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1.
A quantitative estimate of the temperature and salinity variations in the Labrador Sea Water (LSW), the Iceland-Scotland Overflow Water (ISOW), and the Denmark Strait Overflow Water (DSOW) is given on the basis of the analysis of repeated observations over a transatlantic section along 60°N in 1997, 2002, 2004, and 2006. The changes distinguished in the research evidence strong warming and salinification in the layers of the Labrador Sea Water and deep waters at the latitude of the section. The maximum increments of the temperature (+0.35°C) and salinity (+0.05 psu) were found in the Irminger Basin in the core of the deep LSW, whose convective renewal in the Labrador Sea stopped in the mid-1990s. The long-term freshening of the ISOW, which started in the mid-1960s, changed in the mid-1990s to a period of intense stable warming and salinification of this water. By 2005, the salinity in the core of the ISOW in the Iceland Basin increased to the values (～34.99 psu) characteristic of the mid-1970s. In 2002, the warming “signal” of the ISOW reached the Irminger Basin. From 1997 to 2006, the warming and salinification of the columns of the Labrador Sea Water and deep waters became as high as 0.2°C and 0.03 psu, respectively. The character of the long-term variations in the thermohaline properties of the LSW and ISOW from the 1950s evidence that these variations were nearly in-phase and correlated with the low-frequency component of the North Atlantic Oscillation.  相似文献   

2.
Two standard sections across the deep water channel separating the Faroese Plateau from the Scottish continental shelf have been surveyed regularly since the start of the 20th century. There have been significant changes in the characteristics of surface, intermediate and deep water masses during this period. At intermediate depths, the presence of Norwegian Sea Arctic Intermediate Water (NSAIW) was evident as a salinity minimum during the first decade of the century. During the decades 1960–1980 this salinity minimum disappeared, and only four water types were identified in the Channel. Since 1980 the salinity of the intermediate water has again decreased, due to changes in the atmospheric forcing over the Nordic Seas, and it is again evident on a θS curve as a distinct minimum. The salinity of the bottom water in the Channel has also decreased (0.01/decade) linearly since the mid-1970s, although at a slower rate than the intermediate water (0.02/decade). The decline in salinity of the bottom water cannot be accounted for by changes in the salinity of upper Norwegian Sea Deep Water (NSDW), which Faroe Shetland Channel Bottom Water (FSCBW) has traditionally been assumed to be composed of. There is evidence that the upper level of NSDW has become deeper outside the Channel owing to a reduced supply from the Greenland Sea. This has resulted in a change in the composition of FSCBW, from being approximately 60% NSDW during the period 1970–1985 to 40% NSDW since 1990. Thus, the thermohaline circulation of the Nordic Seas has lost its deep water connection. The associated freshening of FSCBW has propagated out through the Channel into the North Atlantic and has resulted in a reduction of the salinity (0.02/decade) and transport (1–7%/decade) of Iceland Scotland Overflow Water (ISOW) into the North Atlantic.  相似文献   

3.
The role of Mediterranean Overflow Water (MOW) in creating subsurface salinity anomalies within the Rockall Trough, a gateway to high latitude areas of deep convection, has been examined closely in recent years. Eulerian investigations of high latitude property fields have suggested that these subsurface anomalies are likely the result of variability in the zonal extent of the eastern limb of the subpolar gyre: when expanded into the eastern North Atlantic, the gyre is presumed to limit the extent to which MOW is able to penetrate northward to subpolar latitudes. However, though the depth of the subsurface salinity anomalies in the Rockall Trough supports the hypothesis that the intermittent presence of MOW is involved in creating the anomalies, MOW pathways to the Rockall Trough have not yet been established. Here, Lagrangian trajectories from floats released in the eastern North Atlantic between 1996 and 1997 and synthetic trajectories launched within an eddy-resolving ocean general circulation model are used to demonstrate that two main density neutral transport pathways lead to the Rockall Trough. One pathway involves the transport of relatively fresh waters as part of the North Atlantic Current and the other involves the transport of relatively salty waters from the eastern reaches of the subtropical North Atlantic. The results from this study indicate that changes in these pathways over time can explain the subsurface salinity variability in the Rockall Trough.  相似文献   

4.
We examine the effect of a northward shift in the position of the southern hemisphere subpolar westerly winds (SWWs) on the vertical and horizontal distribution of temperature and salinity in the world ocean. A northward shift of the SWWs causes a latitudinal contraction of the subpolar gyres in the southern hemisphere (SH). In the Indian and Pacific, this leads to subsurface warming in the subtropical thermocline. As the southern margins of the gyres move into latitudes characterised by warmer surface air temperature (SAT), the layers at mid-depth below 400 m depth become ventilated by warmer water. We characterize the approximation of the ventilated thermocline in our coarse resolution model using a set of passive tracer experiments, and illustrate how the northward shift in the SWWs causes an equatorward shift in the latitude of origin of water ventilating layers deeper than 400 m in the Indian and Pacific, leaving the total surface ventilation of the upper 1200 m unchanged. In contrast, the latitudinal constraint on the Antarctic Circumpolar Current posed by the Drake Passage causes a cooling and freshening throughout the Atlantic thermocline; here, subsurface thermocline water originates from higher latitudes under the wind shift. On longer timescales Atlantic cooling and freshening is reinforced by a reduction in North Atlantic Deep Water (NADW) formation and surface salinification of the Indian and Pacific Oceans. In effect, the latitude of zero wind stress curl in the SWWs regulates the relative importance of the “cold water route” via the Drake Passage and the “warm water route” associated with thermocline water exchange via the Indian Ocean. Thus, a more northward location of the SWWs corresponds with a reduced salinity contrast between the Indian/ Pacific Oceans and the Atlantic. This results in reduced NADW formation. Also, a more northward location of the SWWs facilitates the injection of cool fresh Antarctic Intermediate Water into the South Atlantic subtropical gyre. Beyond these changes, on a millennial timescale, the deep ocean warms throughout the water column in response to the wind shift. Global salinity stratification also becomes less stable, as more saline water remains at the surface and accumulates in the Indian and Pacific thermocline. The freshening of the deep ocean reflects a reduced stirring of the global ocean due to reduced net circulation arising from a misalignment between the westerlies and the topographically constrained ACC. Our results lend support to the idea that a more equatorward location of the SWW maximum during glacial climates contributed to cooler and fresher conditions in the Atlantic, inhibiting NADW.  相似文献   

5.
Hydrographic changes in the Labrador Sea, 1960–2005   总被引：1，自引：0，他引：1  
The Labrador Sea has exhibited significant temperature and salinity variations over the past five decades. The whole basin was extremely warm and salty between the mid-1960s and early 1970s, and fresh and cold between the late 1980s and mid-1990s. The full column salinity change observed between these periods is equivalent to mixing a 6 m thick freshwater layer into the water column of the early 1970s. The freshening and cooling trends reversed in 1994 starting a new phase of heat and salt accumulation in the Labrador Sea sustained throughout the subsequent years. It took only a decade for the whole water column to lose most of its excessive freshwater, reinstate stratification and accumulate enough salt and heat to approach its record high salt and heat contents observed between the late 1960s and the early 1970s. If the recent tendencies persist, the basin’s storages of salt and heat will fairly soon, likely by 2008, exceed their historic highs.The main process responsible for the net cooling and freshening of the Labrador Sea between 1987 and 1994 was deep winter convection, which during this period progressively developed to its record depths. It was caused by the recurrence of severe winters during these years and in its turn produced the deepest, densest and most voluminous Labrador Sea Water (LSW1987–1994) ever observed. The estimated annual production of this water during the period of 1987–1994 is equivalent to the average volume flux of about 4.5 Sv with some individual annual rates exceeding 7.0 Sv. Once winter convection had lost its strength in the winter of 1994–1995, the deep LSW1987–1994 layer lost “communication” with the mixed layer above, consequently losing its volume, while gaining heat and salt from the intermediate waters outside the Labrador Sea.While the 1000–2000 m layer was steadily becoming warmer and saltier between 1994 and 2005, the upper 1000 m layer experienced another episode of cooling caused by an abrupt increase in the air-sea heat fluxes in the winter of 1999–2000. This change in the atmospheric forcing resulted in fairly intense convective mixing sufficient to produce a new prominent LSW class (LSW2000) penetrating deeper than 1300 m. This layer was steadily sinking or deepening over the years following its production and is presently overlain by even warmer and apparently less dense water mass, implying that LSW2000 is likely to follow the fate of its deeper precursor, LSW1987–1994. The increasing stratification of the intermediate layer implies intensification in the baroclinic component of the boundary currents around the mid-depth perimeter of the Labrador Sea.The near-bottom waters, originating from the Denmark Strait overflow, exhibit strong interannual variability featuring distinct short-term basin-scale events or pulses of anomalously cold and fresh water, separated by warm and salty overflow modifications. Regardless of their sign these anomalies pass through the abyss of the Labrador Sea, first appearing at the Greenland side and then, about a year later, at the Labrador side and in the central Labrador Basin.The Northeast Atlantic Deep Water (2500–3200 m), originating from the Iceland–Scotland Overflow Water, reached its historically freshest state in the 2000–2001 period and has been steadily becoming saltier since then. It is argued that LSW1987–1994 significantly contributed to the freshening, density decrease and volume loss experienced by this water mass between the late 1960s and the mid 1990s via the increased entrainment of freshening LSW, the hydrostatic adjustment to expanding LSW, or both.  相似文献   

6.
Growing interest in the dynamics and temporal variability of the deep western boundary current (DWBC) in the northern North Atlantic has led to numerous studies of the modern hydrography and palaeoceanography of this current system. The DWBC is fed by the two dense water-masses that spill over the Greenland–Iceland–Scotland Ridge; Denmark Strait Overflow Water (DSOW) and Iceland Scotland Overflow Water (ISOW). These overflows entrain ambient water masses, primarily Labrador Sea Water (LSW), as they cross the Iceland and Irminger Basins before merging in the vicinity of south-east Greenland. A number of studies have been performed around the Eirik Drift, located off the southern Greenland margin, downstream of this main merging point. However, the relationship between the DWBC and the associated sedimentation at this location has yet to be fully elucidated. New hydrographic data show that the current's main sediment load is carried by only one of its components, the DSOW. Seismic surveys and sediment cores confirm that Holocene sedimentation is limited to areas underlying the most offshore part of the current, where the hydrographic data show the highest concentration of DSOW. Active sedimentation through the Holocene therefore appears to have been controlled by proximity to the sediment-laden DSOW.Our interpretation of new and historic geostrophic transport and tracer data from transects around the southern Greenland margin also suggests that the DWBC undergoes significant growth through entrainment as it flows around the Eirik Drift. We attribute this to multiple strands of ISOW following different depth-dependent pathways between exiting the Charlie Gibbs Fracture Zone and joining the DWBC. Comparison of our new data with other modern hydrographic datasets reveals significant temporal variability in the DWBC, associated with variations in the position, structure and age since ventilation of the current in the vicinity of Eirik Drift. The complexity of the current dynamics in this area has implications for the interpretation of hydrographic and palaeoceanographic data.  相似文献   

7.
Chlorofluorocarbon (CFC) inventories provide an independent method for calculating the rate of North Atlantic Deep Water (NADW) formation. From data collected between 1986 and 1992, the CFC-11 inventories for the major components of NADW are: 4.2 million moles for Upper Labrador Sea Water (ULSW), 14.7 million moles for Classical Labrador Sea Water (CLSW), 5.0 million moles for Iceland–Scotland Overflow Water (ISOW), and 5.9 million moles for Denmark Strait Overflow Water (DSOW). The inventories directly reflect the input of newly formed water into the deep Atlantic Ocean from the Greenland, Iceland and Norwegian Seas and from the surface of the subpolar North Atlantic during the time of the CFC-11 transient. Since about 90% of CFC-11 in the ocean as of 1990 entered the ocean between 1970 and 1990, the formation rates estimated by this method represent an average over this time period. Formation rates based on best estimates of source water CFC-11 saturations are: 2.2 Sv for ULSW, 7.4 Sv for CLSW, 5.2 Sv for ISOW (2.4 Sv pure ISOW, 1.8 Sv entrained CLSW, and 1.0 Sv entrained northeast Atlantic water) and 2.4 Sv for DSOW. To our knowledge, this is the first calculation for the rate of ULSW formation. The formation rate of CLSW was calculated for an assumed variable formation rate scaled to the thickness of CLSW in the central Labrador Sea with a 10 : 1 ratio of high to low rates. The best estimate of these rates are 12.5 and 1.3 Sv, which average to 7.4 Sv for the 1970–1990 time period. The average formation rate for the sum of CLSW, ISOW and DSOW is 15.0 Sv, which is similar to (within our error) previous estimates (which do not include ULSW) using other techniques. Including ULSW, the total NADW formation rate is about 17.2 Sv. Although ULSW has not been considered as part of the North Atlantic thermohaline circulation in the past, it is clearly an important component that is exported out of the North Atlantic with other NADW components.  相似文献   

8.
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9.
Water mass variations in the northeastern Atlantic Ocean along 20°W are analyzed with pentadal resolution over the past 15 years using data from four repeat occupations of a meridional hydrographic section running south from Iceland. The section was sampled in 1988, 1993, 1998, and 2003. The results are interpreted in the context of changes in air–sea forcing, ocean circulation, and water properties associated with the North Atlantic Oscillation (NAO). The NAO index oscillated around zero from 1984 to 1988, was strongly positive from 1989 to 1995, after which it shifted to lower positive, and occasionally negative values from 1996 to 2003. Previously published studies suggest that after the 1995–1996 shift of the NAO, the subpolar gyre largely retreated to the northwest in the northeastern Atlantic Ocean, resulting in an increasingly southeastern character of local water masses with time. Water property changes extending from the SubPolar Mode Water (SPMW) just below the seasonal pycnocline through the density range shared by Mediterranean Outflow Water and SubArctic Intermediate Water (SAIW) along 20°W are consistent with changes in wind-driven ocean circulation and air–sea heat flux associated with shifts in the NAO, especially after accounting for ocean memory. After periods of lower NAO index the SPMW is warmer, saltier, and lighter. At these same times, large increases of apparent oxygen utilization (AOU) and potential vorticity are found at the SPMW base, consistent with SPMW ventilation to lighter densities during lower NAO index periods. Deeper and denser in the water column, the cold, fresh, and dense SAIW signature within the permanent pycnocline that was most strongly present in 1993, near the culmination of a period of high NAO index, is much reduced in 1988 and 1998. In 2003, after a prolonged period of lower NAO index, increasing influence of warmer, saltier subtropical waters is clear within the permanent pycnocline. The deep penetration of the changes implies that they are caused primarily by circulation changes resulting from NAO-associated wind shifts, but changes in air–sea heat flux could also have played a role.  相似文献   

10.
Many of the changes observed during the last two decades in the Arctic Ocean and adjacent seas have been linked to the concomitant abrupt decrease of the sea level pressure in the central Arctic at the end of the 1980s. The decrease was associated with a shift of the Arctic Oscillation (AO) to a positive phase, which persisted throughout the mid 1990s. The Arctic salinity distribution is expected to respond to these dramatic changes via modifications in the ocean circulation and in the fresh water storage and transport by sea ice. The present study investigates these different contributions in the context of idealized ice-ocean experiments forced by atmospheric surface wind-stress or temperature anomalies representative of a positive AO index.Wind stress anomalies representative of a positive AO index generate a decrease of the fresh water content of the upper Arctic Ocean, which is mainly concentrated in the eastern Arctic with almost no compensation from the western Arctic. Sea ice contributes to about two-third of this salinification, another third being provided by an increased supply of salt by the Atlantic inflow and increased fresh water export through the Canadian Archipelago and Fram Strait. The signature of a saltier Atlantic Current in the Norwegian Sea is not found further north in both the Barents Sea and the Fram Strait branches of the Atlantic inflow where instead a widespread freshening is observed. The latter is the result of import of fresh anomalies from the subpolar North Atlantic through the Iceland-Scotland Passage and enhanced advection of low salinity waters via the East Icelandic Current. The volume of ice exported through Fram Strait increases by 20% primarily due to thicker ice advected into the strait from the northern Greenland sector, the increase of ice drift velocities having comparatively less influence. The export anomaly is comparable to those observed during events of Great Salinity Anomalies and induces substantial freshening in the Greenland Sea, which in turn contributes to increasing the fresh water export to the North Atlantic via Denmark Strait. With a fresh water export anomaly of 7 mSv, the latter is the main fresh water supplier to the subpolar North Atlantic, the Canadian Archipelago contributing to 4.4 mSv.The removal of fresh water by sea ice under a positive winter AO index mainly occurs through enhanced thin ice growth in the eastern Arctic. Winter SAT anomalies have little impact on the thermodynamic sea ice response, which is rather dictated by wind driven ice deformation changes. The global sea ice mass balance of the western Arctic indicates almost no net sea ice melt due to competing seasonal thermodynamic processes. The surface freshening and likely enhanced sea ice melt observed in the western Arctic during the 1990s should therefore be attributed to extra-winter atmospheric effects, such as the noticeable recent spring-summer warming in the Canada-Alaska sector, or to other modes of atmospheric circulations than the AO, especially in relation to the North Pacific variability.  相似文献   

11.
Wyville Thomson Ridge Overflow Water (WTOW), which is the only part of the outflow from the Norwegian Sea not to directly enter the Iceland Basin, is shown to be a significant water mass in the northern Rockall Trough. It is found primarily at intermediate depths (600–1200 m) beneath the northward flowing warm Atlantic waters, and above recirculating Mediterranean influenced waters and Labrador Sea Water (LSW). The bottom of the WTOW layer can be identified by a mid-depth inflexion point in potential temperature–salinity plots. An analysis of historical data reveals that WTOW has been present in all but eight of the last 31 years at 57.5°N in the Rockall Trough. A denser component of WTOW below 1500 m has also been present, although it appears to be less persistent (12 out of the 31 years) and limited to the west of the section. The signature of intermediate WTOW was absent in two periods, the mid-1980s and early 1990s, both of which coincided with a freshening, and probable increase in volume, of LSW in the trough. Potential temperature–salinity diagrams from historical observations indicate that WTOW persists at least as far south as 55°N (and as far west as 20°W in the Iceland Basin) although its signature is quickly lost on leaving the Rockall Trough. We suggest that a transport of WTOW down the western side of the trough exists, with WTOW at intermediate depths entering the eastern trough either via a cyclonic recirculation, or as a result of eddy activity. Further, WTOW is seen on the Rockall–Hatton Plateau and in the deep channels connecting with the Iceland Basin, suggesting additional possible WTOW transport pathways. These suggested transport routes remain to be confirmed by further observational or modelling studies.  相似文献   

12.
Meridional ocean freshwater transports and convergences are calculated from absolute geostrophic velocities and Ekman transports. The freshwater transports are analyzed in terms of mass-balanced contributions from the shallow, ventilated circulation of the subtropical gyres, intermediate and deep water overturns, and Indonesian Throughflow and Bering Strait components. The following are the major conclusions:
1.
Excess freshwater in high latitudes must be transported to the evaporative lower latitudes, as is well known. The calculations here show that the northern hemisphere transports most of its high latitude freshwater equatorward through North Atlantic Deep Water (NADW) formation (as in [Rahmstorf, S., 1996. On the freshwater forcing and transport of the Atlantic thermohaline circulation. Climate Dynamics 12, 799-811]), in which saline subtropical surface waters absorb the freshened Arctic and subpolar North Atlantic surface waters (0.45 ± 0.15 Sv for a 15 Sv overturn), plus a small contribution from the high latitude North Pacific through Bering Strait (0.06 ± 0.02 Sv). In the North Pacific, formation of 2.4 Sv of North Pacific Intermediate Water (NPIW) transports 0.07 ± 0.02 Sv of freshwater equatorward.In complete contrast, almost all of the 0.61 ± 0.13 Sv of freshwater gained in the Southern Ocean is transported equatorward in the upper ocean, in roughly equal magnitudes of about 0.2 Sv each in the three subtropical gyres, with a smaller contribution of <0.1 Sv from the Indonesian Throughflow loop through the Southern Ocean. The large Southern Ocean deep water formation (27 Sv) exports almost no freshwater (0.01 ± 0.03 Sv) or actually imports freshwater if deep overturns in each ocean are considered separately (−0.06 ± 0.04 Sv).This northern-southern hemisphere asymmetry is likely a consequence of the “Drake Passage” effect, which limits the southward transport of warm, saline surface waters into the Antarctic [Toggweiler, J.R., Samuels, B., 1995a. Effect of Drake Passage on the global thermohaline circulation. Deep-Sea Research I 42(4), 477-500]. The salinity contrast between the deep Atlantic, Pacific and Indian source waters and the denser new Antarctic waters is limited by their small temperature contrast, resulting in small freshwater transports. No such constraint applies to NADW formation, which draws on warm, saline subtropical surface waters .
2.
The Atlantic/Arctic and Indian Oceans are net evaporative basins, hence import freshwater via ocean circulation. For the Atlantic/Arctic north of 32°S, freshwater import (0.28 ± 0.04 Sv) comes from the Pacific through Bering Strait (0.06 ± 0.02 Sv), from the Southern Ocean via the shallow gyre circulation (0.20 ± 0.02 Sv), and from three nearly canceling conversions to the NADW layer (0.02 ± 0.02 Sv): from saline Benguela Current surface water (−0.05 ± 0.01 Sv), fresh AAIW (0.06 ± 0.01 Sv) and fresh AABW/LCDW (0.01 ± 0.01 Sv). Thus, the NADW freshwater balance is nearly closed within the Atlantic/Arctic Ocean and the freshwater transport associated with export of NADW to the Southern Ocean is only a small component of the Atlantic freshwater budget.For the Indian Ocean north of 32°S, import of the required 0.37 ± 0.10 Sv of freshwater comes from the Pacific through the Indonesian Throughflow (0.23 ± 0.05 Sv) and the Southern Ocean via the shallow gyre circulation (0.18 ± 0.02 Sv), with a small export southward due to freshening of bottom waters as they upwell into deep and intermediate waters (−0.04 ± 0.03 Sv).The Pacific north of 28°S is essentially neutral with respect to freshwater, −0.04 ± 0.09 Sv. This is the nearly balancing sum of export to the Atlantic through Bering Strait (−0.07 ± 0.02 Sv), export to the Indian through the Indonesian Throughflow (−0.17 ± 0.05 Sv), a negligible export due to freshening of upwelled bottom waters (−0.03 ± 0.03 Sv), and import of 0.23 ± 0.04 Sv from the Southern Ocean via the shallow gyre circulation.
3.
Bering Strait’ssmall freshwater transport of <0.1 Sv helps maintains the Atlantic-Pacific salinity difference. However, proportionally large variations in the small Bering Strait transport would only marginally impact NADW salinity, whose freshening relative to saline surface water is mainly due to air-sea/runoff fluxes in the subpolar North Atlantic and Arctic. In contrast, in the Pacific, because the total overturning rate is much smaller than in the Atlantic, Bering Strait freshwater export has proportionally much greater impact on North Pacific salinity balances, including NPIW salinity.
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13.
A time series of a standard hydrographic section in the northern Rockall Trough spanning 23 yr is examined for changes in water mass properties and transport levels. The Rockall Trough is situated west of the British Isles and separated from the Iceland Basin by the Hatton and Rockall Banks and from the Nordic Seas by the shallow (500 m) Wyville–Thompson ridge. It is one pathway by which warm North Atlantic upper water reaches the Norwegian Sea and is converted into cold dense overflow water as part of the thermohaline overturning in the northern North Atlantic and Nordic Seas. The upper water column is characterised by poleward moving Eastern North Atlantic Water (ENAW), which is warmer and saltier than the subpolar mode waters of the Iceland Basin, which also contribute to the Nordic Sea inflow. Below 1200 m the deep Labrador Sea Water (LSW) is trapped by the shallowing topography to the north, which prevents through flow but allows recirculation within the basin. The Rockall Trough experiences a strong seasonal signal in temperature and salinity with deep convective winter mixing to typically 600 m or more and the formation of a warm fresh summer surface layer. The time series reveals interannual changes in salinity of ±0.05 in the ENAW and ±0.04 in the LSW. The deep water freshening events are of a magnitude greater than that expected from changes in source characteristics of the LSW, and are shown to represent periodic pulses of newer LSW into a recirculating reservior. The mean poleward transport of ENAW is 3.7 Sv above 1200 dbar (of which 3.0 Sv is carried by the shelf edge current) but shows a high-level interannual variability, ranging from 0 to 8 Sv over the 23 yr period. The shelf edge current is shown to have a changing thermohaline structure and a baroclinic transport that varies from 0 to 8 Sv. The interannual signal in the total transport dominates the observations, and no evidence is found of a seasonal signal.  相似文献   

14.
We investigate the sensitivity of a coarse resolution coupled climate model to the representation of the overflows over the Greenland–Scotland ridge. This class of models suffers from a poor representation of the water mass exchange between the Nordic Seas and the North Atlantic, a crucial part of the large-scale oceanic circulation. We revisit the explicit representation of the overflows using a parameterisation by hydraulic constraints and compare it with the enhancement of the overflow transport by artificially deepened passages over the Greenland–Scotland ridge, a common practice in coarse resolution models. Both configurations increase deep water formation in the Nordic Seas and represent the large-scale dynamics of the Atlantic realistically in contrast to a third model version with realistic sill depths but without the explicit overflow transport. The comparison of the hydrography suggests that for the unperturbed equilibrium the Nordic Seas are better represented with the parameterised overflows. As in previous studies, we do not find a stabilising effect of the overflow parameterisation on the Atlantic meridional overturning circulation but merely on the overflow transport. As a consequence the surface air temperature in the Nordic Seas is less sensitive to anomalous surface fresh water forcing.Special attention is paid to changes in the subpolar gyre circulation. We find it sensitive to the overflow transport and the density of these water masses through baroclinic adjustments. The analysis of the governing equations confirms the presence of positive feedbacks inherent to the subpolar gyre and allows us to isolate the influence of the overflows on its dynamics.  相似文献   

15.
The surface and subsurface waters of the Angola and Agulhas Current systems significantly influence the Benguela region and its living resources, and it is probable that the movement of Central Water, which plays a key role in the coastal upwelling process, is controlled by circulation of underlying Antarctic Intermediate Water (AIW) as well as by the dynamics of the overlying subtropical water. The movement of AIW can be inferred from a study of the t-s characteristics, and the data holdings and data base of the South African Data Centre for Oceanography facilitated this investigation. Key findings of the investigation, some confirming earlier theories and hypotheses, are as follows. The mean depth of the AIW core in the South-East Atlantic is 750 m, and in the South-West Indian Ocean, 1 100 m. Agulhas Current AIW, which is modified by Red Sea Water, becomes fresher en route because of entrainment and mixing of water from the south and west. Most of the Agulhas Current AIW per se retroflects east of 18°E. A poleward movement of AIW along the West Coast to around 32°S may be inferred from the salinity and oxygen data, with a freshening en route analogous to the Agulhas Current. Relatively fresh AIW (s < 34,35 × 10?3) is present off the South-Western Cape, the only part of the Benguela where the overlying virgin Central Water upwells. No statistically significant seasonal differences could be resolved.  相似文献   

16.
Water properties measured by the central mooring in the Line W mooring array southeast of Cape Cod document a large character shift during the period of November 2001 to April 2008. The observed temperature, salinity and planetary potential vorticity (PPV) anomalies manifest changes in the formation region of the water masses present at Station W, specifically upper Labrador Sea Water (uLSW), deep Labrador Sea Water (dLSW) and Overflow Water (OW). During the observation period, the minimum in the PPV anomaly field relative to the record mean PPV profile migrated from 1500 m, where it was originally found, to 700 m. Temporal changes in the vertical distribution of temperature and salinity were correlated with the PPV changes. This suggests a dLSW-dominated first half of the record versus an uLSW-dominated second half. The structure of these anomalies is consistent with observations within the Labrador Sea, and their transit time to Line W agrees well with tracer-derived times for signals spreading along the western boundary. In that context, the observed water properties at Line W in the early 2000s reflected the intense deep convection in the Labrador Sea in the mid-1990s, with less intense convection subsequently affecting lighter isopycnals. The observed velocity field is dominated by high-frequency (periods of days to months) fluctuations, however, a fraction of the velocity variability is correlated with changes in water mass properties, and indicate a gradual acceleration of the southwestward flow, with a corresponding increase in Deep Western Boundary Current transport.  相似文献   

17.
Inter-annual to inter-decadal changes of hydrographic structure and circulation in the subpolar North Atlantic are studied using a coarse resolution ocean circulation model. The study covers 1949 through 2001, inclusive. A “time-mean state nudging” method is applied to assimilate the observed hydrographic climatology into the model. The method significantly reduces model biases in the long-term mean distribution of temperature and salinity, which commonly exist in coarse-resolution ocean models. By reducing the time-mean biases we also significantly improve the model’s representation of inter-annual to inter-decadal variations. In the central Labrador Sea, the model broadly reproduces the heat and salt variations of the Labrador Sea Water (LSW) as revealed by hydrographic observations. Model sensitivity experiments confirm that the low-frequency hydrographic changes in the central Labrador Sea are closely related to changes in the intensity and depth of deep convection. Changes in surface heat flux associated with the winter North Atlantic Oscillation (NAO) index play a major role in driving the changes in T–S and sea surface height (SSH). Changes in wind stress play a secondary role in driving these changes but are important in driving the changes in the depth-integrated circulation. The total changes in both SSH and depth-integrated circulation are almost a linear combination of the separate influences of variable buoyancy and momentum fluxes.  相似文献   

18.
本文利用Argo海水盐度资料、海流同化数据和同期大气再分析数据,探讨热带太平洋盐度趋势变化和相关动力过程。Argo资料显示,2015?2017年热带太平洋出现显著的盐度异常(SAE),这是改变长期趋势的主要原因,表现为表层显著淡化和次表层咸化特征。这种盐度异常具有明显的区域性特征和垂直结构的差异,体现在热带太平洋北部海区(NTP)和南太平洋辐合区(SPCZ)表层淡化,盐度最大变幅为0.71～0.92,淡化可以达到混合层底;热带太平洋南部海区(STP)次表层咸化,最大变幅为0.46,主要发生在温跃层附近,期间盐度异常沿着等位密面从西向东扩展。平流和挟卷是与SAE密切相关的海洋动力过程,两者在NTP淡化海域有着持续而较为显著的影响,在SPCZ淡化、STP咸化海域后期贡献也较大,其中盐度平流对热带太平洋海区盐度变化起主要贡献。NTP淡化海区表层淡水通量和STP咸化海区密度补偿引起的混合也是SAE的重要影响因素。  相似文献   

19.
The Antarctic Intermediate Water (AAIW) exhibits a decadal variability during recent years, i.e., salinification before 1997 and freshening thereafter, with the maximum anomalies locating at the region of Brazil and Malvinas currents confluence. Our study proposed that the local mesoscale eddies may play an important role in triggering this decadal oscillation. The eddy activity intensification (weakening) leads to the increase (decrease) of poleward cross-frontal eddy salinity flux and upward eddy buoyancy flux, which results in the weakening (strengthening) of the subsurface stratification and potential vorticity (PV). The PV anomalies facilitate (block) the poleward transport of warm saline subtropical water, while the stratification weakening favors the further downward transmission of salinity anomalies by processes of eddy flux as well as mean-flow advection (the stratification strengthening inhibits the vertical transport), then initiates the decadal change of the AAIW property. The whole process of the eddy-related propagation of salinity anomalies takes about 4 to 6 years.  相似文献   

20.
In July–September 1997 two hydrographic lines were done in the western N. Atlantic along longitudes of 52 and 66°W as part of the WOCE one-time hydrographic survey of the oceans. Each of these two lines approximately repeated earlier ones done during the International Geophysical Year(s) (IGY) and the mid-1980s. Because of this repeated sampling, long-term hydrographic changes in the water masses can be examined. In this report, we focus on temperature and salinity changes within the subtropical gyre mainly between latitudes of 20 and 35°N and compare our results to those presented by Bryden et al. (1996), who examined changes along a zonal line at 24°N, most recently occupied in 1992. Since this most recent 24°N section in 1992, substantial changes have occurred in the western part of the subtropical gyre at the depths of the Labrador Sea Water (LSW). In particular, we see clear evidence for colder, fresher Labrador Sea Water throughout the gyre on our two recent sections that was not yet present in 1992 at similar longitudes along 24°N. At shallower depths inhabited by waters that are an admixture of Mediterranean (MW) and Antarctic Intermediate Waters (AAIW), our recent survey shows an increase in salinity, which can only be attributed to changes in water masses on potential temperature or neutral density surfaces. Furthermore, waters above the MW/AAIW layer and into the deeper part of the main pycnocline have continued to become saltier and warmer throughout the 40-year period spanned by our sections. These latter changes have been dominantly due to a vertical sinking of density surfaces as T/S changes in density surfaces are small, but depths of individual T/S horizons have increased with time. The net change since the IGY shows a mean temperature increase between 800 and 2500 m depth at a rate of 0.57°C/century with a corresponding steric sea level rise of 1 mm/yr, and a net downward heave with small values near the top and bottom, and a maximum rate of −2.7 m/yr at 1800 m depth. Changes in the deep Caribbean indicate a warming since the IGY due to temperature increases of the inflowing source waters in the subtropical gyre at 1800m depth, but no significant change in the deep salinity.  相似文献   

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