摘要:

Satellite imagery over a 3‐year period is used to provide statistical data on the position of the Agulhas Current, frontal features, and meanders off the southeast coast of South Africa. Data recorded during a cruise undertaken over the continental shelf off Algoa Bay in May 1987 provided subsurface temperature data of a large frontal feature and is used to correlate in situ measurements with satellite imagery. It is found that the inshore thermal front of the Agulhas Current lay, on average, at the shelf break northeast of Algoa Bay and seaward of it to the west. Its thermal core was about 26 km further out. The Agulhas Current influences the ocean structures over the shelf area by longer‐term Natal Pulses and shorter term warm surface frontal plume fluctuations. A case study of a rapidly propagating meander (average speed, 42 km/day) showed that an inshore plume formed a thin wedge over the cooler shelf water, and it illustrated that the surface signature as seen by satellite imagery does not necessarily correspond to the subsurface structure. The formation of a large shear‐edge eddy could also be followed and was possibly accentuated by a severe

ISSN:0148-0227    

DOI:10.1029/JC095iC01p00667

年代:1990

数据来源: WILEY

摘要:

Evidence of a warm, salty surface current flowing poleward along the Iberian Peninsula is presented using a sequence of satellite infrared images and concomitant in situ hydrographic data obtained during the winter of 1983–1984. The current, which flows over 1500 km along the upper continental slope‐shelf break zone off western Portugal, northwest and northern Spain, and southwest France, is 25–40 km wide, about 200 m deep, and characterized by velocities of 0.2–0.3 m s−1. According to the hydrographic data acquired during late November and early December 1983, the current's salinity signal off Portugal is about 0.2 practical salinity units, and its waters are ∼0.5°C warmer than the surrounding ones. The satellite observations, however, which span a longer time period and cover a much larger area, indicate that the current's typical thermal signature is 1°–1.5°C. The current's associated geostrophic volume transports show an increase from about 300×103m3s−1near 38°3′N to 500–700×103m3s−1at 41°–42°N. The origin of this poleward flow and the causes for its increasing transport off western Iberia are investigated. Onshore Ekman convergence induced by southerly winds along the Portuguese west coast provides about one fifth of the computed transports in the correct direction. A mechanism giving better quantitative agreement with the observations is the geostrophic adjustment of the eastward oceanic flow driven by the large‐scale meridional baroclinic pressure gradient in the eastern North Atlantic as the flow reaches the continental slope of the western Iberian Peninsula. Topographic trapping by the bathymetric step existing along the shelf break explains both the width and the path of the observed current. The role of “dam break” type mechanisms is discarded owing to strong discrepancies between the available models and the present observations. Since satellite images reveal that similar situations occurred during many winters, the flow identified here appears as a characteristic feature of the winter circulation off southwest Europe. Furthermore, the occurrence of analogous poleward flows in eastern boundary layers of the subtropical and mid‐latitude oceans suggests that these currents are typical featur

ISSN:0148-0227    

DOI:10.1029/JC095iC01p00679

年代:1990

数据来源: WILEY

摘要:

In the present paper, the probability for a mesoscale oceanic rings to be seen by a perfect satellite altimeter is investigated. First, the motion of a ring center is determined using a stochastic differential equation. The problem of radar altimeter sampling is then investigated, first in the one‐dimensional case, then for two dimensions. The probability of a ring to be seen by a satellite is then computed and is shown to depend on the satellite period and fundamental interval, the mean and standard deviation of the ring speed, and the distance between the ring center and the nearest track when the ring appears. The maximum of this probability and the time at which the maximum arises give an estimate of the satellite ability to sense the rings. The observation of Gulf Stream (GS) warm and cold core rings (WCR/CCR) by the Topex‐Poseidon and Geosat altimeters are then presented as examples. For both satellites and ring events, the probability for the ring center to lie on a satellite track (i.e., the entire ring signal is seen) is very low (less than 5%). However, more than 90% of the GS WCRs and 65% of the GS CCRs are seen at least once (i.e., the distance between the ring center and a satellite track is less than the ring radius) with a probability over 80%. The mean probability for the satellite to see a significant part of the ring signal (the distance between the ring center and a satellite track is less than half the ring radius) drops to about 50%. Aliasing between ring motion and satellite sampling should be taken into account during the definition of altimetric data assimilation methods for numerical models. On the other hand, the probability of view is high enough to indicate that altimetric data could improve climatological and statistical studies of mesoscale edd

ISSN:0148-0227    

DOI:10.1029/JC095iC01p00693

年代:1990

数据来源: WILEY

摘要:

Data from five cruises during 1985–1986 along 63.55°W, across the eastern Caribbean Sea, are examined for seasonal variability of gcostrophic transport. A biannual maximum transport signal appears to correspond to that measured in the Florida Straits as well as to variations in the regional wind stress curl. Similar seasonal variations in the properties of water masses, entering the eastern margin of the basin and concentrated through the passages of the Lesser Antilles, are also indicated. A permanent system of strong westward flowing streams, interspersed by weaker eastward flowing streams, is shown to exi

ISSN:0148-0227    

DOI:10.1029/JC095iC01p00699

年代:1990

数据来源: WILEY

摘要:

Long‐wave theory and simple turbulence closures have been used to show that three distinct types of circulation (highly stratified, weakly stratified, and partially mixed) arise in narrow, shallow estuaries from the finite amplitude of the tide and the interaction of stratification with vertical mixing. Each type has a different dominant process causing the vertical exchange of salt and fresh water on the flood, and each gives rise to a characteristic residual circulation. The tidal circulation in highly stratified shallow estuaries, described herein, is the result of a finite amplitude internal motion driven by the barotropic tide; shear instabilities at the interface are the major vertical exchange mechanism. The residual circulation is caused primarily by ebb‐flood asymmetry in interface position and thickness. A model based on our theoretical analysis shows that the interface thickness on flood is much less than the depth of flow up to a critical tidal amplitude at which the two‐layer flow is destroyed, and correctly predicts the tidal amplitude of the neap‐spring transition in the Columbia River Estuary. Tidal frequency internal wave motion is not found in weakly stratified and partially mixed estuaries. Because of the reduced tidal shear, shear instabilities are weak or absent, and the residual circulation assumes a very different character, as described in

ISSN:0148-0227    

DOI:10.1029/JC095iC01p00711

年代:1990

数据来源: WILEY

摘要:

Long‐wave theory is used herein to analyze circulation in weakly stratified and partially mixed estuaries. Unlike the highly stratified systems considered in part 1, the flows considered here have only a minimal tidal‐frequency internal wave component. These estuaries may therefore be modeled as the sum of weakly interacting barotropic and baroclinic modes. The dominant factors driving the residual flow are finite amplitude barotropic effects in weakly stratified estuaries and a combination of barotropic effects and steady horizontal density gradient forcing in partially mixed estuaries. The dominant vertical exchange mechanism in the weakly stratified case is bottom boundary‐induced turbulence, and that in partially mixed estuaries is believed to be random internal wave interactions. A model of the Columbia River Estuary under weakly stratified conditions accurately predicts the observed residual velocity and salinity fields, and the tidal amplitude at which a transition to a highly stratified state takes place. The partially mixed state is unstable in the Columbia River Estuary because the tides and buoyancy input are both too s

ISSN:0148-0227    

DOI:10.1029/JC095iC01p00733

年代:1990

数据来源: WILEY

摘要:

Molecular diffusion sustains the flux of soluble gases on the water side of the air‐sea interface. The “handover” of this flux to more efficient eddy mixing begins with the smallest eddies, of size l;, which interact with the surface diffusion boundary layer (DBL), of thickness δ. Owing to the discrepancy of the scales, δ ≪l, the flow field on the δ scale consists of horizontal motions of a velocity constant with depth and varying horizontally on thelscale. The vertical velocity is proportional to the divergence of the horizontal flow and increases linearly with depth. An exact solution of the advection‐diffusion equation for the simple model of divergent stagnation point flow shows the mass transfer coefficient (velocity)kto be proportional to (aD1/2) and DBL thickness δ to be proportional to (D/a1/2), whereais divergence,Ddiffusivity. Over a solid wall a similar model of Hiemenz flow yields a more complex relationship, also involving viscosity. These models reveal the mechanism by which the DBL is kept thin. The most intense surface divergences on a wind‐blown sea surface are associated with rollers on breaking wavelets. Vorticity and divergence in the rollers are both proportional to u*2/v;, whereu* is friction velocity andvis viscosity. The mass transfer coefficient resulting from divergences of this magnitude is then given byk= const u*Sc−1/2, whereScis Schmidt number. Exact solutions of the advection‐diffusion equation for model rollers reveal the details of the handover process. A thin DBL is maintained over divergences by the upward velocity. At convergences, narrow downward plumes convey DBL fluid into the turbulent interior. Flux lines (analogous to streamlines) are horizontal over divergences and dive down under convergences. Application to the sea surface requires a parameter quantifying the surface density of divergences. Laboratory data imply that a substantial fraction of the surface is covered by the divergences at higher wind speeds. However, in the open ocean straining by the large waves, and especially whitecapping, may significantly reduce the density of divergences and with it the area‐average gas transfer rate. On the other hand, bubble and droplet production in whitecaps may diminish this effe

ISSN:0148-0227    

DOI:10.1029/JC095iC01p00749

年代:1990

数据来源: WILEY

摘要:

This study examines the utility of remotely sensed Seasat A satellite scatterometer (SASS) winds for driving a spectral ocean wave model. The SASS winds are integrated with the conventional ship winds using an objective analysis scheme which employs the method of successive correction, modified to account for the variable data density, data quality, and the application of an elliptical weighting function. The analyzed winds are produced initially on a Cartesian grid over the North Atlantic and are then transformed onto a spherical orthogonal grid for input to a spectral ocean wave model. The model products are examined and validated against the observed ship and remotely sensed wave data to assess the impact of SASS winds on ocean wave analysis and modeling. Our study demonstrates that the integration of SASS winds into the wind analyses can provide improved wind inputs for validating wave models in a hindcast mode. The model wave heights when validated against the comparison data show a reduction in the root mean square error of up to 25% with the assimilation of SASS winds in the wind analyses. The ultimate goal of this study is to develop suitable algorithms for incorporating satellite‐sensed wind and wave data into operational ocean wave model

ISSN:0148-0227    

DOI:10.1029/JC095iC01p00761

年代:1990

数据来源: WILEY

摘要:

A coupled ice/ocean dynamics model is developed to provide Arctic offshore operators with 5‐ to 7‐day forecasts of ice motions, ice conditions, and ice edge motions. An adaptive grid is introduced to follow the ice edge, and the grid may move independently of the ice motion. The grid can be Lagrangian or Eulerian at different locations away from the ice edge. Ice stress is described using an elastic‐plastic model with strength determined by the ice conditions. The ocean dynamics model describes time‐dependent, three‐dimensional behavior, including wind‐driven currents and barotropic and baroclinic flows. The thermal energy budget of the ice cover is coupled to the ocean, with mass and salt interchange accompanying freezing or melting. Near the marginal ice zone (MIZ), surface winds (determined by reducing and turning the geostrophic winds) are enhanced to reflect observed behavior. The model was tested by simulating ice edge motions observed during the 1983 Marginal Ice Zone Experiment‐West and during drilling of the 1983 north Aleutian shelf Continental Offshore Stratigraphic Test well. Simulations of ice edge movement in the Bering Sea compare with observed data to within about 5 km/d. The model correctly describes mixed‐layer evolution in the marginal ice zone as fresh meltwater is mixed downward by turbulence. Along‐edge baroclinic flows due to density gradients across the ice edge are simulated by the model, in agreement with observations. Increased ice drift speeds generate higher melt rates due to increased turbulence levels, with the result that ice edge advance is moderated in spite of higher

ISSN:0148-0227    

DOI:10.1029/JC095iC01p00775

年代:1990

数据来源: WILEY

摘要:

The coming opportunity to extract quantitative sea ice data from routine synthetic aperture radar imagery requires the development of automated image processing techniques. An algorithm is described for measuring the opening and the closing of leads by comparing two sequential synthetic aperture radar (SAR) digital images. The pair of images is classified into leads and ice, correcting for variation in brightness with range. Displacements are determined automatically on a regular 2‐km grid (on a 100‐km square scene) by cross‐correlation. An algorithm groups together cells which cover a lead “event.” The lead area change is estimated for each cell group by counting lead pixels before and after deformation. All positive changes are summed to obtain the amount of opening, and all negative changes give closing. Errors are estimated by comparison with area changes determined from manually digitized lead boundaries and are about 0.2% of the area of the scene. For comparison, the measured opening and closing are in the range of 1 to 3% of the scene area. In ice models the openings and closings of leads are parameterized in terms of the large‐scale deformation. SAR allows us to study these parametric relations by giving us access both to direct observations of opening and closing and to detailed kinematic data from which to estimate the mean deformation. In this image pair for which the deformation is nearly pure shear, we find relatively small amounts of opening and closing, consistent with previous observations, and with parameterizations in most models. The large values of opening and closing predicted by the Poisson‐Gauss kinematic model are no

ISSN:0148-0227    

DOI:10.1029/JC095iC01p00789

年代:1990

数据来源: WILEY