摘要:

Oceanic flow phenomena vary in such large ranges of time and spatial scales that even with the fastest and largest computer to‐date, one cannot, at reasonable costs, compute large‐scale circulation and at the same time resolve mesoscale features like fronts and/or eddies; yet these features are dynamically important, and their inclusion may determine how well we can model the mean flow, including the deep‐ocean circulation. A two‐way interactive, nested‐grid, primitive‐equation model is developed here for coastal ocean applications. Notable features of the model are (1) nesting can be specified on any subregion of a coarse‐grid, large domain, and there can be more than one nest if required; (2) the nested region can be “hot‐started” from earlier calculation results of the coarse‐grid region, that is, the code automatically (by a change of an input flag) generates topography, wind forcing, climatology, currents, density and other prognostic variables in the nest and steps forward in time; (3) a time‐splitting integration, with small timesteps in the nest and large timesteps in the coarse‐grid domain, is used; and (4) nested variables are driven by coarse‐grid solutions around the nest's boundaries, where a flow relaxation scheme may be used, and at the same time drive the coarse‐grid evolution through its averaged action in the overlapped region. To demonstrate its robustness, the model is applied in a February/March 1988 real‐time simulation of meanders and eddies in the Norwegian Coastal Current, initialized from a 585 days' quasi‐equilibrium calculation. The simulation includes meteorological forcings, inflows/outflows across the open boundaries (inflow of the North Atlantic warm water in particular), tides, coastal and Baltic discharges, and wintertime hydrography for depths>500 m. From March 20 to 31, the development of a meander between the Froya and the Halten Banks is simulated. The timing and location of the meander agr

ISSN:0148-0227    

DOI:10.1029/92JC01991

年代:1992

数据来源: WILEY

摘要:

A three‐dimensional, primitive‐equation simulation of the circulation in the northeast Atlantic shelves and seas, defined by 51°–76°N latitudes and 20°W–22°E longitudes, has been conducted for the period February‐March 1988. The simulation was initialized from a 585‐day quasi‐equilibrium calculation and included realistic meteorological forcing, inflows/outflows across the open boundaries (inflow of the North Atlantic warm water in particular), tides, coastal and Baltic discharges, and relaxation to wintertime climatology for model depths>500 m. The calculation is the first part of an overall effort to nest a high‐resolution region for simulation of eddies and fronts in the Norwegian Coastal Current (NCC). This paper presents detailed simulation strategies and discusses results from the coarse‐grid region to study the larger‐scale model response induced by atmospheric forcing, so that its effects on flow dynamics in the nested grid can be better understood. The mean and variability of the simulated flow field are compared, whenever possible, with published observations. In particular, we examine in detail the simulated wind‐induced response in the Skagerrak transport, which produces blocking and outbreak of the Skagerrak and North Sea waters. These transport variabilities can be expected to be important in the development of the NCC meanders

ISSN:0148-0227    

DOI:10.1029/92JC01990

年代:1992

数据来源: WILEY

摘要:

In the present paper (part 1 of 2), the product of the Semtner and Chervin general circulation model (GCM) is compared with available observations in the frontal areas of the Brazil/Malvinas and the Kuroshio/Oyashio confluences. The dimensionality of the systems studied is reduced by using the empirical orthogonal functions (EOF) and frontal density methods. The two sets of data utilized to validate the model are the sea surface temperature (SST) from the satellite observations and temperature fields product from the GCM at levels 1 (12.5 m), 2 (37.5 m) and 6 (160 m). Comparisons are made between the dominant empirical modes and the locus of maximum probability for observations and model product. The model reproduces intense thermal fronts at the surface and in the upper layers. In the upper layer (level 1) they are induced by the internal dynamics of the model and not by the restoring of the model to climatology alone. The variability of these fronts is less pronounced in the model than in the observations. The dominant period in the observations is annual with contributions of semiannual and high frequency oscillations. In the model, the dominant variability is also annual at all analyzed levels. A semiannual oscillation contributed to a lower degree and is related to eddies that, in the model, have an annual and semiannual periodicity. For the regions examined, the location of the fronts are reproduced in the model within differences of 4° to 5° with observations. In the Brazil/Malvinas region, the Confluence front is reproduced approximately 4° towards the west of the observed front. This appears to be due to the resolution of the model that, in a 0.5° × 0.5° grid, does not resolve the sharp slope at the edge of the Argentine continental shelf. The maximum southward penetration of the warm tongue of Brazil waters occurs in the model approximately 4° towards the north. This is related to the fact that, in the model, the Malvinas transport doubles the one derived from the observations. This might be due to the effect of a large modeled transport for the Circumpolar Current or, again, to a poor resolution of the topography. In the Kuroshio area, the Oyashio front, which in observations is more pronounced at the surface than in the lower layers, is well reproduced in the surface temperature field. On the contrary, the Kuroshio front, more intense in the lower layers but still marked in the satellite observations, is visible in the model only below 160 m. The front is not present in the surface temperature field but, as a consequence of the thermal wind balance, an intense eastward flow at the location of the Kuroshio Extension is observed in the model velocity field. When compared with the observations, the location of the Extension is shifted approximately 5° towards the south. This indicates a shift in latitude between the modeled and observed latitude of separation. The resolution of the model is marginal to reproduce the process of eddy formation, but large scale eddies are observed in the model in both analyzed areas. They are generated as a pinch of the main flow with an annual and semiannual periodicity. We conclude that some of the differences between model and observations, like the differences found in the locations of the fronts, and the diminished variability, will decrease with a higher res

ISSN:0148-0227    

DOI:10.1029/92JC02222

年代:1992

数据来源: WILEY

摘要:

The energy of the model transient eddies at 37.5 m is compared with Geosat altimeter observations, for the South Atlantic Ocean and for the Kuroshio system. The model shows areas of transient motions overlapping the ones obtained from Geosat altimeter data. For the South Atlantic Ocean, the modeled eddy kinetic energy is smaller than the one observed with Geosat, by a factor of 3 for area average on the whole South Atlantic region, and by a factor of 4 for its western boundary. On the Agulhas system, transient eddy activity develops in the region where the Agulhas current retroflects. In the western South Atlantic, the modeled eddy activity is concentrated on the Confluence front; observed variability along a more extended region following the topography is not resolved in the model. For the Kuroshio region, the energy level of the modeled transient motions is comparable with Geosat observations, but the model eddy activity is more concentrated in the Kuroshio Current and not in the Kuroshio extension. The observations show the opposite. For the South Atlantic Ocean, a comparison is also done between model eddy kinetic energy (defined as including standing and transient eddy contributions) with values obtained from surface drifters. The analysis shows differences in the western boundary, and good agreement across the South Atlantic Ocean between 35°S and 45°S. In this formulation, the model mean energy level is smaller than the observed with drifters from the First GARP Global Experiment; differences might be due to an overestimation in the values obtained with the drifter

ISSN:0148-0227    

DOI:10.1029/92JC01395

年代:1992

数据来源: WILEY

摘要:

We report the results of laboratory experiments on the formation and survival of internally stratified subsurface eddies in a rotating fluid. The eddies were created by injecting a dense turbulent plume at the surface of a linearly stratified environment. The relative vorticity of the lenses was always negative but larger than that of homogeneous lenses created by laminar injection. During the first 100 revolutions, the eddies shed fluid in two symmetric arms. The shedding which is believed to result from shear instabilities always resulted in a stationary axisymmetric eddy. After the eddy had spun down, the remnant fluid persisted for thousands of rotations as a circular feature with internal stratification identical to that of the environment. We created eddies with and without double diffusive convective instabilities and compared the volume of dyed fluid and the evolution of their aspect ratios. Sugar and salt were used as laboratory analogues of salt and heat, respectively. The Burger number of the lenses decreased rapidly within the first 200 revolutions and then much more slowly to reach a value between 0.2 and 0.4, These latter values are larger than those predicted by Gill (1981) for a homogeneous lens due to the internal stratification of the lenses. Radial spreading of the lens due to double diffusive intrusions was found to be larger, but of the same order of magnitude, as that induced by the vertical exchange of momentum in the absence of double diffusive convection. We formed eddies internally stratified in the diffusive sense (stable sugar gradient and unstable salt gradient) or doubly stable (stable sugar and salt gradients) by changing the ratio of the volume flux at the source to the volume flux at the spreading level as described by Bormans and Turner (1990). When the stratification in the eddies was doubly stable, three distinctive regions were observed: a region of convective layers and diffusive density interfaces at the top, a central region with no apparent structure and a bottom region dominated by fingers sheared by rotation. This type of eddy is compared to the particular case of a Meddy.

ISSN:0148-0227    

DOI:10.1029/92JC01973

年代:1992

数据来源: WILEY

摘要:

Sea level variations in the Indian Ocean north of 20°S are analyzed from Geosat satellite altimeter data in 1987–1988. These observed variations are compared with numerical simulations from a reduced‐gravity model forced by observed winds over the same period. The first complex empirical orthogonal function of observed and simulated variations is an annual signal. For this signal, observations and simulations are highly correlated in both time and space. Off‐equatorial sea level variations propagate westward and poleward as Rossby waves. The strongest annual variations occur in the southeastern tropical Indian Ocean. The maximum amplitude (∼12 cm) is located at about 90°E, 12°S, although the wind stress curl is weak there and east of it. The signal propagates from the eastern boundary to the southwest across almost all

ISSN:0148-0227    

DOI:10.1029/92JC01961

年代:1992

数据来源: WILEY

摘要:

Steady solutions from a one‐layer, wind‐driven, primitive equation model are analyzed to determine the importance of wind forcing, pressure gradient force due to the climatological density distribution and bottom form drag on circulation in the Southern Ocean. Five simulations are discussed: three wind‐forced simulations with differing bathymetry (flat bottom, 15% bathymetry, and full bathymetry), one case with full bathymetry forced with the density‐induced pressure force, and one case with full bathymetry forced by both wind and density‐induced pressure gradients. The simulations presented here confirm the previous speculation (Munk and Palmén, 1951) that form drag is effective in balancing the driving force due to the surface wind stress. In fact, it has such a strong effect that bathymetry with only 15% of the true amplitude reduces the transport from over 480 × 106m3s−1to about 190 × 106m3s−1. If the true bathymetry is used, the total transport is reduced to a value around 20 × 106m3s−1. Analysis of the zonally integrated momentum in the unblocked latitudes of the Southern Ocean shows that the bottom form drag balances the surface forcing, even for simulations that have viscosities that are in the upper range of acceptable values. The vertically integrated pressure gradient due to the climatological density distribution produces a body force that accelerates the Antarctic Circumpolar Current, producing a transport of about 250 × 106m3s−1. Therefore the pressure gradient produced by the density structure of the Southern Ocean is an integral part of the dynamics of the Antarctic Circumpolar Current. It forces the flow across bathymetry that would, in the absence of the spatially varying density field, block the circulation. This result is in contrast to mid‐latitude gyres in which the steady, wind‐driven circulation is insulated from the influence of bathymetry by stratification (And

ISSN:0148-0227    

DOI:10.1029/92JC02058

年代:1992

数据来源: WILEY

摘要:

A cross‐spectral analysis is made of subtidal (0.01–0.5 cpd) sea level (SSL), atmospheric pressure and wind fluctuations observed during 1966–1988 in the southern reaches of the fjord system formed by the Straits of Juan de Fuca/Georgia and Puget Sound. It shows that observed SSL can be largely explained by quasi‐steady inverse barometer compensation to atmospheric pressure and the propagation of adjusted sea level fluctuations from the nearby continental shelf, with some local weather forcing in summer. Variations in coherence, amplitudes and relative phase between seasons, locations and frequencies support previous work that both barotropic and baroclinic processes have important influences on SSL within the fjord

ISSN:0148-0227    

DOI:10.1029/92JC01988

年代:1992

数据来源: WILEY

摘要:

We have recently developed a three‐dimensional semi‐analytical mixed layer remote sensing model which includes the horizontal advection and diffusion processes in our previous one dimensional model (Yan et al., 1991a). The three‐dimensional (3‐D) model can be used as a supplement to the one‐dimensional (1‐D) model in the area where advection is important and for a finer resolution grid prediction. The 3‐D thermal inertia model is based on the 3‐D thermal energy conservation equation. Using sine transformation and inverse sine transformation methods, we obtained the solution for mixed layer thermal inertia which, in turn, can be used to calculate mixed layer depth using a similar method to that of Yan et al. (1991a). In the solution, the thermal inertia (which is proportional to the mixed layer depth) is expressed as functions of sea surface temperature changes (δSST), heat flux (Q) and surface layer velocity (u, v, and w). The vertical entrainment and wind effects are accounted for in the model by eddy diffusivity while changes of δSST result from entrainment, e.g., if vertical entrainment is larger, δSST will be smaller and vice versa. The advantages of this model are that it can be easily forced by remotely sensed data and that it is much simpler to compute than numerical 3‐D mixed layer turbulence closure models. The model was tested using the data from the advanced very high resolution radiometer/multichannel sea surface temperature data set and the Cooperative Ocean Atmosphere Data Set for the North Pacific Ocean. The model‐predicted mixed layer depths compared favorably with the mixed layer depths calculated from 14 years of Volunteer Observing Ship/expendable bathythermograph observation data. The model/data comparison also exhibited a number of mesoscale features of seasonal changes and anomalies of

ISSN:0148-0227    

DOI:10.1029/92JC01833

年代:1992

数据来源: WILEY

摘要:

The evolving upper ocean response excited by the passage of hurricane Gilbert (September 14–19, 1988) was investigated using current and temperature observations acquired from the deployment of 79 airborne expendable current profilers (AXCPs) and 51 airborne expendable bathythermographs from the National Oceanic and Atmospheric Administration WP‐3D aircraft in the western Gulf of Mexico. The sea surface temperatures (SSTs), mixed layer depths, and bulk Richardson numbers were objectively analyzed to examine the spatial variability of the upper ocean response to Gilbert. Net decreases of the SSTs of 3°–4°C were observed by the profilers as well as by the airborne infrared thermometer (AIRT) along the flight tracks and advanced very high resolution radiometer (AVHRR) imagery. The AXCPs indicated a marked cooling from 29°C to about 25.5°C on September 17, 1988, which was about 1.2 inertial periods (IP) following storm passage. This pool of cooler water (3.5°) was located further downstream in the hurricane wake by September 19 (2.7 IP following the storm) as a result of the near‐inertial currents in the mixed layer. While there was a bias of about 0.6°C and 1.7°C between the in situ and AVHRR‐derived SSTs, respectively, both the AVHRR images and the objectively analyzed fields indicated a rightward bias in the upper ocean cooling that extended from the storm track to about 4Rmax(whereRmax, the radius of maximum winds, is equal to 50 km). The larger SST offset of 1.7°C was due to the difference between the time of the AVHRR image and the time of the aircraft experiment on September 19. The SSTs derived from the AVHRR images and the AIRT also indicated large gradients between the cold wake and the warm eddy in the central Gulf of Mexico. The mixed layer deepened by about 30–35 m on the right side of the track during the storm and 1.2 IP later, with little evidence of continued deepening afterward. The mixed layer current vectors demonstrate that a strong, near‐inertially rotating current was excited by the passage of Gilbert, with maxima of about 1–1.4 m s−1. The currents, observed during and subsequent to (1.2 IP) the storm, diverged from the storm track, whereas the mixed layer current vectors 2.7 IP after storm passage converged toward the track, with relative maxima of 0.8–1 m s−1. This alternating pattern of convergence and divergence of the mixed layer current was associated with the upwelling and downwelling cycles of the baroclinic response. Considerable current shear existed between the mixed layer and the thermocline currents in the cool wake between the storm track and the 4Rmax. Estimates of the bulk Richardson numbers ranged between 0.2 and 1.0 during Gilbert and at 1.2 IP, which suggests that enhanced current shears were responsible for some

ISSN:0148-0227    

DOI:10.1029/92JC01586

年代:1992

数据来源: WILEY