currents at the two shelf moorings (alongshore separation 29–37 km) are correlated with each other and with the local wind stress, and these relationships do not exhibit a seasonal variation. Denser arrays of wind and current sensors during the two summers indicated that there were strong, persistent variations in the wind field over relatively small scales (tens of kilometers) with corresponding variations in the shelf current field during the summer of 1981 but not during the summer of 1982. Over the slope, the largest EOFs account for 56–72% of the current variance and have a vertical structure which is relatively uniform with depth during the summer, fall, and winter of 1981 but is strongly sheared in the upper 100 m during the summer of 1982. Current observations over the slope are not correlated with either the shelf currents or the local wind stress during any season. In contrast, the temperature variability, as described by EOFs, does exhibit a seasonal variation in vertical structure, with the variability concentrated in the upper 20–30 m in summer and more uniform with depth in winter. The near‐surface temperature variability over the slope is correlated with the shelf temperature variability. Estimates of the cross‐shelf heat flux due to the wind‐driven Ekman transport accounts for a large part of the shelf temperature variability. Observations from the midshelf mooring maintained through the fall and winter of 1982–1983 showed evidence of the 1982–1983 El Niño with persistent onshore and poleward currents and water temperature

ISSN:0148-0227    

DOI:10.1029/JC094iC09p12571

年代:1989

数据来源: WILEY

摘要:

Two recent cruises of the R/VShikmonaprovided detailed coverage of a large portion of the Levantine Basin of the eastern Mediterranean. The first cruise was conducted in October 1985, corresponding to late summer, and the second was in March 1986, corresponding to the following winter. An objective analysis of the combined conductivity‐temperature‐depth and expendable bathythermograph data from these cruises reveals a variety of warm core and cold core mesoscale eddies during both seasons. Some of these features are unique to each cruise while others appear to retain their identities from summer to winter. Of the latter, there are two intense anticyclonic eddies. One of these appears to have formed off the coast of Egypt as a meander of the North African current. The other is a rather persistent quasi‐stationary feature to the southeast of Cyprus which has been seen near this location for over 3 years. Lack of adequate satellite images for the period and region covered led us to use water mass properties to identify and track the development of these two eddies. Below the layer of seasonal influence it was found that each eddy had a unique signature in its TS diagram which changed very little over the 6 months covered by the two cruises and the period between them. The water mass properties also provide additional evidence as to the possible formation regions for these two e

ISSN:0148-0227    

DOI:10.1029/JC094iC09p12593

年代:1989

数据来源: WILEY

摘要:

Moored current observations were obtained from a small region of Shelikof Strait during 1986–1987. Results from two sites11 km, however, correlations were quite weak. A new feature, intense 13‐day spectral peaks of narrow horizontal and vertical scale, was also revealed. The feature appears to be a baroclinic, residual flow linked to tidal curre

ISSN:0148-0227    

DOI:10.1029/JC094iC09p12603

年代:1989

数据来源: WILEY

摘要:

A simple numerical model is developed to study the three‐dimensional structure of the steady circulation off Newfoundland. The model uses linear dynamics, a vertical eddy viscosity to parameterize the vertical mixing, and a linear parameterization of bottom friction in which the bottom stress is related to the bottom velocity. Realistic geometry and bottom topography are included. The results presented here are for an ocean of uniform density. The model is driven by specifying the sea level along the northern boundary, the inflow corresponding to the inshore and offshore branches of the Labrador Current. The model can reproduce the circulation pattern found by Greenberg and Petrie (1988), who used a vertically integrated nonlinear primitive equation model. It also enables dynamical interpretation to be given in terms of flow along f/h contours and frictional “diffusion” across these contours. In this connection, we investigate the dependence on the friction parameters of the transport through several sections. We also examine the onshore‐offshore mass exchange over the southeastern edge of the Grand Banks. We find that the model Labrador Current flowing southward along the shelf break leads to an onshelf flux in the upper part of the water column and an offshelf flux in the bottom Ekman layer. This is caused by the frictional torque and leads to an along‐slope decay of the current amplitude. Finally the limitations of the model and plans for its further development are

ISSN:0148-0227    

DOI:10.1029/JC094iC09p12607

年代:1989

数据来源: WILEY

摘要:

In winter, tidal outflow from the Leschenault Estuary forms a buoyant sheared stratified plume in Koombana Bay on each outgoing tide. The turbulence in the plume and the local entrainment into the plume are examined based on measurements collected in an intensive field program. Coherency measurements between horizontally separated sensors are used to examine the anisotropy of the turbulence. The effects of viscosity and buoyancy on the turbulence are studied using an activity diagram which is also an indicator of the mixing efficiency and total mixing. The Osborne‐Cox model is tested and found to be valid in the plum

ISSN:0148-0227    

DOI:10.1029/JC094iC09p12619

年代:1989

数据来源: WILEY

摘要:

A survey of relevant numerical, laboratory, and observational studies, combined with our own laboratory experiments, suggests that the surface outflow from a strait generally forms a gyre in the adjacent sea if the exit corner is sharp. The gyre grows from an initial “separation bubble” of radiusu/f, whereuis the current speed andfthe Coriolis frequency. The criterion for the maintenance of a coastal jet along a curved coast, instead of separation and gyre formation, seems to be that the Rossby numberu(fRw)−1<1, withRwthe radius of curvature of the exit corner, so that the Coriolis force can hold the current to the coast. This synthesis disregards factors such as the variation ofuacross the current but, applied to the flow through the Strait of Gibraltar into the Alboran Sea, suggests that a gyre should always form as the exit corner is sharp. We suggest, though, that the relevant curvature is that where the interface between the two layers intersects the seafloor rather than where the sea surface meets the coast. If this is the case, supercritical flow or some subcritical flows at the eastern end of the Strait should still always lead to gyre formation, but the observed replacement of the gyre by a coastal jet along the Moroccan coast could be related to subcritical flow that is reduced below its normal value by a barotropic fluctuation or by changes in the interface depth in the Mediterr

ISSN:0148-0227    

DOI:10.1029/JC094iC09p12637

年代:1989

数据来源: WILEY

摘要:

An efficient numerical model is used to solve the linear barotropic equations of motion with North Pacific bottom topography and seasonal wind forcing (that is, monthly mean wind stresses with the annual mean removed). The model domain extends from 10°S to 60°N and from 120°E to 100°W with 1°×1° resolution. The model‐predicted seasonal transport and sea level variations are described and compared with the available data. It is found that at the Tokara Strait, to the south of Japan, the model‐predicted seasonal transport variations are very similar to those of the observed annual cycle of sea level difference across the Strait, although the amplitude of the model response is less than that suggested by the data. Both model and data indicate enhanced flow of the Kuroshio through the Strait in summer with a minimum in that flow in the winter. This is 6 months out of phase with the seasonal transport variations predicted by the flat‐bottomed Sverdrup relation. The model response is very sensitive, however, to the smoothing of the topography, with additional peaks in transport being found in the spring and the fall when the smoothing is increased. This suggests that care in handling the topography is required if a general circulation model is to reproduce this seasonal cycle. Away from the western boundary, the model‐predicted transport variations are quite similar to those in a flatbottomed ocean. The model also shows some success at reproducing features of the observed annual cycle of sea level corrected for both atmospheric pressure variations and the steric density effect. An interesting feature of the results is that the model‐calculated sea level at the offshore edge of the coastal waveguide is remarkably similar all the way along the North American coast from the Aleutian Islands to California. This shows a peak in model sea level in the summer and a minimum in the winter and indicates that it is the seasonal fluctuations of the subtropical gyre which dominate the model response even in the northern latitudes. However, the amplitude of this signal is somewhat less than that observed at the coast which, as previous studies have shown, is strongly influenced by coastal effects. The importance of wind forcing of the coastal waveguide is also apparent in the simple model described here, as is demonstrated by comparing model‐calculated sea level at the coast with that

ISSN:0148-0227    

DOI:10.1029/JC094iC09p12645

年代:1989

数据来源: WILEY