大洋环流的诊断计算

采用二维的全球高分辨率（1／4°×1／4°）的自由表面诊断模型结合动力计算估算全球大洋环流,模拟结果与其他模拟结果非常相似。流函数的分布表明,全球大洋中的主要流系均得到体现,包括大洋环流的西向强化的现象（黑潮、湾流等）。黑潮主轴的流量约54Sv（1Sv=10^6m^3／s）,非常接近实测值：各层水平流场分布情况显示,各大洋的一些基本流系都能得到很好的再现。如黑潮和南极绕极流可深达底层。湾流不能到达深层,大约在1000～2000m之间海流即已转向。     

Formation of extreme surface turbulent heat fluxes from the ocean to the atmosphere in the North Atlantic

The role of extreme surface turbulent fluxes in total oceanic heat loss in the North Atlantic is studied. The atmospheric circulation patterns enhancing ocean–atmosphere heat flux in regions with significant contributions of the extreme heat fluxes (up to 60% of the net heat loss) are analyzed. It is shown that extreme heat fluxes in the Gulf Stream and the Greenland and Labrador Seas occur in zones with maximal air pressure gradients, i.e., in cyclone–anticyclone interaction zones.     

Low-frequency variations of the Gulf-Stream transport: description and mechanisms

On the basis of the contemporary array of oceanographic and hydrometeorological data, we compute the characteristics of variations of the Gulf-Stream transport in 1950–2004. The role played by the low-frequency oscillations of vorticity of the wind field and turbulent heat fluxes in the North Atlantic in the formation of the analyzed variations is estimated. We reveal a significant (on a 5% confidence level) positive linear trend of the monthly average Gulf-Stream transport manifested in the increase in the Gulf-Stream transport by 13 Sv for the investigated period. On the basis of the established estimates, we make a conclusion that about a quarter of the interannual variations of the Gulf-Stream transport is caused by the low-frequency oscillations of vorticity of the wind field in the Subtropical Atlantic. Moreover, the Gulf-Stream transport is delayed relative to the wind oscillations by about 2 yr. An important role in the changes in the Gulf-Stream transport is played by the response of the system of west boundary currents to the quasiperiodic action of turbulent heat fluxes on the surface of the ocean connected with the North-Atlantic Oscillation. The intensification of turbulent heat fluxes in the Northern Subpolar Cyclonic Gyre and their weakening in the north part of the Subtropical Anticyclonic Gyre are accompanied by the intensification of the Gulf Stream observed after 3–5 yr. The anomalies of turbulent heat fluxes of the opposite sign are followed by weakening of the Gulf Stream also after a period of 3–5 yr. We also mention a potentially important role played the Pacific decadal oscillation in maintaining the decadal variations of the intensity of Gulf Stream. The influence of this oscillation on the Gulf-Stream transport is realized both via the changes in the wind field in different phases of oscillations and due to its influence on the heat exchange of the ocean with the atmosphere.     

Response of the Gulf Stream transport to characteristic high and low phases of the North Atlantic Oscillation

We investigate an ambiguity in the current understanding of the Gulf Stream (GS) transport in response to the North Atlantic Oscillation (NAO). While some investigations (discussed herein) suggest enhanced transport during low NAO phases, other studies suggest enhanced transport in high NAO phases. NAO-induced variability in the western North Atlantic is studied by using a 1/6°-resolution basin-scale Regional Ocean Modeling System (ROMS) model. Results indicate that the western boundary current limb of the GS, upstream of Cape Hatteras, exhibit enhanced transport during low-NAO phases. However, further downstream of Cape Hatteras, after the GS separates from the coast, diminished GS transport is seen during low-NAO phases. The converse is true for high NAO phases for both segments of the GS system. Model results show the Deep Western Boundary Current (DWBC), the northern recirculation gyre and the southern recirculation gyre intensify (weaken) during the high (low) NAO periods.     

Numerical study of the storm-induced circulation on the Scotian Shelf during Hurricane Juan using a nested-grid ocean model

A nested-grid ocean circulation modelling system is used to assess the upper ocean response of the Scotian Shelf and adjacent slope to Hurricane Juan in September 2003. The nested-grid system consists of a fine-grid inner model covering the Scotian Shelf/slope and a coarse-grid outer model covering the northwest Atlantic Ocean. The model-calculated upper ocean response to Hurricane Juan is characterized by large divergent surface currents forced by the local wind forcing under the storm, and intense near-inertial currents in the wake of the storm. The sea surface temperature (SST) cooling produced by the model is biased to the right of the storm track and agrees well with a satellite-derived analysis. Over the deep water, off the Scotian Shelf, some of the near-inertial energy input by the storm is advected eastward by the Gulf Stream away from the storm track. The hurricane also generates shelf waves that propagate equatorward with the coastline on their right. In comparison with the outer model results, the inner model captures more meso-scale structures, greater SST cooling and stronger near-inertial currents in the study region.     

Continental slope sea level and flow variability induced by lateral movements of the Gulf Stream in the Middle Atlantic Bight

As described by [Csanady, G.T., Hamilton, P., 1988. Circulation of slope water. Continental Shelf Research 8, 565–624], the flow regime over the slope of the southern Middle Atlantic Bight (MAB) includes a current reversal in which southwestward flow over the upper and middle slope becomes entrained in the northeastward current adjacent to the Gulf Stream. In this paper we use satellite-derived data to quantify how lateral motions of the Gulf Stream impact this current system. In our analysis, the Gulf Stream’s thermal front is delineated using a two-year time series of sea surface temperature derived from NOAA/AVHRR satellite data. Lateral motions of the Gulf Stream are represented in terms of temporal variations of the area, east of 73°W, between the Gulf Stream thermal front and the shelf edge. Variations of slope water flow within this area are represented by anomalies of geostrophic velocity as derived from the time series of the sea level anomaly determined from TOPEX/POSEIDON satellite altimeter data. A strong statistical relationship is found between Gulf Stream displacements and parabathic flow over the continental slope. It is such that the southwestward flow over the slope is accelerated when the Gulf Stream is relatively far from the shelf edge, and is decelerated (and perhaps even reversed) when the Gulf Stream is close to the shelf edge. This relationship between Gulf Stream displacements and parabathic flow is also observed in numerical simulations produced by the Miami Isopycnic Coordinate Model. In qualitative terms, it is consistent with the notion that when the Gulf Stream is closer to the 200-m isobath, it is capable of entraining a larger fraction of shelf water masses. Alternatively, when the Gulf Stream is far from the shelf-break, more water is advected into the MAB slope region from the northeast. Analysis of the diabathic flow indicates that much of the cross-slope transport by which the southwestward flow entering the study region is transferred to the northeastward flow exiting the region occurs in a narrow band roughly centered at 36.75°N, order 150 km north of Cape Hatteras. This transport, and thus the cyclonic circulation of the southern MAB, strengthens when the Gulf Stream is relatively close to the shelf edge, and weakens when the Gulf Stream is far from the shelf edge.     

Float trajectories in the deep western boundary current and deep equatorial jets of the tropical Atlantic

Fourteen neutrally buoyant SOFAR floats at a nominal depth of 1800 m were tracked acoustically for 3.7 yr in the vicinity of the western boundary and the equator of the Atlantic Ocean. The trajectories revealed a swift, narrow, southward-flowing deep western boundary current (DWBC) extending from 7N across the equator. Two floats crossed the equator in the DWBC and went to 10S. Two other floats left the DWBC and drifted eastward in the equatorial band (3S–3N). Three floats entered the DWBC from the equatorial current system and drifted southward. These results suggest that at times the DWBC flows directly southward across the equator with a mean velocity of 8–9 cm/s averaged over long distances (∼2800 km). At other times DWBC water is diverted eastward near the equator for long periods (2–3 yr), which can reduce the mean along-boundary velocity to 1–2 cm/s. This is much less than the instantaneous along-boundary velocities in the DWBC, which are often above 25 cm/s and occasionally exceed 50 cm/s. Mean eastward-flowing jets were observed near 2N and 2S bounding a mean westward jet centered on the equator (1S–1N). The southern jet at 2S coincides with a CFC-rich plume centered south of the equator. The CFC plume is inferred to have been advected by the southern jet across the Atlantic and into the Gulf of Guinea.     

The connection between Labrador Sea buoyancy loss, deep western boundary current strength, and Gulf Stream path in an ocean circulation model

The sensitivity of the North Atlantic gyre circulation to high latitude buoyancy forcing is explored in a global, non-eddy resolving ocean general circulation model. Increased buoyancy forcing strengthens the deep western boundary current, the northern recirculation gyre, and the North Atlantic Current, which leads to a more realistic Gulf Stream path. High latitude density fluxes and surface water mass transformation are strongly dependent on the choice of sea ice and salinity restoring boundary conditions. Coupling the ocean model to a prognostic sea ice model results in much greater buoyancy loss in the Labrador Sea compared to simulations in which the ocean is forced by prescribed sea ice boundary conditions. A comparison of bulk flux forced hindcast simulations which differ only in their sea ice and salinity restoring forcings reveals the effects of a mixed thermohaline boundary condition transport feedback whereby small, positive temperature and salinity anomalies in subpolar regions are amplified when the gyre spins up as a result of increased buoyancy loss and convection. The primary buoyancy flux effects of the sea ice which cause the simulations to diverge are ice melt, which is less physical in the diagnostic sea ice model, and insulation of the ocean, which is less physical with the prognostic sea ice model. Increased salinity restoring ensures a more realistic net winter buoyancy loss in the Labrador Sea, but it is found that improvements in the Gulf Stream simulation can only be achieved with the excessive buoyancy loss associated with weak salinity restoring.     

Attribution of Deep Western Boundary Current variability at 26.5°N

Observed variations in the Deep Western Boundary Current (DWBC) at 26.5°N, which carries the deep limb of the Atlantic Meridional Overturning Circulation (MOC), have been shown to greatly exceed in magnitude the variations of the overall basin-wide MOC, with strong variability at a range of time scales from weeks to multiple-months. Attribution of these strong DWBC variations will be crucial for understanding variations in the MOC itself. Nevertheless, despite many years of moored observations of the DWBC at 26.5°N, understanding of these variations has been elusive. Two years of observations from a high horizontal resolution array of pressure-equipped inverted echo sounders are used together with output from a modern high-resolution numerical model to investigate the mechanisms behind these ±20×10⁶ m³ s⁻¹ volume transport variations. The model and observational results together suggest that the strongest variations cannot be explained solely via either of the two most commonly proposed mechanisms – meandering or pulsation of the DWBC. The dominant mechanism appears to be propagation of Rossby Wave-like structures into the region from the east, and it is the impact of these features in the region that yield the largest transport anomalies. These waves have been observed and discussed in the past – however their key role as the dominant source of DWBC variability has not previously been recognized. The implications of these results are also discussed in the context of future observing systems for the DWBC.     

The structure of shelf and gulf stream motions in the Georgia bight

The structure of the current and temperature fields along 30°N over the mid-shelf and western Blake Plateau in the South Atlantic Bight has been investigated by combining two moored instrument experiments in the summer of 1981. The shelf moorings were part of the second Georgia Bight Experiment (GABEX-II) and the Gulf Stream mooring data on the Blake Plateau have been described by LEE and WADDELL (1983). Empirical Orthogonal Functions (EOF) in the frequency domain are used to extract shelf and Gulf Stream coherent current and temperature fluctuations in the two- to 14-day period band. Three modes are found, of which the first two are interpreted as Gulf Stream meander and frontal eddy circulations. The difference between them is chiefly in the shelf motions; the first mode is primarily restricted to the shelf edge, whereas the second mode penetrates to the 40m isobath. The third mode dominates at mid-shelf and is the only mode that shows strong coherence with the windstress and local sea-level fluctuations. The relationship of the modes to the occurrence of mid- and inner-shelf cold sub-surface intrusions, generated by shelf-edge Gulf Stream frontal eddies, is examined. All three modes are found to play a role in the initiation, growth and decay of these structures.     

利用GOCE重力场模型确定全球稳态海面地形及表层地转流

稳态海面地形(MDT)是大地测量学家和海洋学家共同关心的一个重要物理量。该文基于WHU2009全球平均海面高模型和GO_CONS_GCF_2_TIM_R3纯GOCE重力场模型,采用几何法经高斯滤波处理后确定了全球稳态海面地形,与CLS09及DTU10M DTs相比,其差值均方根RMS均小于8 cm,表明该文结果具有较高的精度;根据地转流方程计算了相应的表层地转流,与GRACE重力场模型GGM03S结果相比,GOCE重力场模型所确定的表层地转流在墨西哥湾流、黑潮及厄加勒斯海流等海域均体现了更强的流速和更多的细部特征,验证了GOCE在洋流探测中的优势。     

Monitoring the deep western boundary current in the western North Pacific by echo intensity measured with lowered acoustic Doppler current profiler

Oxidation of iron and manganese ions is predominant in the oxygen-rich deep western boundary current (DWBC) within the Pacific Ocean. By the faster removal of particulate iron hydroxide and manganese oxide, densities of the particulate matters are considered to be lower in the DWBC than the interior region. To detect the density variation of suspended particles between the DWBC and interior regions, we analyzed echo intensity (EI) measured in the western North Pacific by hydrographic casts with a 300 kHz lowered acoustic Doppler current profiler (LADCP) in a whole water column. At depths greater than 3000 m (~ 3000 dbar), EI is almost uniformly low between 12°N and 30°N but peaks sharply from 30°N to 35°N to a maximum north of 35°N. EI is found to be anomalously low in the DWBC compared to the background distribution. The DWBC pathways are identifiable by the low EI and high dissolved oxygen concentration. EI data by LADCPs and other acoustic instruments may be used to observe the temporal variations of the DWBC pathways.

     

Influence of the interaction between the Atlantic and the Arctic Ocean on the Gulf Stream

Numerical experiments with the circulation model of the North Atlantic based on the splitting algorithms in the σ-coordinate system with a spatial resolution allowing for reproducing synoptic eddies were performed in two versions: with the Arctic Ocean and without it (boundary along 78°N). They showed that the account for the water exchange with the Arctic is fundamentally important for reproducing jet dynamics at the western boundary of the Atlantic down to the subtropical zone. The influence of the conditions at the liquid boundary that separates the Atlantic and the Arctic extends not only over the subarctic area [29] but is also “transferred” by the Labrador Current and the Slope Water Current (SWC) to the area of the Gulf Stream proper. One cannot properly describe the detachment of the Gulf Stream from the coast without adequate reproducing of the Labrador Current and SWC. An hypothesis is posed that the location of the detachment region at 35°N is caused by strong vertical motions at the interface between the SWC and the Gulf Stream jet with horizontal velocities that are almost equal to those at the exit from the Florida Strait. A comparison of the model circulation with that retrieved from the hydrological data and the drift of neutral buoyancy floats [14, 22] showed both qualitative and quantitative coincidences of the features of the northward warm water transfer such as the streamline around the so-called northwestern “corner” (motion “along the topography”) and the jet-wise transport of these waters from Labrador to the northeast inside a kind of “pipeline,” which is limited in the upper baroclinic layer 1 km thick by mean velocity contour lines of about 10 cm/s. A comparison between the experimental [19] and model fields of the ocean level showed that, at the absence of direct representation of the water (mass) exchange between the Atlantic and the Arctic Ocean, the decrease of the gradient velocities in the Gulf Stream may reach 30%.     

Dissolved iodine across the Gulf Stream Front and in the South Atlantic Bight

The distributions of iodide, iodate and total iodine were determined along a transect from the Sargasso Sea and across the Gulf Stream to the continental shelf of the South Atlantic Bight during November 1990. The western boundary of the Gulf Stream at the outer shelf-upper slope was characterized by steeply sloping isotherms and isopleths of iodide and iodate, resulting from a dome of cold water that was rich in iodate and nearly devoid of iodide at the slope. Both the mid and the inner shelf were relatively well mixed vertically. The concentration of iodate in the surface waters decreased shoreward from >0.3 μM in the Sargasso Sea/Gulf Stream/outer shelf, to 0.29 μM in the midshelf, 0.19 μM in the outer-inner shelf and 0.11 /IM in the inner-inner shelf. Concomitantly, the concentration of iodide increased from <161 nM to 175 nM, 257 nM and 300 nM. The concentration changes were more abrupt in the inner-inner shelf within about 30 km from the shore. There was no evidence of significant concentrations of organic iodine. These distributions of iodide and iodate suggest that the South Atlantic Bight may act as a geochemical processor of dissolved iodine. Iodate is added to the shelf during topographically induced upwelling and frontal exchange with the Gulf Stream. In the shelf waters, iodate is reduced to iodide in situ. Iodide is exported from the shelf to the Gulf Stream which may eventually further transport it to the ocean interior. A ☐ model calculation suggests that 28% and 43% of the iodate added to the Bight and the inner shelf, respectively, are converted to another form in these waters, almost all of which is iodide. About a third of the reduction of iodate to iodide in the Bight occurs in the inner shelf. Thus, the inner shelf may be the most geochemically active zone within the Bight. The residence times of iodide relative to its production and that of iodate relative to its removal are 3.1 and 3.6 months in the Bight and 0.9 and 1.8 months in the inner shelf.     

On the three-dimensional numerical modeling of the deep circulation around Ángel de la Guarda Island in the Gulf of California

A three-dimensional nonlinear baroclinic model is used to model the circulation in the Ballenas Channel, Gulf of California, México, which was inferred from current meter observations over three sills that surround the area. The suggested circulation consists of deep inflow that follows two paths: the first one is a direct spill of water through San Lorenzo sill into Ballenas Channel, the second one, a larger route that starts at San Esteban sill, then flows north of the island passing over Tiburón and Delfín basins, and then turns to the south reaching the North Ballenas Channel sill and then spills into Ballenas Channel. Following the latter result, a previous modeling effort to reproduce the circulation was partially obtained, the long path was not reproduced and it was believed that finer horizontal resolution was needed. In this work, the bathymetric resolution was increased by a factor of three and the full path of this deep circulation is now obtained and corroborated.     

Vertical transport of particulate organic matter in the deep Bering Sea and gulf of Alaska

Sediment trap arrays were deployed at two deep ocean stations, one in the Bering Sea and the other in the Gulf of Alaska, in the summer of 1975. The sediment trap was constructed of a pair of polyethylene cylinders (0.185 m² opening) with funnel-shaped bases. The trap is equipped with a lid which is closed before recovery by a tripping messenger system triggered by an electric time release. 37–68% of the total organic carbon fluxes (37–38% in the Bering Sea; 48–68% in the Gulf of Alaska) were represented by large particles (67µm<) such as fecal matter and fecal pellets which contributed minor fractions to the total particulate organic matter concentration in sea water. The total fluxes were 11.1 and 14.2 mg C m^–2d^–1 at 1,510 and 2,610 m respectively at the station (3,800 m) in the Bering Sea, and were 7.60, 4.66 and 3.27 mg C m^–2d^–1 at 900, 1,500 and 1,875 m respectively at the station (4,150 m) in the Gulf of Alaska. The former values are several times greater than the latter, suggesting that there is a regional variation in the vertical carbon flux in deep layers. The fluxes were approximately equivalent to 1 to 3% of primary productivity in the overlying surface layers. These observations suggest that deep-water ecosystems may be influenced by relatively rapid sinking of large particles such as fecal matter and fecal pellets from near surface production.     

Circulation modelling

Summer upwelling on the continental shelf north of Cape Canaveral, Florida, has been previously observed to result from wind forcing. A two-layer, finite element model reproduces reasonably well the characteristics of the wind-driven upwelling in respect to location and magnitude. Model investigation also shows that upwelling results from offshore current forcing which is imposed through an along-shelf sea level slope. This sea level slope, which has been found to be of the order of −10⁻⁷, represents a mean Gulf Stream effect. The results suggest that the strongest upwelling events near Cape Canaveral occur when the wind and Gulf Stream forcings act together.     

On gyre interactions

The principal meeting point of the subtropical and subpolar gyres of the North Atlantic is at the Tail of the Grand Banks where the two western boundary currents, the Gulf Stream and Labrador Current, join forces as the North Atlantic Current, which flows northeast almost 10° in latitude before turning east as the Subpolar Front, ultimately feeding the Labrador and Nordic Seas and the thermohaline overturning. After the Gulf Stream turns into the North Atlantic Current at the Grand Banks, its role shifts from a wind-driven current to a link in the large-scale thermohaline circulation. The processes governing this transition, in particular the continued transport north of mass and heat, are questions of considerable climatic importance. The North Atlantic Current is a very unusual western boundary current in that its mass transport decreases in the downstream direction.The mean path and annual shifting of the eastward flowing Gulf Stream is conjectured to result from a time-varying shelf-Slope Water overflow of waters from the Labrador shelf. As the volume transport increases in fall and deepens the Slope Water pycnocline, it forces the Gulf Stream south and deepens the Sargasso Sea thermocline as well. The timing of these steps governs the June maximum in baroclinic transport. There is some evidence that this ‘back-door’ gyre interaction may operate on interannual time scales as well. The question then arises whether the shelf-to-Slope Water Sea transport also plays a role in governing the separation of the Gulf Stream.The widely observed robustness of the width of the Gulf Stream appears to result from a tight balance between the release of available potential energy and the kinetic energy of the current. A broader current would release more energy than can be ‘disposed of’, while a narrower current requires more kinetic energy than is available to sustain it. It is shown that for plausible dissipation rates in the recirculation gyres, the amount of energy that needs to be expelled from the Gulf Stream is such a small fraction of that advected through as to be vitually undetectable, hence the stiffness of the current.     

Coupling global and regional circulation models in the coastal Gulf of Alaska

As part of the US GLOBEC NE Pacific program, we are simulating currents in the Coastal Gulf of Alaska (CGOA) to explore sources of interannual and interdecadal variability. To do so, we have developed a coupled modeling system composed of linked regional and global circulation models. The regional model, configured with 13–22 km resolution in the CGOA, is forced at the surface by observed heat fluxes and wind stresses, at the continental boundaries by observed runoff, and at the open ocean boundaries by a combination of tracer climatologies and sub-tidal velocity and tidal elevation provided by a global finite element model. In this communication, we describe the coupled system, including its present method of intermodel coupling, describe a series of multi-year model hindcasts, compare hindcast results with Eulerian and Lagrangian field data obtained in the CGOA in fall 1996, and assess the impact of global information (barotropic sub-tidal velocities and tidal elevations) on the regional model under the present coupling strategy. We find that the regional model produces appropriate current systems (Alaskan Stream, Alaska Coastal Current) and scalar fields, but with mesoscale variability (of SSH and velocities) at somewhat reduced strength relative to data, and with temperature gradients somewhat larger than those observed. Barotropic sub-tidal information from the global model penetrates the regional model interior, supplying additional mesoscale variability, and modifying regional velocity and scalar fields in both shallow and deep areas. Tidal information exerts a significant influence on sub-tidal scalar and velocity structure only in specific shallow areas, where the tides (and tidal mixing) are strongest. Pending the exploration of alternate coupling schemes, we infer from these results that on a time scale of months, purely barotropic information from outside the CGOA will have a modest impact on its mean regional circulation, but a potentially stronger impact on the statistics and details of mesoscale eddies.     

Model estimates for the mean gas exchange between the ocean and the atmosphere under the conditions of the present-day climate and its changes expected in the 21st century

Annual mean fluxes of CO₂ and oxygen across the sea surface are estimated with the use of numerical modeling for several regions located in the Gulf Stream and Kuroshio zones. The present-day climatic conditions and the climatic conditions expected in the middle and at the end of the 21st century are considered. Specific features of gas exchange under a strong wind that are associated with gas exchange by bubbles and with changes in the air-water difference of the gas concentrations were taken into account in the calculations. The estimates obtained differ substantially from the results based on the traditional approach, which disregards the above features. A considerable increase in the absorption of CO₂ by the ocean, which is mainly caused by the continuing increase in the CO₂ concentration in the air during its small changes in the ocean, is expected in the 21st century. At the same time, no trends are revealed in the annual mean fluxes of oxygen across the ocean surface. The conclusion is made that, in calculations of CO₂ absorption by the world ocean, it is necessary to take into account both specific features of gas transfer under a strong wind and an increase in the atmospheric concentration of CO₂.