摘要:

The influence of submarine canyons on shelf currents is studied using the Rossby adjustment method for a homogeneous, inviscid fluid on anfplane. The canyon in the model is assumed to have vertical edges and constant width. The geostrophic flow around a canyon is found to be dependent upon two geometric parameters: the ratio of the depth of the canyon to the depth of the shelf and the ratio of the width of the canyon to the Rossby radius over the canyon. Moreover, a single parameter determines most of the properties of the geostrophic state. This parameter is called the canyon number and is a combination of the two basic geometric parameters. In the geostrophic state an infinitely long flat‐bottom canyon will act as a complete barrier to an approaching shelf flow. The approaching flow is asymmetrically diverted along the canyon, and a net flux is generated to the left of the flow in the northern hemisphere. If the canyon cuts a shelf between the shelf break and the coast and connects to a strait (the geometry of Juan de Fuca Canyon) an in‐canyon (out‐canyon) current will be generated when the shelf break current flows keeping the shelf at its left (right) in the northern hemisphere. If the canyon has a stepped or sloped bottom, the geostrophic flow has a singularity where the step or slope meets the left canyon edge (looking upcanyon) in the northern hemisphere. Flow can cross the canyon edge through the singularity, so the canyon is no longer a complete barrier to the approaching shelf flow. In this case, as above, a net flux is generated to the left of the approaching shelf

ISSN:0148-0227    

DOI:10.1029/96JC01149

年代:1996

数据来源: WILEY

摘要:

Using the Mellor‐Yamada level II closure scheme, a numerical study is made of the bottom boundary layer on a uniform, stratified shelf with slope α. For constant interior along‐slope currentsugand molecular background diffusion, a new timescale for shutdown of bottom stress is derived for the downwelled layer as well as a new estimate for the maximum height of the upwelled layer,hU= (CD/fN)1/2ug(1 +S1/2)−1whereCD= 2.5 × 10−3is a drag coefficient,fandNare the Coriolis and buoyancy frequencies, andS= (Nα/f)2is the Burger number. The Richardson number is also shown to fix the thermal‐wind shear at the top of the upwelled layer to be approximatelyuz≃N(RiS)−1, while within the layer, a depth‐averaged form of the geostrophic balance is shown to hold. In the case of large interior vertical diffusivities (10−4m2s−1), the fluxes of momentum and buoyancy into the interior are quantified and shown to result in the arrest (enhancement) of shutdown in the upwelling (downwelling) boundary layers. Advective restratification of mixed water is not found to occur, and for an 8‐day periodic interior currentug(t), the thermal‐wind shear that remains during the downwelling and upwelling phases can result in a 1.2‐day lag of interior current with bottom stress. Bottom stress and the cross‐slope Ekman flux are found to be surprisingly symmetric during the upwelling and downwelling phases of the interior current. The lag with bottom stress and effects of shutdown are shown to persist in the presence of tidal currents and large background diffusion and may well be signif

ISSN:0148-0227    

DOI:10.1029/96JC01272

年代:1996

数据来源: WILEY

摘要:

This paper deals with frontogenesis in the presence of ageostrophic vertical current shears and horizontal density gradients. The problem has broad application to the situation encountered in tidal fronts and current system meanders, but specific focus here is on Gulf Stream meander crests and filaments that advance onto the continental shelf just north of Cape Hatteras. These occur typically every few days as Gulf Stream meanders progress northeastward through the South Atlantic Bight and past Cape Hatteras. We model the submesoscale evolution of the interface between the continental shelf water and these Gulf Stream features while they are on the continental shelf. We assume the region to be characterized by an initial condition consisting of a horizontal density transition region and an ageostrophic, surface‐intensified horizontal flow. The ensuing frontogensis process is modeled numerically with anfplane calculation employing the full nonlinear equations in the depth/cross‐front plane; flow is assumed out of this plane (along the front), but no variation of the flow in this direction is allowed. A pseudospectral model is employed using trigonometric functions in the horizontal and Chebyshev polynomials in the vertical. Many different scenarios are investigated by changing the width, shape, and relative positions of the density transition and velocity jet. In the majority of cases a propagating hydraulic jump is formed. Simultaneously, the initial surface jet evolves to a subsurface‐intensified jet while it weakens and ultimately changes directions. The presence of this strong velocity jet can substantially enhance the rate of jump formation or completely inhibit frontogenesis. Supporting analytical calculations are used to show that the presence of vertical ageostrophic shear can augment or oppose the usual frontogenesis mechanism present when the collapsing horizontal density gradient is acted on by the resulting convergent surface current. The outcome of the shear/density gradient interaction depends upon the position of each field with respect to the other. In the vicinity of the nose of the hydraulic jump for the cases investigated, the density is seen to have a qualitatively similar dependence upon the stream function in the translating frame, irrespective of the initial condition from which it ev

ISSN:0148-0227    

DOI:10.1029/96JC01423

年代:1996

数据来源: WILEY

摘要:

We investigate how well the 1975–1992 sea level interannual variability in the tropical Pacific is captured by dynamic height from temperature profiles. For each temperature profile, a surface dynamic height relative to 300 m is estimated, assuming a constant temperature‐salinity relationship. After multiplication by a latitudinally varying factor and the removal of a seasonal cycle, the dynamic height deviations fit the tide gauge sea level variability to within the sampling errors, except at a few sites near the equator west of the date line, where surface salinity variability is large. The dynamic height data are assimilated into a wind‐forced linear numerical model of the sea level in the tropical Pacific, applying a Kalman filter in a space of reduced dimension. A limited number of empirical orthogonal functions of the unfiltered run (1975–1992) define the reduced space, into which the Kalman Filter covariance evolution calculation is done [Cane et al., 1996]. Experiments indicate that results are better with 32 functions than with a smaller number but are not improved by retaining more functions. The resulting analyzed fields of sea level are compared to withheld dynamic height estimates from moorings, sea level data from tide gauges, and sea level analyses made with the same Kalman filter formalism applied to tide gauge measurements. The comparisons to observations suggest that the temperature profiles were usually sufficient to constrain the monthly analyzed fields to be close to the observed sea level with errors typically less than 3 cm near the equator. The comparison to tide gauge sea level reveals that this analysis is often more accurate than the analysis of tide gauge sea level data with which it shares many characteristics. Near the equator west of the date line, salinity variations are large and their neglect in estimating dynamic height has a negative impact on the analysis. The analyzed signal is underestimated in the southwest Pacific and at more than 20° off the equator. The reanalysis of the temperature data done with a primitive equation model at the National Meteorological Center (NMC) [Ji et al., 1995;Enfield and Harris, 1995] does not share this problem. On the other hand, NMC reanalysis (RA4) departs more from the observations elsewhere, although more data were included than in our

ISSN:0148-0227    

DOI:10.1029/96JC01532

年代:1996

数据来源: WILEY

摘要:

Intensive observations in the northeast Indian Ocean (Bay of Bengal) during the presouthwest and northeast monsoon seasons of 1991 reveal that freshwater discharge from rivers of the Indian subcontinent exerts the dominant control over total carbon dioxide (TCO2) and pCO2distributions in surface waters. Low pCO2levels occur within the low‐salinity zones, with a large area in the northwestern bay acting as a sink for atmospheric CO2. Only a part of the observed pCO2variation can be accounted for by the effect of salinity, and biological production supported by external nutrient inputs in conjunction with strong thermohaline stratification may be more important in lowering surface water pCO2by>100 μatm relative to that in the atmosphere. The pCO2distribution is seasonally variable and appears to be controlled by the spreading of fresher waters by the prevailing surface circulati

ISSN:0148-0227    

DOI:10.1029/96JC01452

年代:1996

数据来源: WILEY

摘要:

Meteorological observations from an array of stations deployed along the periphery of the Gulf of Mexico, between 1990 and 1993, are used to describe the seasonal fluctuations in patterns of atmospheric variables from a contemporary set of measurements. Seasonal maps of wind stress based on these measurements resemble wind stress maps based on ship observations, as published byElliott[1979], rather than maps based on analyses of numerical weather forecasts, as published byRhodes et al.[1989], particularly near the western boundary of the gulf. Seasonal maps of wind stress curl are characterized by positive curls over the western and southwestern gulf. The central result of this study is to document the important role of the mountain chain which extends along the southwestern section of the gulf in channeling the wind toward the Isthmus of Tehuantepec.

ISSN:0148-0227    

DOI:10.1029/96JC01442

年代:1996

数据来源: WILEY

摘要:

Geosat altimeter data were used to study a region to the south and east of South Africa, which includes the Agulhas Current System and the Antarctic Circumpolar Current (ACC). The sea surface height variability was determined from 2 years of Geosat altimeter data by collinear techniques. Strong association was found between variability and changes in topography, even at scales of 100–200 km. The height anomaly profiles were used to determine geostrophic current speeds of the order of 20 cm s−1for the ACC and up to 138 cm s−1for the Agulhas Current System. Correlation length scales of the height anomalies were found to agree with the first internal Rossby radius of deformation. Differences appear to be related to physical mechanisms such as wind forcing and topographic stretching which alter the vorticity balance of the Agulhas Current S

ISSN:0148-0227    

DOI:10.1029/96JC01147

年代:1996

数据来源: WILEY

摘要:

Between October 1982 and May 1983 the deep water in the northern Red Sea was ventilated through slope convection. The deep water became cooler, fresher, and more oxygenated. Dense bottom water formed in the Gulf of Suez during winter cooling sank down the continental slope, thereby entraining near‐surface and intermediate depth waters. From the application of a one‐dimensional plume model, a vertical transport of 0.58 × 106m3s−1over a period of 7 months was estimated. Long‐term time series of atmospheric heat fluxes show that such a major convection event can occur every 4–7 years. The corresponding renewal time of the Red Sea deep water is thus of the order of 40–90 years, in agreement with previo

ISSN:0148-0227    

DOI:10.1029/96JC01148

年代:1996

数据来源: WILEY

摘要:

We hypothesize that a formation mechanism of anticyclonic eddies (lenses) is the outflow of intermediate waters down the canyons of the continental shelf. We reproduced in the laboratory the horizontal injection of fluid into the rotating stratified surroundings at the equilibrium density level. The experiments demonstrate that such an injection forms an anticyclonic eddy. Measurements of the velocity field show that the core of the eddy is almost axisymmetric and is in solid body rotation. The periphery of the eddy is formed by the jet flow. Cyclonic satellites were also observed at the periphery of the eddy. The main features of the laboratory flow are consistent with those of the “young” eddy observed recently in the Gulf of Ca

ISSN:0148-0227    

DOI:10.1029/96JC01294

年代:1996

数据来源: WILEY

摘要:

Open‐ocean deep‐water formation involves the interplay of two dynamical processes; plumes (≤1 km wide), driven by “upright” convection, and geostrophic eddies (≥5 km wide), driven by baroclinic instability. Numerical “twin” experiments are used to address two questions about the plumes: Can they be represented by a simple mixing process in large‐scale models? If so, is it important that the mixing occurs over a finite timetmix, or would instantaneous mixing produce the same effect on large‐scale properties? In numerical simulations which resolve the geostrophic eddies, we represent the plumes with a “slow” convective adjustment algorithm which is broadly equivalent to an enhanced vertical diffusivity of density in statically unstable regions. The diffusivity κ depends ontmix, the mixing timescale. The fidelity of the plume parameterization is then evaluated by comparison with plume‐resolving simulations of open‐ocean deep convection. Integral properties of the plumes, such as the temperature census of the convected water and the strength of the rim current that encircles the convecting region, are all accurately reproduced by the slow adjustment scheme. The importance of choosing an appropriate finite value fortmixis explored by settingtmix= 12 hours in some experiments, in accordance with scaling considerations, andtmix= 0 in others, corresponding to instantaneous adjustment, the conventional assumption. In the case of convection into a moderately or strongly stratified ocean the behavior does not significantly depend ontmix. However, in neutral conditions the slow adjustment does improve the parametric representation. Our experiments confirm the picture of plumes homogenizing the

ISSN:0148-0227    

DOI:10.1029/96JC00861

年代:1996

数据来源: WILEY