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1.
A two-cell circulation associated with a front observed in coastal upwelling regions is studied numerically in a three-dimensional level model. An ocean with a flat bottom is forced by the wind stress with a longshore variation. Upwelling is induced in the region next to the coast. In association with the upwelling, the pycnocline slopes up toward the coast and intersects the sea surface forming a front. After that, downwelling is induced just inshore-side of the front and upwelling offshore-side. The transverse circulation in the present model seems to reproduce the observed two-cell circulation. It is found that the generation of the two-cell circulation is due to deviations of the longshore flow from the thermal-wind relation (geostrophy). The deviations are caused by the onshore-offshore movements of the front. Although no vorticity input through the wind stress is assumed, several barotropic vortices are induced by the effect of the inclination of the pycnocline and grow as long as the winds continue to blow. The observed poleward undercurrent may be interpreted as a combination of motions of the internal mode associated with the front and a barotropic flow associated with a cyclonic vortex.  相似文献   

2.
A simple three-dimensional model of a time-dependent coastal upwelling is discussed for time scales of several days to a week, with the linear, two-layer, flat-bottom and ?-plane approximation. Emphasis is placed on the effects of longshore scales determined by the longshore variabilities in the wind stress distributions. The responses of the inshore motions are shown to depend critically on the longshore scales. For a certain wide range of the scales, the system reveals dominantly baroclinic responses and a full development of the poleward coastal undercurrent without β effect. Somewhat detailed discussions are given on the coastal upwelling, the coastal jet and the poleward undercurrent, which are interpreted simply as the orbital velocities of the forced Kelvin-type waves.  相似文献   

3.
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4.
The timing and intensity of the effects of the 1997–98 El Niño on sea-surface temperature (SST) and coastal sea level along the US west coast are examined using in situ time-series measurements, and the effects on upper ocean currents on the continental shelf and slope off Oregon and northern California are examined using repeated shipborne ADCP transects, a mid-shelf mooring off Newport Oregon and an HF surface current radar. An initial transient positive anomaly was observed in both adjusted sea level and SST during May–June 1997, followed by anomalously high coastal sea levels, generally strongest during September 1997 through February 1998 and abruptly returned to normal in late February 1998, and by positive temperatures anomalies over the mid-shelf that persisted longer, into April 1998. Low-frequency coastal sea-level anomalies propagated poleward at 2.1 m/s. Poleward flow over the shelf and slope was enhanced at most depths during the El Niño, compared with following years. Northward currents in the upper 12 m over the continental shelf off Newport, Oregon averaged 13.7 cm/s stronger during August 1997 through February 1998 than during the same period the following year. Enhanced poleward flow was present at all latitudes sampled during November 1997 and February 1998, particularly over the continental slope. These transects also provided clear views of a fall/winter equatorward undercurrent, which was both strongest and had the most alongshore similarity of form, during the ENSO. Finally, subsurface-intensified anticyclonic eddies originating in the poleward undercurrent appear to be a recurrent feature of the circulation off Newport late in the upwelling season.  相似文献   

5.
Effect of the typhoon passage on the western boundary region of a two-layer ocean with bottom topography is studied. The ocean is initially at rest and is set in motion by a typhoon passing parallel to the west coast. Equations that represent barotropic and baroclinic modes of motions are solved numerically by means of the method of finite differences. Motions of the barotropic mode are assumed to be horizontally non-divergent. In this mode, an elongated vortex is produced by the typhoon and propagates toward the south after passage of the typhoon. Behavior of the vortex may be interpreted as continental shelf waves. It is found that the formation and propagation of continental shelf waves are hardly affected by the density stratification. As for the baroclinic response, the typhoon causes considerable interface displacements along its track. The interface displacements are associated with geostrophic motions and remain for long time, though they are formed on the continental slope. Besides the large scale baroclinic response, internal Kelvin waves are induced along the artificial east wall.  相似文献   

6.
Effects of the longshore variation of the coastline geometry and the bottom topography on coastal upwelling are discussed. Longshore variations of the topography cause local enhancing or weakening of upwelling in the process of the generation and propagation of internal Kelvin and the shelf waves.  相似文献   

7.
Simple numerical experiments on two-dimensional coastal upwelling are made with emphasis on the role of non-geostrophic solenoidal field of density in the formation of double-celled circulation and multi-celled density front. Geometry of shelf and slope is not taken into account. Existence of poleward undercurrent presumably caused by the longshore variation of the large scale pressure field is also suppressed for the sake of simplicity.The results are, (1) double-celled circulation revealed in the present experiment is closely related with the internal frictional layer, where the horizontal density gradient balances with the vertical gradient of the longshore velocity and the vertical diffusion of the vorticity. (2) density front formed by the emergence of the pycnocline to the sea surface is successively advected offshoreward by the Ekman transport. (3) the pycnocline intersecting the sea surface forms the density front which is nearly vertical on account of the small scale convection. The surface currents converge at the front and construct an anti-clockwise circulation (viewed from the lee side). (4) small coefficient of eddy viscosity and strong wind stress lead the Ekman transport unstable and form a multi-celled structure in the frontal region.  相似文献   

8.
A high-resolution, multi-level, primitive equation ocean model is used to examine the response of the coastal region from 22.5°S to 35°S of the Chile Current System to both equatorward and climatological wind forcing. The results from both types of forcing show that an equatorward surface current, a poleward undercurrent, upwelling, meanders, filaments and eddies develop in response to the predominant equatorward wind forcing. When climatological wind forcing is used, an offshore branch of the equatorward surface current is also generated. These features are consistent with available observations of the Chile Current System. The model results support the hypothesis that wind forcing is an important mechanism for generating currents, eddies and filaments in the Chile eastern boundary current system and in other eastern boundary current regions which have predominantly equatorward wind forcing.  相似文献   

9.
Past observations and theories have indicated the importance of the constitution of the lowest-mode of shelf waves to the velocity field. However, significant contributions of the higher mode waves to current velocity fluctuations in the vicinity of the coast are suggested in observational results obtained along the Fukushima coast in Pt. I of this study (Kubota et al., 1981). To understand the importance of the higher modes, the generation of shelf waves is investigated theoretically by two methods. First, the generation of long shelf waves by monochromatic forcing is examined, and it is concluded that near the coast the second mode's contribution to the longshore velocity is the largest for the Fukushima coast. Second, the response of shelf waves to broad-band forcing is investigated by taking the dispersive characteristics of shelf waves into consideration. It is concluded that shelf waves with zero group velocity are selectively excited if the forcing has a broad-band spectrum. According to observational results obtained along the Fukushima coast, the wind spectrum has a broad peak at about 100 hours (Kubota et al., 1981). Since the third mode of shelf waves has zero group velocity around the period of 100 hours, the third mode can be selectively generated off the Fukushima coast. From this it is suggested that the Fukushima coast is in the forced region and that observed current fluctuations are motions associated with the second- and third-mode shelf waves.  相似文献   

10.
The results of a hydrological survey of the Karamea Bight, on the west coast of the South Island, are presented and discussed. The hydrology of the region was dominated by the Westland Current, the river outflow and wind‐induced upwelling. The existence of a southwards flowing undercurrent associated with the upwelling is postulated at 150 m depth over the continental slope, and the name of Westland Undercurrent is proposed for this feature.  相似文献   

11.
Yoshida (1967) pointed out that the coastal upwelling region may not coincide with the intense longshore wind region and shift poleward. In order to clarify this poleward shift from the existing data, the monthly mean distributions of the offshore Ekman transport and the coastal upwelling intensity are estimated along the California coast from U. S. Daily Weather Maps and from the CCOFI data in 1949, respectively. The results show that the center of the coastal upwelling region is generally shifted to the north from the position of the maximum offshore Ekman transport. The detailed discussions are given for the case of August 1949 when the shift is seen most clearly.  相似文献   

12.
Equilibrium conditions in anf-plane ocean evolve as follows after the sudden onset of winds parallel to a coast. At first the flow is two-dimensional-spatial variations are confined to a plane perpendicular to the coast-and the salient features in the forcing region are acceleration of a coastal jet in the surface layers in the wind direction, and offshore Ekman drift that causes coastal upwelling. Kelvin waves excited at the edge of the forced region establish equilibrium conditions by creating an alongshore pressure gradient that balances the wind so that the acceleration stops. The vertical structure corresponding to each vertical mode differs from that of the wind-driven coastal jet so that the arrival of the barotropic Kelvin wave starts to accelerate a coastal undercurrent in a direction opposite to that of the wind. Subsequent baroclinic Kelvin waves modify the vertical structure of the coastal current so that the undercurrent in the subsurface layer is accelerated. In an inviscid model there is a singularity in the surface layers at the coast ast→∞ because the Kelvin modes with small offshore and vertical scales travel slowly and take a very long time to make their contribution to the establishment of equilibrium conditions. A modest amount of friction eliminates this problem. Nonlinearities are important in the heat equation and affect sea surface temperatures significantly but their effect on the momentum balance is secondary.  相似文献   

13.
The abrupt depth increase which characterises the edge of many continental shelves determines a reduced horizontal length scale and a localised transition from shelf seas to the deep ocean. Particular forms of motion which may arise from the steep slopes include topographically guided currents along the slope, shelf-break upwelling, topographic Rossby waves and internal lee waves in the tidal current. The ocean/shelf mismatch may lead to a clear separation of water types, substantial reflection (from the shelf-edge neighbourhood) of all oceanic and shelf motions with periods greater than a few hours, and interaction between barotropic and baroclinic motions. Unstable longshelf currents, interleaving water masses, strong internal tides and internal waves, and narrow canyons enhance mixing across the shelf edge.  相似文献   

14.
Runoff from abundant rainfall on the watersheds along the east coast of Nicaragua results in a well-defined nearshore current, extending 20–40 km out from the coast. Important terms in the controlling dynamical balance are the Coriolis forces, and eddy frictional forces. Calculations of the diabathic surface slope 0(10?5) show a smooth setup of 4–5 cm from the outer edge of the current to the shoreline. A longshore surface slope of 0(10?8) appears to be set up by the longshore wind stress, and further computations allow an estimate of ～ 6 gr cm?1 s?1 for the dynamic eddy viscosity. An analytical expression including diabathic surface slope and density gradient, parabathic surface slope, wind stress, and quadratic bottom friction reproduces the salient observed features of the nearshore current. These include the pronounced maximum in the parabathic (along-shelf) current about 10 km off the coast, a complex diabathic velocity structure, and a shelf countercurrent just seaward of the outer edge. Further calculations suggest that the dominant driving arises from the freshwater-induced density gradients, accounting for upward of 80% of the flow velocity. As suggested by Royer (1982), the prevailing trade wind exerts an onshore wind stress that serves the important role of maintaining the integrity of the density gradients via the convergence of a surface Ekman layer toward the coast.  相似文献   

15.
Continuous CTD data from a series of recent cruises show that the distribution of the water mass characteristics in the central Benguela region from the Orange River mouth (28°38'S) to alvis Bay (22°57'S) is discontinuous in the central and intermediate waters at about the latitude of Lüderitz (26°40'S), Namibia. The central and intermediate water masses at the shelf edge and shelf break north of the Lüderitz upwelling cell have a high salinity relative to the potential temperature compared to similar waters south of the upwelling cell. It is shown that the feed waters for the wind-induced upwelling on the shelf to the north and south of the Lüderitz discontinuity are different in character and source. The distribution of the water masses shows that the shelf-edge poleward undercurrent provides low-oxygen water from different regions in the Atlantic Ocean to be upwelled onto the shelf. North of th Lüderitz upwelling cell, the central and intermediate waters come from the oxygen-depleted Angola Basin, whereas south of the discontinuity those waters are from the interior of the adjacent Cape Basin, which is less oxygen-deficient. This has implications for the dispersion of low-oxygen water and the triggering of anoxic events, and consequences for the biota on the shelf, including commercially important fish species.  相似文献   

16.
The textural and compositional characteristics of the surficial shelf sediments north and south of the Orange River Delta are reviewed and compared. Sediments are fractionated and dispersed both north- and southwards of the Orange River mouth by wave action, longshore drift and subsurface currents. The mean grain size decreases both offshore and southwards in response to decreased wave influence at the seabed and the competence of the weak poleward undercurrent respectively. The increasing dominance of marine biogenic components in sediments south of the prodelta indicates a greater marine influence, modifying previous inferences that the Namaqualand mudbelt is primarily derived from the southward transport of Orange River sediments. A sharp distinction can be drawn between sediments of the Orange Shelf to the south and the Walvis Shelf to the north. Foraminiferarich deposits that dominate the Orange middle shelf and slope indicate that upwelling is an inner-shelf phenomenon. On the Walvis Shelf, foraminiferal sediments are confined to the slope and outer shelf. Fish debris is more common in Walvis Shelf sediments. Although phosphorite and glauconite sands often occur together in the same deposits on the Orange Shelf, the two minerals are concentrated in separate deposits on the Walvis Shelf.  相似文献   

17.
Toxic Gymnodinium catenatum blooms usually occur in the Galician Rias at the end of the upwelling season, which necessitates a ban on harvesting shellfish extraction, with subsequent economic losses for this sector. One of the possible causes cited in the literature is the advection of populations from outside the area but no evidence was available to substantiate this.Oceanographic conditions at the end of the upwelling season in the NW coast of the Iberian Peninsula (39°–43°N) have been studied for the years 1986, 1990, 1995 and 1998. Sea surface temperature data from satellite images, wind data, drifter tracks and in situ oceanic data from the Galician Rias have been combined to clarify the oceanographic structures in the area at the commencement of the blooms. On the inner shelf, an inshore poleward current advecting warm water has been identified after the cessation of upwelling. On the middle and outer shelf, a tongue of cold water as a remnant of the previous upwelling continued to move southward. On the slope and offshore, the poleward counter current reported by several authors was detected carrying warm oceanic water northwards.It is suggested that the inshore poleward current, not previously reported in the literature, could advect initial populations of dinoflagellates to the Rias from northern Portuguese waters. This would explain why blooms such as G. catenatum have been found usually in Portuguese waters several weeks before the Galician Rias, showing an apparent northward movement, but cells of this toxic dinoflagellate species have not been found in waters of the offshore poleward counter current.  相似文献   

18.
普里兹湾水文特征与变化   总被引：2，自引：1，他引：2  
根据澳大利亚学者分别在１９８２年１２月和１９８７年２月普里兹湾的调查资料,并结合中国＂极地＂号南极考察船１９９０～１９９１年间的调查资料印证,提出在中国＂中山站＂所在地普里兹湾在初夏与晚夏期间的水文特征。  相似文献   

19.
High primary productivity on the Pacific coast of the Baja California Peninsula is usually related to coastal upwelling activity that injects nutrients into the euphotic zone in response to prevailing longshore winds (from the northwest to north). The upwelling process has maximum intensity from April to June, with the coastal upwelling index varying from 50 to 300 m3/s per 100 m of coastline. Along the entire coast of the peninsula, the upwelling intensity changes in accordance with local wind conditions and bottom topography. Spatial variability can also be modulated by the influence of mesoscale meanders of the California Current. We have identified the seasonal and synoptic variability of upwelling signatures on the Baja California shelf, using averaged monthly and weekly sea surface temperature (SST) distributions obtained from remote sensing imagery from the Advanced Very High Resolution Radiometer in the period from 1996 to 2001. Analysis of SST distribution and direct experimental data on temperature and nutrient concentration shows that the areas with the coldest SST anomalies were closely related to the bottom slope, shelf width, and coastline orientation relating to wind direction. We also assume that the nutrient transport into the coastal lagoons may be forced by the coupling of coastal upwelling and tidal pumping of surface waters into the lagoon system. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

20.
The intensification of upwelling front and two-cell circulation is studied numerically in a two dimensional level model. Upwelling front is set initially with longshore geostrophic flow. The uniform wind stress forces the ocean which has an infinite north-south coast line. Two-cell circulation, downwelling just inshore-side of the front and upwelling offshore-side, is induced, and the front is intensified. It is found that the intensification is occurred in the inshore-side of the front, and the intensification is basically due to the deviation from the thermal-wind balance, as is shown bySuginohara (1977). It is found that the inshore-side cell intrudes under the pycnocline. It seems to reproduce the observed two-cell circulation.  相似文献   

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