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1.
A review is presented of the ocean circulation along Australia’s southern shelves and slope. Uniquely, the long, zonal shelf is subject to an equatorward Sverdrup transport that gives rise to the Flinders Current - a small sister to the world’s major Western Boundary Currents. The Flinders Current is strongest near the 600 m isobath where the current speeds can reach 20 cm/s and the bottom boundary layer is upwelling favourable. It is larger in the west but likely intermittent in both space and time due to possibly opposing winds, thermohaline circulation and mesoscale eddies. The Flinders Current may be important to deep upwelling within the ubiquitous canyons of the region.During winter, the Leeuwin Current and local winds act to drive eastward currents that average up to 20-30 cm/s. The currents associated with the intense coastal-trapped wave-field (6-12 day band) are of order 25-30 cm/s and can peak at 80-90 cm/s. Wintertime winds and cooling also lead to downwelling to depths of 200 m or more and the formation of dense coastal water within the Great Australian Bight and the South Australian Sea. Within the Great Australian Bight, the thermohaline circulation associated with this dense water is unknown, but may enhance the eastward shelf-edge, South Australian Current. The dense salty water formed within Spencer Gulf is known to cascade as a gravity current to depths of 200 m off Kangaroo Island. This dense water outflow and meanders in the shelf circulation also fix the locations of a sequence of quasi-permanent mesoscale eddies between the Eyre Peninsula and Portland.During summer, the average coastal winds reverse and surface heating leads to the formation of warm water in the western Great Australian Bight and the South Australian Sea. No significant exchange of shelf water and gulf water appears to occur due to the presence of a dense, nutrient-rich (sub-surface) pool that is upwelled off Kangaroo Island. The winds lead to weak average coastal currents (<10 cm/s) that flow to the north-west. In the Great Australian Bight, the wind stress curl can lead to an anticyclonic circulation gyre that can result in shelf-break downwelling in the western Great Australian Bight and the formation of the eastward, South Australian Current. In the east, upwelling favourable winds and coastal-trapped waves can lead to deep upwelling events off Kangaroo Island and the Bonney Coast that occur over 3-10 days and some 2-4 times a season. The alongshore currents here can be large (∼40 cm/s) and the vertical scales of upwelling are of order 150 m (off Kangaroo Island) and 250 m (off the Bonney Coast).Increasing evidence suggests that El Nino events (4-7 year period) can have a major impact on the winter and summer circulation. These events propagate from the Pacific Ocean and around the shelf-slope wave-guide of West Australia and into the Great Australian Bight. During winter El Nino events, the average shelf currents may be largely shut-down. During summer, the thermocline may be raised by up to 150 m. The nature and role of tides and surface waves is also discussed along with uncertainties in the general circulation and future research.  相似文献   

2.
The distribution and circulation of water masses in the region between 6°W and 3°E and between the Antarctic continental shelf and 60°S are analyzed using hydrographic and shipboard acoustic Doppler current profiler (ADCP) data taken during austral summer 2005/2006 and austral winter 2006. In both seasons two gateways are apparent where Warm Deep Water (WDW) and other water masses enter the Weddell Gyre through the Lazarev Sea: (a) a probably topographically trapped westward, then southwestward circulation around the northwestern edge of Maud Rise with maximum velocities of about 20 cm s−1 and (b) the Antarctic Coastal Current (AntCC), which is confined to the Antarctic continental shelf slope and is associated with maximum velocities of about 25 cm s−1.Along two meridional sections that run close to the top of Maud Rise along 3°E, geostrophic velocity shears were calculated from CTD measurements and referenced to velocity profiles recorded by an ADCP in the upper 300 m. The mean accuracy of the absolute geostrophic velocity is estimated at ±2 cm s−1. The net baroclinic transport across the 3°E section amounts to 20 and 17 Sv westward for the summer and winter season, respectively. The majority of the baroclinic transport, which accounts for ∼60% of the total baroclinic transport during both surveys, occurs north of Maud Rise between 65° and 60°S.However, the comparison between geostrophic estimates and direct velocity measurements shows that the circulation within the study area has a strong barotropic component, so that calculations based on the dynamic method underestimate the transport considerably. Estimation of the net absolute volume transports across 3°E suggests a westward flow of 23.9±19.9 Sv in austral summer and 93.6±20.1 Sv in austral winter. Part of this large seasonal transport variation can be explained by differences in the gyre-scale forcing through wind stress curl.  相似文献   

3.
The first global ocean reanalysis with focus on the Asian-Australian region was performed for the period October 1992 to June 2006. The 14-year experiment assimilated available observations of altimetric sea-level anomaly, satellite SST and quality-controlled in situ temperature and salinity profiles from a range of sources, including field surveys and the Argo float array. This study focuses on dominant circulation patterns in the South-East Asian/Australian region as simulated by an eddy-resolving and data-assimilating ocean general circulation model. New estimates of the ocean circulation are provided which are largely in agreement with the limited number of observations. Transports of key currents in the region are as follows: The total (top-to-bottom) annual mean Indonesian Throughflow transport and its standard deviation are 9.7 ± 4.4 Sv from the Pacific to the Indian Ocean with a minimum in January (6.6 Sv) and a maximum in April (12.3 Sv). The Leeuwin Current along the west coast of Australia is dominated by eddy structures with a mean southward transport of 4.1 ± 2.0 Sv at 34°S. Along the southern coast of Australia a narrow shelf edge current known as the South Australian Current advects 4.5 ± 2.6 Sv eastward at 130°E. The South Australian Current converges east of Tasmania with the eddy-rich extension of East Australian Current. At 32°S this current transports 36.8 ± 18.5 Sv southward. A dominating feature of the circulation between north-eastern Australia and Papua-New Guinea is the strong and quasi-permanent Coral Sea Gyre. This gyre is associated with the highly variable Hiri Current which runs along the south coast of Papua-New Guinea and advects 8.2 ± 19.1 Sv into the Western Pacific Ocean. All of these transport estimates are subject to strong eddy variability.  相似文献   

4.
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5.
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6.
The SAGE iron addition experiment was conducted from R.V. Tangaroa east of South Island, New Zealand, in late March-early April 2004. A desktop survey of climatological data was completed before the experiment, providing information to inform site selection and experiment design. The desktop survey is presented here in updated and enhanced form in order to explain the site selection and describe the conditions expected at the site during the experiment in comparison with those actually encountered.The experiment site was in Subantarctic waters between the Subtropical and Subantarctic Fronts. These waters are characterised by high surface macronutrient concentration, low iron concentration and low chlorophyll. The preferred site based on the desktop survey was in the vicinity of 173.5°E, 47.5°S, in Southern Bounty Trough. The actual release location was chosen immediately before the release and was 112 km to the northwest of this at 172°32′E, 46°44′S. The surface water here has typically come from the southwest (over the northern Campbell Plateau) or the southeast (through Pukaki Gap) and the mean current is directed towards ENE at ∼0.1 m s−1. The release location is well removed from regions of high eddy kinetic energy to the east (where the Subantarctic Front reaches its northern limit) and the west (where fine-scale instabilities develop on the Southland Front, which flows along the continental shelf). Typical conditions at the release site at the end of March are: surface temperature 12 °C; mixed layer depth 40 m; surface chlorophyll concentration ∼0.3 mg m−3; surface photosynthetically active radiation (PAR) 23 E m−2 d−1; surface nutrient concentrations 8-10 mmol m−3 (nitrate), 0.5-0.8 mmol m−3 (phosphate), 1-2 mmol m−3 (silicate) and 0.1-0.5 nM (iron); 99th percentile wind speed 19-21 m s−1. At this time of year, surface PAR is well below its summer maximum, the mixed layer is beginning its seasonal deepening and the silicate concentration is at its seasonal minimum. These factors may have limited the phytoplankton response to iron addition and were compounded in March-April 2004 by strong winds early in the experiment (substantially exceeding the 99th percentile in speed), lower than the average SST, larger than the average mixed layer depth, silicate concentration at the bottom end of the expected range and initially low PAR.  相似文献   

7.
High-resolution data collected southeast of the Canary Islands during late winter 2006 are analyzed to describe the hydrography and three-dimensional circulation in the coastal transition zone off NW Africa. The data are optimally interpolated over a regular grid, the geostrophic velocity field is calculated and the Q-vector formulation of the omega equation is used to compute the quasi-geostrophic (QG) mesoscale vertical velocity. The coastal transition zone is divided into upwelling, frontal and offshore regions with distinct physical and dynamic characteristics. The upwelling region is characterized by cold and weakly stratified waters flowing towards the equator, with a poleward undercurrent of approximately 0.05 m s−1 over the continental slope. The frontal region exhibits a southwestward baroclinic jet associated with cross-shore raising isopycnals; the jet transport is close to 1 Sv, with maximum velocities of 0.18 m s−1 at surface decreasing to 0.05 m s−1 at 300 db. Vertical sections across the frontal region show the presence of deep eddies probably generated by the topographic blocking of the islands to the southward current, as well as much shallower eddies that likely have arisen as instabilities of the baroclinic upwelling jet. The QG mesoscale vertical velocity field is patchy, estimated to range from −18 to 12 m day−1, with the largest absolute values corresponding to an anticyclonic eddy located south of Fuerteventura Island. These values are significantly larger than estimates for other vertical velocities: diapycnal vertical velocities associated with mixing in the frontal region (a few meters per day), and wind-induced vertical velocities (non-linear Ekman pumping arising from the interaction between the wind stress and the background vorticity, maximum values of a few meters per day; linear Ekman pumping due to the divergence of Ekman transport, a fraction of a meter per day; or the coastal constraint in the upwelling region, about 0.7 m day−1). However, the patchiness in both the QG mesoscale vertical velocity and the non-linear Ekman pumping velocity cause their integrated vertical transports to be one order of magnitude smaller than either coastal Ekman transport (0.08 Sv), integrated linear Ekman pumping (−0.05 Sv) or diapycnal transfer (about 0.1–0.2 Sv). The pattern of the near-surface fluorescence field is a good indicator of these different contributions, with large homogeneous values in the coastal upwelling region and a patchy structure associated with the offshore mesoscale structures.  相似文献   

8.
Observations of topographic Rossby waves (TRW), using moored current meters, bottom pressure gauges, and Lagrangian RAFOS floats, are investigated for the deep basin of the Gulf of Mexico. Recent extensive measurement programs in many parts of the deep gulf, which were inspired by oil and gas industry explorations into ever deeper water, allow more comprehensive analyses of the propagation and dissipation of these deep planetary waves. The Gulf of Mexico circulation can be divided into two layers with the ∼800-1200 m upper layer being dominated by the Loop Current (LC) pulsations and shedding of large (diameters ∼300-400 km) anticyclonic eddies in the east, and the translation of these LC eddies across the basin to the west. These processes spawn smaller eddies of both signs through instabilities, and interactions with topography and other eddies to produce energetic surface layer flows that have a rich spectrum of orbit periods and diameters. In contrast, current variability below 1000 m often has the characteristics of TRWs, with periods ranging from ∼10-100 days and wavelengths of ∼50-200 km, showing almost depth-independent or slightly bottom intensified currents through the weakly stratified lower water column. These fluctuations are largely uncorrelated with simultaneous upper-layer eddy flows. TRWs must be generated through energy transfer from the upper-layer eddies to the lower layer by potential vorticity adjustments to changing depths of the bottom and the interface between the layers. Therefore, the LC and LC eddies are prime candidates as has been suggested by some model studies. Model simulations have also indicated that deep lower-layer eddies may be generated by the LC and LC eddy shedding processes.In the eastern gulf, the highest observed lower-layer kinetic energy was north of the Campeche Bank under the LC in a region that models have identified as having strong baroclinic instabilities. Part of the 60-day TRW signal propagates towards the Sigsbee Escarpment (a steep slope at the base of the northern continental slope), and the rest into the southern part of the eastern basin. Higher energy is observed along the escarpment between 89°W and 92°W than either under the northern part of the LC or further south in the deep basin, because of radiating TRWs from the western side of the LC. In the northern part of the LC, evidence was found in the observations that 20-30-day TRWs were connected with the upper layer through coherent signals of relative vorticity. The ∼90° phase lead of the lower over the upper-layer relative vorticity was consistent with baroclinic instability. Along the Sigsbee Escarpment, the TRWs are refracted and reflected so that little energy reaches the lower continental slope and a substantial mean flow is generated above the steepest part of the escarpment. RAFOS float tracks show that this mean flow continues along the escarpment to the west and into Mexican waters. This seems to be a principal pathway for deepwater parcels to be transported westward. Away from the slope RAFOS floats tend to oscillate in the same general area as if primarily responding to the deep wave field. Little evidence of westward translating lower-layer eddies was found in both the float tracks and the moored currents. In the western gulf, the highest deep energy levels are much less than in the central gulf, and are found seaward of the base of the slope. Otherwise, the situation is similar with TRWs propagating towards the slope, probably generated by the local upper-layer complex eddy field, being reflected and forcing a southward mean flow along the base of the Mexican slope. Amplitudes of the lower-layer fluctuations decay from the northwest corner towards the south.  相似文献   

9.
The structure and functioning of nanoplanktonic assemblages in coastal upwelling areas have usually been overlooked in explorations of the productivity of these areas. As part of a multidisciplinary, time-series station in the coastal area off Concepción, seasonal variations (upwelling and non-upwelling) in the abundance and biomass of these assemblages were investigated. Hydrographic measurements and biological samples were taken monthly over a 2-year period (18 August 2004-28 July 2006). Nanoflagellates dominated the total integrated abundance (3-317 × 109 cells m−2; 0-80 m). Diatoms and dinoflagellates usually contributed to a lesser degree (<20%) but sporadically made important contributions to the total integrated nanoplankton biomass (0.02-10.6 g C m−2). Most of the nanoplankton was concentrated in surface waters (<30 m) during all the samplings and no seasonal differences in abundance or biomass were found in this layer, although the mean values and dispersions around them were highest during the upwelling period along with maximum integrated (0-80 m) chlorophyll-a values, as total or in the <20 μm fraction. Changes in nanoplankton abundance were significantly but weakly (r < 0.4) correlated with changes in the hydrographic variables; the highest correlation values were positive for temperature and oxygen, factors that varied with depth and date. The potential grazing rates of heterotrophic nano-predators (flagellates and dinoflagellates) on prokaryotic prey, estimated with a generic model, ranged from 3 to 242 bacterioplankton predator−1 h−1 and from 0.1 to 14 cyanobacteria predator−1 h−1. Our results imply a small impact of seasonal hydrographic variability on the abundance and biomass of nanoplanktonic assemblages and suggest that grazing by nanoheterotrophs might control the prokaryotic picoplankton populations in the upwelling area off Concepción.  相似文献   

10.
Vessel-based observations of the oceanic surface layer during the 14-day 2004 SAGE ocean fertilization experiment were conducted using ADCP, CTD and temperature microstructure in a frame of reference moving with a patch of injected SF6 tracer. During the experiment the mixed layer depth zmld ranged between 50 and 80 m, with several re-stratifying events that brought zmld up to less than 40 m. These re-stratifying events were not directly attributable to local surface-down development of stratification and were more likely associated with horizontal variation in density structure. Comparison between the CTD and a one-dimensional model confirmed that the SAGE experiment was governed by 3-d processes. A new method for estimating zmld was developed that incorporates a component that is proportional to density gradient. This highlighted the need for well-conditioned near-surface data which are not always available from vessel-based survey CTD profiles. A centred-displacement scale, Lc, equivalent to the Thorpe lengthscale, reached a maximum of 20 m, with the eddy-centroid located at around 40 m depth. Temperature gradient microstructure-derived estimates of the vertical turbulent eddy diffusivity of scalar (temperature) material yielded bin-averaged values around 10−3 m2 s−1 in the pycnocline rising to over 10−2 m2 s−1 higher in the surface layer. This suggests transport rates of nitrate and silicate at the base of the surface layer generate mixed layer increases of the order of 38 and 13 mmol/m2/day, respectively, during SAGE. However, the variability in measured vertical transport processes highlights the importance of transient events like wind mixing and horizontal intrusions.  相似文献   

11.
Zooplankton sampling at Station 18 off Concepción (36°30′S and 73°07′W), on an average frequency of 30 days (August 2002 to December 2005), allowed the assessment of seasonal and inter-annual variation in zooplankton biomass, its C and N content, and the community structure in relation to upwelling variability. Copepods contributed 79% of the total zooplankton community and were mostly represented by Paracalanus parvus, Oithona similis, Oithona nana, Calanus chilensis, and Rhincalanus nasutus. Other copepod species, euphausiids (mainly Euphausia mucronata), gelatinous zooplankton, and crustacean larvae comprised the rest of the community. Changes in the depth of the upper boundary of the oxygen minimum zone indicated the strongly seasonal upwelling pattern. The bulk of zooplankton biomass and total copepod abundance were both strongly and positively associated with a shallow (<20 m) oxygen minimum zone; these values increased in spring/summer, when upwelling prevailed. Gelatinous zooplankton showed positive abundance anomalies in the spring and winter, whereas euphausiids had no seasonal pattern and a positive anomaly in the fall. The C content and the C/N ratio of zooplankton biomass significantly increased during the spring when chlorophyll-a was high (>5 mg m−3). No major changes in zooplankton biomass and species were found from one year to the next. We concluded that upwelling is the key process modulating variability in zooplankton biomass and its community structure in this zone. The spring/summer increase in zooplankton may be largely the result of the aggregation of dominant copepods within the upwelling region; these may reproduce throughout the year, increasing their C content and C/N ratios given high diatom concentrations.  相似文献   

12.
利用多源卫星遥感资料及再分析资料,对台风泰利(Talim)对中尺度冷涡的影响进行了分析研究。结果表明,台风泰利过境中尺度冷涡时,冷涡显著增强。最大海面高度异常出现在冷涡中心区域,达到-32.7cm,最大海表温度降幅达到3℃以上,叶绿素平均浓度增长了10倍。通过计算冷涡海域的罗斯贝数、水体拉伸度和离散度等动力学参数,可以进一步发现,台风过境冷涡所在海域时,局地非地转效应和混合效应显著增强,罗斯贝数、水体拉伸和离散度均增加了2倍以上,最大水体拉伸度和离散度发生在冷涡南侧靠近台风路径的区域。本研究对深入理解涡-台风相互作用,以及由此产生的海洋生态环境效应具有重要的意义和作用。  相似文献   

13.
An inverse model of the large scale circulation in the South Indian Ocean   总被引：1，自引：0，他引：1  
An overview of the large-scale circulation of the South Indian Ocean (SIO) (10°S-70°S/20°E-120°E) is proposed based on historical hydrographic data (1903-1996) synthesized with a finite-difference inverse model. The in situ density, potential temperature and salinity fields of selected hydrographic stations are projected on the basis of EOFs. Then the EOF coefficients (the projected values) are interpolated on the model grid (1° in latitude, 2° in longitude) using an objective analysis whose spatial correlation functions are fitted to the data set. The resulting fields are the input of the inverse model. This procedure filters out the small-scale features. Twelve modes are needed to keep the vertical structures of the fields but the first three modes are sufficient to reproduce the large-scale horizontal features of the SIO: the Subtropical Gyre, the Weddell Gyre, the different branches of the Antarctic Circumpolar Current.The dynamics is steady state. The estimated circulation is in geostrophic balance and satisfies mass, heat and potential vorticity conservation. The wind and air-sea heat forcing are annual means from ERS1 and ECMWF, respectively.The main features of the various current systems of the SIO are quantified and reveal topographic control of the deep and bottom circulation. The cyclonic Weddell Gyre, mainly barotropic, transports 45 Sv (1 Sv = 106m3/s), and has an eastern extension limited by the southern part of the Antarctic Circumpolar Current.The bottom circulation north of 50°S is complex. The Deep Western Boundary Currents are identified as well as cyclonic recirculations. South east of the Kerguelen Plateau, the bottom circulation is in good agreement with previous water mass analysis. The comparison between some recent regional analysis and the inverse estimation is limited by the model resolution and lack of deep data.The meridional overturning circulation (MOC) is estimated from the finite difference inverse model. Between 26°S and 32°S the reversal of the current deepens and reaches 1400 m at 32°S. The major part of the deep meridional transport at 32°S is located between the African coast and the Madagascar Ridge, carried by the Agulhas Undercurrent. The mean value for this meridional thermohaline recirculation is 8.8 ± 4.4 Sv between 26°S and 32°S. The Agulhas Undercurrent (11 Sv) is associated with a weak Agulhas Current (55 Sv). The MOC is thus trapped in the western margin of the Southwest Indian Ridge. The corresponding vertical velocity along 32°S between 30°E and 42°E is 7.2 × 10−5 ± 8.9 × 10−5 cm s−1. The net meridional heat flux represents −0.53 PW at 18°S and −0.33 PW at 32°S (negative values for southward transports). The intensity of the meridional heat flux is linked to the intensity of the Agulhas Current and to the vertical mixing.  相似文献   

14.
Spatial and temporal characteristics of the water masses and the dispersion of the suspended particulate matter were investigated using current meter, hydrographic and nephelometric observations, gathered during the ECOFER experiment (1989–1991) in the Cap-Ferret Canyon on the Aquitanian margin of the Bay of Biscay. While characteristics of the deep water masses were stable from one year to another, large hydrographic change in the upper 500 m related to winter renewal induced by poleward advection of warm and saline water along the continental slope. The slope circulation and seasonal eddy activity appear as important dynamical mechanisms that control the entrainment and the dispersion of the suspended particulate matter from the neritic domain to the deep ocean. A predominantly northward along-slope current with occasional reversal characterizes this circulation. The nephelometric structures also showed seasonal changes in the overall suspended particulate matter content, but recurrent features, such as the presence of intermediate nepheloid layers at the shelf-break depth and various depths along the slope (∼500, 1000 and 2000 m), were observed. These nepheloid layers extended off the slope to about 10–30 km, but especially laterally along the slope. Their presence indicated that suspended particulate matter exchanges between the shelf and the slope occurred mainly in the head of the canyon and along the southern open slope. The intermediate nepheloid layers around 500 m depth detached from the slope particularly in regions where the bottom slope is close to critical for the M2 internal tide.  相似文献   

15.
We use a simple approach to estimate the present-day thermal regime along the northwestern part of the Western Indian Passive Margin, offshore Pakistan. A compilation of bottom borehole temperatures and geothermal gradients derived from new observations of bottom-simulating reflections (BSRs) allows us to constrain the relationship between the thermal regime and the known tectonic and sedimentary framework along this margin. Effects of basin and crustal structure on the estimation of thermal gradients and heat flow are discussed. A hydrate system is located within the sedimentary deep marine setting and compared to other provinces on other continental margins. We calculate the potential radiogenic contribution to the surface heat flow along a profile across the margin. Measurements across the continental shelf show intermediate thermal gradients of 38–44 °C/km. The onshore Indus Basin shows a lower range of values spanning 18–31 °C/km. The Indus Fan slope and continental rise show an increasing gradient from 37 to 55 °C/km, with higher values associated with the thick depocenter. The gradient drops to 33 °C/km along the Somnath Ridge, which is a syn-rift volcanic construct located in a landward position relative to the latest spreading center around the Cretaceous–Paleogene transition.  相似文献   

16.
The research vessel Warreen obtained 1742 planktonic samples along the continental shelf and slope of southeast Australia from 1938-42, representing the earliest spatially and temporally resolved zooplankton data from Australian marine waters. In this paper, Warreen observations along the southeast Australian seaboard from 28°S to 38°S are interpreted based on synoptic meteorological and oceanographic conditions and ocean climatologies. Meteorological conditions are based on the NOAA-CIRES 20th Century Reanalysis Project; oceanographic conditions use Warreen hydrological observations, and the ocean climatology is the CSIRO Atlas of Regional Seas. The Warreen observations were undertaken in waters on average 0.45 °C cooler than the climatological average, and included the longest duration El Niño of the 20th century. In northern New South Wales (NSW), week time-scale events dominate zooplankton response. In August 1940 an unusual winter upwelling event occurred in northern NSW driven by a stronger than average East Australian Current (EAC) and anomalous northerly winds that resulted in high salp and larvacean abundance. In January 1941 a strong upwelling event between 28° and 33°S resulted in a filament of upwelled water being advected south and alongshore, which was low in zooplankton biovolume. In southern NSW a seasonal cycle in physical and planktonic characteristics is observed. In January 1941 the poleward extension of the EAC was strong, advecting more tropical tunicate species southward. Zooplankton abundance and distribution on the continental shelf and slope are more dependent on weekly to monthly timescales on local oceanographic and meteorological conditions than continental-scale interannual trends. The interpretation of historical zooplankton observations of the waters off southeast Australia for the purpose of quantifying anthropogenic impacts will be improved with the use of regional hindcasts of synoptic ocean and atmospheric weather that can explain some of the physically forced natural variability.  相似文献   

17.
Using inverse methods a circulation for a new section along 32°S in the Indian Ocean is derived with a maximum in the overturning stream function (or deep overturning) of 10.3 Sv at 3310 m. Shipboard and Lowered Acoustic Doppler Current Profiler (ADCP) data are used to inform the choice of reference level velocity for the initial geostrophic field. Our preferred solution includes a silicate constraint (−312 ± 380 kmol s−1) consistent with an Indonesian throughflow of 12 Sv. The overturning changes from 12.3 Sv at 3270 m when the silicate constraint is omitted to 10.3 Sv when it is included. The deep overturning varies by only ±0.7 Sv as the silicate constraint varies from +68 to −692 kmol s−1, and by ±0.3 Sv as the net flux across the section, driven by the Indonesian throughflow, varies from −7 to −17 Sv with an appropriately scaled silicate flux constraint. Thus, the overturning is insensitive to the size of the Indonesian throughflow and silicate constraint within their apriori uncertainties. We find that the use of the ADCP data adds significant detail to the horizontal circulation. These resolved circulations include the Agulhas Undercurrent, deep cyclonic gyres and deep fronts, features evidenced by long term integrators of the flow such as current meter and float measurements as well as water properties.  相似文献   

18.
On the basis of hydrographic data obtained from 28 November to 27 December, 1998, the three-dimensional structure of circulation in the South China Sea (SCS) is computed using a three-dimensional diagnostic model. The combination of sea surface height anomaly from altimeter data and numerical results provides a consistent circulation pattern for the SCS, and main circulation features can be summarized as follows: in the northern SCS there are a cold and cyclonic circulation C1 with two cores C1-1 and C1-2 northwest of Luzon and an anticyclonic eddy (W1) near Dongsha Islands. In the central SCS there is a stronger cyclonic circulation C2 with two cores C2-1 and C2-2 east of Vietnam and a weaker anticyclonic eddy W2 northwest of Palawan Island. A stronger coastal southward jet presents west of the eddy C2 and turns to the southeast in the region southwest of eddy C2-2, and it then turns to flow eastward in the region south of eddy C2-2. In the southern SCS there are a weak cyclonic eddy C3 northwest of Borneo and an anti-cyclonic circulation W3 in the subsurface layer. The net westward volume transport through section CD at 119.125°E from 18.975° to 21.725°N is about 10.3 × 106 m3s−1 in the layer above 400 m level. The most important dynamic mechanism generating the circulation in the SCS is a joint effect of the baroclinicity and relief (JEBAR), and the second dynamical mechanism is an interaction between the wind stress and relief (IBWSR). The strong upwelling occurs off northwest Luzon.  相似文献   

19.
The Newport Hydrographic (NH) Line along 44.65°N off central Oregon was sampled seasonally during two epochs: 1961-1971 through the TENOC program and 1997-2003 through the GLOBEC Northeast Pacific Long Term Observations Program (LTOP); some observations are available for 2004 and 2005. During TENOC, the line extended 305 km offshore to 128°W, with stations 18 km apart over the continental shelf and 36 km offshore. During LTOP, the line was shorter (to 126°W) with closer station spacing over the continental shelf (9 km apart) and slope (18 km apart). LTOP cruises included biochemical sampling and underway current measurements. During both TENOC and LTOP, the seasonal cycle is very strong (accounting for >50% of the variance in surface layer properties), with rapid transitions in spring and fall. The summer regime is subject to coastal upwelling driven by southward winds, equatorward surface currents, and advection of low-salinity waters from the Columbia River. The winter regime off Newport is subject to coastal downwelling and poleward surface currents driven by northeastward winds. Comparison between TENOC and LTOP summer regimes shows the near-surface layer (0-100 m) at most locations is significantly warmer and fresher during LTOP than TENOC, and steric heights over the continental margin are significantly higher. Comparison of LTOP and TENOC winters shows that average differences at most locations were not statistically significant, but that the variance of steric height and shelf-break temperatures was significantly higher during LTOP than TENOC. Interannual variability of climate indices is also stronger during LTOP, which included a rare Subarctic invasion in 2002 as well as the strong 1997-1998 El Niño. During both TENOC and LTOP, interannual variability of steric height is closely related to the El Niño/La Niña cycle. Nutrient concentrations and nitrate-to-phosphate ratios of upwelling-source waters vary inversely with halocline temperature. Both reflect alongshore advection by coastal currents: southward currents bring cool, nitrate-rich waters in summer (especially during the Subarctic invasion), and northward currents bring relatively warm, nitrate-poor waters to the NH line in winter (especially during El Niño). Seasonal and interannual variations in the nutrient level of upwelling-source water are reflected in time series of vertically-integrated chlorophyll over the LTOP survey region (about 150 km by 300 km). Seasonal variations in chlorophyll and currents are congruent with seasonal variations in copepod biomass and diversity. We were not successful in establishing a clear connection between chlorophyll levels and interannual variations in copepod biomass or diversity, nor in explaining the large decrease in the survival rate of coho salmon between TENOC (6%) and LTOP (3%).  相似文献   

20.
Vertical profiles of temperature and salinity have been measured for 50 years along Line P between the North American west coast and mid Gulf of Alaska. These measurements extend 1425 km into the gulf at 13 or more sampling stations. The 10-50-m deep layer of Line P increased in temperature by 0.9 °C from 1958 to 2005, but is significant only at the 90% level due to large interannual variability. Most of this increase in temperature accompanies the 1977 shift in wind patterns. Temperature changes at 100-150 m and salinity changes in both layers are not statistically significant. Much of the variance in temperature is in the upper 50 m of Line P, and temperature changes tend to be uniform along Line P except for waters on the continental margin. Salinity changes are dominated by variability in the halocline between 100 and 150 m depth and are less uniform along Line P. Largest oscillations in temperature and salinity are between 1993 and 2003. These events can be understood by considering changes in eastward wind speed and wind patterns that are revealed in the first two modes of the Pacific Decadal Oscillation. Changes in these patterns are indicators for both Ekman surface forcing (Surface ocean currents flow to the right of the wind direction) and Ekman pumping (Surface waters diverge away from regions of positive wind stress curl, leading to upwelling of colder saltier water). Changes in temperature along the nearshore part of Line P suggest Ekman surface forcing is the stronger of the two processes in the upper layer. The change in salinity anomalies in the halocline along the seaward end of Line P, following the wind shift in 1977, is in agreement with enhanced upwelling caused by stronger Ekman pumping in this region.  相似文献   

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