首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
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   

2.
The Wyville Thomson Ridge forms part of the barrier to the meridional circulation across which cold Nordic Sea and Arctic water must traverse to reach the Atlantic Ocean. Overflow rates across the ridge are variable (but can be dramatic at times), and may provide a subtle indicator of significant change in the circulation in response to climate change. In spring 2003, a series of CTD sections were conducted during a large overflow event in which Norwegian Sea Deep Water (NSDW) cascaded down the southern side of the ridge into the Rockall Trough at a rate of between 1 and 2 Sv. The NSDW was partially mixed with overlying North Atlantic Water (NAW), and comprised about 1/3rd of the cascading water. The components of NAW and NSDW in the overflow were sufficiently large that there must have been a significant divergence of the inflow through the Faroe-Shetland Channel, and of the outflow through the Faroe Bank Channel.As the plume descended, its temperature near the sea bed warmed by over 3 °C in about a day. Although the slope was quite steep (0.03), the mean speed of the current (typically 0.36 m s−1) was too slow for significant entrainment of NAW to occur (the bulk Richardson number was of order 5). However, very large overturns (up to 50 m) were evident in some CTD profiles, and it is demonstrated from Thorpe scale estimates that the warming of the bottom waters was due to mixing within the plume. It is likely that some of the NSDW had mixed with NAW before it crossed the ridge. The overflow was trapped in a gully, which caused it to descend to great depth (1700 m) at a faster rate, and with less modification due to entrainment, than other overflows in the North Atlantic. The water that flowed into the northern part of the Rockall Trough had a temperature profile that ranged from about 3 to 8 °C. Water with a temperature of >6 °C probably escaped into the Iceland Basin, between the banks that line the north-western part of the Trough. Colder water (< 6 °C) must have travelled down the eastern side of the Rockall Bank, and may have had a volume flux of up to 1.5 Sv.  相似文献   

3.
Between 1996 and the mid-2000s the upper waters (200–700 m) of the Rockall Trough became warmer (+0.72 °C), saltier (+0.088) and reduced in nitrate and phosphate (−2.00 µM and −0.14 µM respectively). These changes, out-with calculated errors, can be explained by the varying influence of southern versus subpolar water masses in the basin as the Subpolar Gyre weakened and contracted. Upper water properties strongly correlate with a measure of the strength of the Subpolar Gyre (the first principal component of sea surface height over the Subpolar North Atlantic) prior to the mid-2000s. As the gyre weakens, the upper layers of the trough become warmer (r−0.85), more saline (r−0.86) and reduced in nitrate and phosphate (r+0.81 and r+0.87 respectively). Further the proportion of subpolar waters in the basin decreases from around 50% to less than 20% (r+0. 88). Since the mid−2000s the Subpolar Gyre has been particularly weak. During this period temperatures decreased slightly (−0.21 °C), salinities remained near constant (35.410±0.005) and phosphate levels low and stable (0.68±0.02 µM). These relative lack of changes are thought to be related to the maximum proportion of southern water masses within the Rockall Trough having been reached. Thus the upper water properties are no longer controlled by changes in the relative importance of different water masses in the basin (as prior to the mid-2000s), but rather a different process. We suggest that when the gyre is particularly weak the interannual changes in upper water properties in the Rockall Trough reflect changes in the source properties of the southern water masses. Since the early-2000s the Subpolar Gyre has been weaker than observed since 1992, or modelled since 1960–1970. Hence upper waters within the Rockall Trough may be warmer, saltier and more depleted in nitrate and phosphate than at any time in the last half century.  相似文献   

4.
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   

5.
A water-mass analysis is carried out in Fram Strait, between 77.15 and 81.15°N, based on three-dimensional large-scale potential temperature and salinity distributions reconstructed from the MIZEX 84 hydrographic data collected in summer 1984. Combining these distributions with the geostrophic flow field derived from the same data in a companion paper (Schlichtholz and Houssais, 1999), the heat, fresh water and volume transports are estimated for each of the water masses identified in the strait. Twelve water masses are selected based on their different origins. Among them, the Polar Water (PW) enters Fram Strait from the Arctic Ocean both over the Greenland Slope and over the western slope of the Yermak Plateau. In the Atlantic Water (AW) range, four modes with distinct geographical distributions are indentified. In the Deep Water range, the Eurasian Basin Deep Water (EBDW) is confined to the Lena Trough and to the Molloy Deep area where it is involved in a cyclonic circulation. The warm and shallower mode of the Norwegian Sea Deep Water (NSDW), concentrated to the west, is mainly seen as an outflow from the Arctic Ocean while the cold and deeper mode, essentially observed to the east, enters the strait from the Greenland Sea. Apart from the EBDW, there is a tendency for all water masses of polar origin to flow along the Greenland Slope. The two most abundant water masses, the AW and the NSDW, occupy as much as 67% of the total water volume. The southward net transport of PW through Fram Strait is about 1 Sv at 78.9°N. At the same latitude, the net transport of AW is southward and equal to about 1.7 Sv. Only the transport of the warm mode (AWw) is northward, amounting to 0.2 Sv. The overall net outflow of the Deep Waters to the Greenland Sea is about 2.6 Sv. Two upper water masses, the fresh (AWf) and the cold (AWc) mode of the AW, and one deep-water mass, the NSDW, appear to be produced in the strait, with production rates, between 77.6 and 79.9°N, of about 0.2, 1.0 and 1.7 Sv, respectively. A southward net fresh-water transport through the strait of about 2000 km3 yr−1 (relative to a salinity of 34.93) is mainly due to the PW. The net heat transport relative to −0.1°C is northward, but undergoes a rapid northward decrease, suggesting an area-averaged surface heat loss of 50–100 W m−2 in the strait.  相似文献   

6.
In the period 1991–1996 the WOCE hydrographic section A1E/AR7E between Greenland and Ireland was repeated five times. The observed thermohaline changes altered the baroclinic structure along the eastern margin of the subpolar gyre significantly. Between June 1995 and August 1996 an overall increase of the temperature and thickness and a decrease of the density of the Subpolar Mode Water (SPMW) layer were observed, accompanied by an increase of its salinity east of the Reykjanes Ridge and a decrease of its salinity in the Irminger Sea. The changes were most pronounced in the Iceland Basin, where the Subarctic Front retreated westwards, coinciding with a strong weakening of the Westerlies as determined by the North Atlantic Oscillation. They are related to a local reduction of the Ekman upwelling and the ocean-to-atmosphere heat flux on the one hand and to the advection of anomalies from the subtropics on the other hand.The eastward spreading of the different Labrador Sea Water (LSW) vintages led to a corresponding cooling of the LSW in the Irminger Sea and in the Iceland Basin in the period 1991–1996. The renewal of the LSW in the Rockall Trough occurred more sporadically, indicating that the North Atlantic Current (NAC) impedes the southward spreading of LSW in the eastern Atlantic. The changes in 1996 seem to have also counteracted this spreading.  相似文献   

7.
Hydrographic, current meter and ADCP data collected during two recent cruises in the South Indian Ocean (RRS Discovery cruise 200 in February 1993 and RRS Discovery cruise 207 in February 1994) are used to investigate the current structure within the Princess Elizabeth Trough (PET), near the Antarctic continent at 85°E, 63–66°S. This gap in topography between the Kerguelen Plateau and the Antarctic continent, with sill depth 3750 m, provides a route for the exchange of Antarctic Bottom Water between the Australian–Antarctic Basin and the Weddell–Enderby Basin. Shears derived from ADCP and hydrographic data are used to deduce the barotropic component of the velocity field, and thus the volume transports of the water masses. Both the Southern Antarctic Circumpolar Current Front (SACCF) and the Southern Boundary of the Antarctic Circumpolar Current (SB) pass through the northern PET (latitudes 63 to 64.5°S) associated with eastward transports. These are deep-reaching fronts with associated bottom velocities of several cm s-1. Antarctic Bottom water (AABW) from the Weddell–Enderby Basin is transported eastwards in the jets associated with these fronts. The transport of water with potential temperatures less than 0°C is 3 (±1) Sv. The SB is shown to meander in the PET, caused by the cyclonic gyre immediately west of the PET in Prydz Bay. The AABW therefore also meanders before continuing eastwards. In the southern PET (latitudes 64.5 to 66°S) a bottom intensified flow of AABW is observed flowing west. This AABW has most likely formed not far from the PET, along the Antarctic continental shelf and slope to the east. Current meters show that speeds in this flow have an annual scalar mean of 10 cm s-1. The transport of water with potential temperatures less than 0°C is 20 (±3) Sv. The southern PET features westward flow throughout the water column, since the shallower depths are dominated by the flow associated with the Antarctic Slope Front. Including the westward flow of bottom water, the total westward transport of the whole water column in the southern PET is 45 (±6) Sv.  相似文献   

8.
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   

9.
In this paper we use a temperature and salinity based mixing model to assess the dilution of Antarctic Bottom Water (AABW) as it moves away from the Weddell Sea and into the Southwest Indian Ocean. By combining these results with CFC tracer measurements we have been able to make direct estimates of the large-scale translation rates of AABW in this region. We confirm that there is a major northward flow of AABW via a gap in the Southwest Indian Ridge at 30°E, and thence across the Agulhas Basin into the Mozambique Basin, with a translation rate from the Greenwich Meridian of 0.8–1.0 cm s−1 and a volume transport between the two basins of 1.5×106 m3 s−1. A second, smaller flow cuts the Del Cano Rise through the Prince Edward Fracture Zone but is indistinguishable from the general bottom waters once on the northern side of the rise. The third flow moves eastward along the southern flank of the Del Cano Rise to pass north of the Conrad Rise. This has bottom velocities of 0.7 cm s−1 and a volume transport of 1.6×106 m3 s−1. This water is probably the source of the AABW-rich Circumpolar Deep Water that flows through the gap to the west of Crozet Island, and which is traceable again at stations on the northern flanks of the ridge. Flow between the Conrad Rise and the Del Cano Rise is complicated by the influence of a fourth flow, the AABW that passes south of the former and thence into the Crozet Basin via the Crozet-Kerguelen Gap. We suggest that a portion of this flow loops into the channel between the Del Cano Rise and the Conrad Rise, modifying the bottom waters at the easternmost stations within this channel. We will go on in Part 2 of this paper to use these results to estimate the dissolution rates of silica in the SWINDEX area.  相似文献   

10.
One of the major pathways in the northern part of the Meridional Overturning Circulation (MOC) is that of the deep water in the Nordic Seas that runs through the Faroe-Shetland Channel (FSC) and Faroe Bank Channel (FBC), as well as crossing the Wyville Thomson Ridge (WTR), on its way into the Atlantic Ocean. The WTR overflow cascades down the southern side of the ridge via the narrow Ellett Gully to the Cirolana Deep (CD) which, at 1700 m, is the deepest hole in the extreme north of the Rockall Trough. The overflow accounts for nearly 1/10th of the total Faroe-Shetland Channel Bottom Water (FSCBW) discharged through the Faroese channels and is an important intermediate water mass in the Rockall Trough. Over a period of only seven days in April 2003 bottom water temperatures cooled dramatically, from 4.46 to 3.03 °C in the CD and from 3.93 to 2.54 °C in the Ymir Trough (YT). A numerical general circulation model (MITgcm) has been applied in order to reproduce the details of this dense water overflow event. Model results were consistent with the observed cooling and total water transport. It was found that the descending gravity current forms a pair of mesoscale eddies with cyclonic and anticyclonic vorticity at the exit to the CD. Analysis of mixing processes were obtained when a passive tracer was included in the model. It was found that downstream flow is characterized by an explosive detrainment regime in the CD. The model sensitivity runs revealed that the final depth to which the overflow descends depends on the initial upstream velocity of the overflow, as well as the buoyancy difference. It is argued that models of overflows need to have realistic representations of the density structure of the overflow, and sufficiently fine vertical resolution, for the subsequent fate of the overflow to be accurately represented.  相似文献   

11.
Time series of profiles of potential temperature, salinity, dissolved oxygen, and planetary potential vorticity at intermediate depths in the Labrador Sea, the Irminger Sea, and the Iceland Basin have been constructed by combining the hydrographic sections crossing the sub-arctic gyre of the North Atlantic Ocean from the coast of Labrador to Europe, occupied nearly annually since 1990, and historic hydrographic data from the preceding years since 1950. The temperature data of the last 60 years mainly reflect a multi-decadal variability, with a characteristic time scale of about 50 years. With the use of a highly simplified heat budget model it was shown that this long-term temperature variability in the Labrador Sea mainly reflects the long-term variation of the net heat flux to the atmosphere. However, the analysis of the data on dissolved oxygen and planetary potential vorticity show that convective ventilation events, during which successive classes of Labrador Sea Water (LSW) are formed, occurring on decadal or shorter time scales. These convective ventilation events have performed the role of vertical mixing in the heat budget model, homogenising the properties of the intermediate layers (e.g. temperature) for significant periods of time. Both the long-term and the near-decadal temperature signals at a pressure of 1500 dbar are connected with successive deep LSW classes, emphasising the leading role of Labrador Sea convection in running the variability of the intermediate depth layers of the North Atlantic. These signals are advected to the neighbouring Irminger Sea and Iceland Basin. Advection time scales, estimated from the 60 year time series, are slightly shorter or of the same order as most earlier estimates, which were mainly based on the feature tracking of the spreading of the LSW94 class formed in the period 1989-1994 in the Labrador Sea.  相似文献   

12.
Data collected from hydrographic stations occupied within the Venezuelan and Columbian basins of the Caribbean Sea from 1922 through 2003 are analyzed to study the decadal variability of deep temperature in the region. The analysis focuses on waters below the 1815-m sill depth of the Anegada–Jungfern Passage. Relatively dense waters (compared to those in the deep Caribbean) from the North Atlantic spill over this sill to ventilate the deep Caribbean Sea. Deep warming at a rate of over 0.01 °C decade–1 below this sill depth appears to have commenced in the 1970s after a period of relatively constant deep Caribbean Sea temperatures extending at least as far back as the 1920s. Conductivity–temperature–depth station data from World Ocean Circulation Experiment Section A22 along 66°W taken in 1997 and again in 2003 provide an especially precise, albeit geographically limited, estimate of this warming over that 6-year period. They also suggest a small (0.001 PSS-78, about the size of expected measurement biases) deep freshening. The warming is about 10 times larger than the size of geothermal heating in the region, and is of the same magnitude as the average global upper-ocean heat uptake over a recent 50-year period. Together with the freshening, the warming contributes about 0.012 m decade–1 of sea level rise in portions of the Caribbean Sea with bottom depths around 5000 m.  相似文献   

13.
Increased values of trichlorofluoromethane (CFC-11), tritium and stable tritium in the depth range from 2500 to 3500 m at the eastern flank of the Mid-Atlantic Ridge at 48°N (WHP section A2) indicate an influence of newly ventilated water. Water with similar Θ, S and tracer properties is found on the WHP section A1 (55°N) situated north of the Gibbs Fracture Zone in the Iceland Basin. The high tracer concentrations are due to the influence of Iceland Scotland Overflow Water (ISOW). The ISOW-influenced water found in the Iceland Basin partially passes by the Gibbs Fracture Zone (52°N) and flows southward along the topography of the Mid-Atlantic Ridge. A quantitative analysis of the transport from the Iceland Basin to the Westeuropean Basin is carried out based on the assumption that the water with enhanced tracer values is a two-component mixture of recirculating North East Atlantic Deep Water from the eastern part of the Westeuropean Basin and ISOW-influenced water as found on A1 in the Iceland Basin (NEADWIB). The composition of the mixture and the transport time for the NEADWIB are deduced from the temporal evolution of the tracer values. From the distance between the two sections and the area with enhanced tracer values, a transport of NEADWIB from the Iceland Basin to the Westeuropean Basin of 1.63±0.32 Sv1 is calculated for the density range 41.37<σ3<41.475. Transports between 2.4 and 3.5 Sv result if the transport in the former density range is extrapolated to 41.35<σ3<41.52 (corresponding to σΘ>27.8) in different ways.  相似文献   

14.
We conducted full-depth hydrographic observations between 8°50′ and 44°30′N at 165°W in 2003 and analyzed the data together with those from the World Ocean Circulation Experiment and the World Ocean Database, clarifying the water characteristics and deep circulation in the Central and Northeast Pacific Basins. The deep-water characteristics at depths greater than approximately 2000 dbar at 165°W differ among three regions demarcated by the Hawaiian Ridge at around 24°N and the Mendocino Fracture Zone at 37°N: the southern region (10–24°N), central region (24–37°N), and northern region (north of 37°N). Deep water at temperatures below 1.15 °C and depths greater than 4000 dbar is highly stratified in the southern region, weakly stratified in the central region, and largely uniform in the northern region. Among the three regions, near-bottom water immediately east of Clarion Passage in the southern region is coldest (θ<0.90 °C), most saline (S>34.70), highest in dissolved oxygen (O2>4.2 ml l?1), and lowest in silica (Si<135 μmol kg?1). These characteristics of the deep water reflect transport of Lower Circumpolar Deep Water (LCDW) due to a branch current south of the Wake–Necker Ridge that is separated from the eastern branch current of the deep circulation immediately north of 10°N in the Central Pacific Basin. The branch current south of the Wake–Necker Ridge carries LCDW of θ<1.05 °C with a volume transport of 3.7 Sv (1 Sv=106 m3 s?1) into the Northeast Pacific Basin through Horizon and Clarion Passages, mainly through the latter (～3.1 Sv). A small amount of the LCDW flows northward at the western boundary of the Northeast Pacific Basin, joins the branch of deep circulation from the Main Gap of the Emperor Seamounts Chain, and forms an eastward current along the Mendocino Fracture Zone with volume transport of nearly 1 Sv. If this volume transport is typical, a major portion of the LCDW (～3 Sv) carried by the branch current south of the Wake–Necker and Hawaiian Ridges may spread in the southern part of the Northeast Pacific Basin. In the northern region at 165°W, silica maxima are found near the bottom and at 2200 dbar; the minimum between the double maxima occurs at a depth of approximately 4000 dbar (θ～1.15 °C). The geostrophic current north of 39°N in the upper deep layer between 1.15 and 2.2 °C, with reference to the 1.15 °C isotherm, has a westward volume transport of 1.6 Sv at 39–44°30′N, carrying silica-rich North Pacific Deep Water from the northeastern region of the Northeast Pacific Basin to the Northwest Pacific Basin.  相似文献   

15.
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   

16.
The fine-scale circulation around the Heard and McDonald Islands and through the Fawn Trough, Kerguelen Plateau, is described using data from three high-resolution CTD sections, Argo floats and satellite maps of chlorophyll a, sea surface temperature (SST) and absolute sea surface height (SSH). We confirm that the Polar Front (PF) is split into two branches over the Kerguelen Plateau, with the NPF crossing the north-eastern limits of our survey carrying 25 Sv to the southeast. The SPF was associated with a strong eastward-flowing jet carrying 12 Sv of baroclinic transport through the deepest part of Fawn Trough (relative to the bottom). As the section was terminated midway through the trough this estimate is very likely to be a lower bound for the total transport. We demonstrate that the SPF contributes to the Fawn Trough Current identified by previous studies. After exiting the Fawn Trough, the SPF crossed Chun Spur and continued as a strong north-westward flowing jet along the eastern flank of the Kerguelen Plateau before turning offshore between 50°S and 51.5°S. Measured bottom water temperatures suggest a deep water connection between the northern and southern parts of the eastern Kerguelen Plateau indicating that the deep western boundary current continues at least as far north as 50.5°S. Analysis of satellite altimetry derived SSH streamlines demonstrates a southward shift of both the northern and southern branches of the Polar Front from 1994 to 2004. In the direct vicinity of the Heard and McDonald islands, cool waters of southern origin flow along the Heard Island slope and through the Eastern Trough bringing cold Winter Water (WW) onto the plateau. Complex topography funnels flow through canyons, deepens the mixed layer and increases productivity, resulting in this area being the preferred foraging region for a number of satellite-tracked land-based predators.  相似文献   

17.
Winter convection in the Irminger Sea leading to the formation of Labrador Sea Water (LSW) is analyzed using CTD data collected along the 59.5° N transatlantic section in 2004–2014, winter Argo data from 2012–2014, and daily North American regional reanalysis (NARR). The interannual variability of LSW in the Irminger Sea is investigated. The dissolved oxygen saturation rate of 93% is used to indicate maximal local convection depth. It is shown that the deepest convection (up to 1000 m) resulting in the largest LSW volume that formed in the Irminger Sea in 2008 and 2012. These years were characterized by numerous storms with anomalously strong turbulent heat loss from the ocean to the atmosphere and negative air temperature to the east of the southern tip of Greenland in January–March. LSW became warmer by 0.42°C, saltier by more than 0.03 PSU, and more oxygenated by 8 µmol/kg between 2004 and 2014. A strong LSW decay in the Iceland Basin is also noted.  相似文献   

18.
Seven years (2001–2008) of dissolved organic carbon (DOC) vertical profiles were examined in order to assess the main processes determining DOC concentration and distribution in the meso- and bathypelagic layers of the Mediterranean Sea. As expected, DOC showed high and highly variable concentrations in the surface layer of 57–68 μM (average values between 0 and 100 m), with a decrease to 44–53 μM between 200 and 500 m. Deep DOC distribution was strongly affected by deep-water formation, with a significant increase to values of 76 μM in recently ventilated deep waters, and low concentrations, comparable to those observed in the open oceanic waters (34–45 μM), where the oldest, deep waters occurred. In winter 2004/2005 a deep-water formation event was observed and the consequent DOC export at depth was estimated to range between 0.76–3.02 Tg C month–1. In the intermediate layer, the main path of the Levantine Intermediate Water (LIW) was followed in order to estimate the DOC consumption rate in its core. Multiple regression between DOC, apparent oxygen utilization (AOU), and salinity indicated that 38% of the oxygen consumption was related to DOC mineralization when the effect of mixing was removed. In deep waters of the southern Adriatic Sea a DOC decrease of 6 μM, together with an AOU increase of 9 μM, was observed between the end of January 2008 and the end of June 2008 (5 months). These data indicate a rate of microbial utilization of DOC of about 1.2 μM C month−1, with 92% of the oxygen consumption due to DOC mineralization. These values are surprisingly high for the deep sea and represent a peculiarity of the Mediterranean Sea.  相似文献   

19.
The intermediate and deep waters of the Labrador Sea are dominated by recently ventilated water masses (ventilation ages <20 yr). Atmospheric gases such as CO2 and chlorofluorocarbons are incorporated into these water masses at the time of formation and subsequently transported via boundary currents into the North Atlantic interior. Recent measurements of total carbonate were used in tandem with total alkalinity and oxygen to estimate the levels of anthropogenic carbon dioxide in the Labrador Sea region. Upper water column anthropogenic CO2 estimated in this manner showed good agreement with levels calculated from CO2 increase in the atmosphere. In spring 1997, anthropogenic contributions to total carbonate (CTant) were 40±3 μmol/kg in water penetrated by deep convection the previous winter and slightly lower (37±2 μmol/kg) in the deeper convective layer formed in the winters of 1992–1994. Consistent with the concurrent profiles of CFC-11, levels decrease into the older NEADW (North East Atlantic Deep Water) with levels of 30±3 μmol/kg and then increase near bottom within the layer of DSOW (Denmark Strait Overflow Water). The distribution of CTant shows the flow of new LSW southwards with the western boundary current and also eastwards into the Irminger Sea. We estimate that 0.15–0.35 Gt carbon of anthropogenic origin flow through the Labrador Sea within the Western Boundary Undercurrent per year.  相似文献   

20.
The trophic ecology, energy and reproductive states of the deep-water shrimp Aristaeomorpha foliacea, widely distributed along the slopes of the Mediterranean Sea Basins, were analysed in eight areas spread along ca. 3000 km in order to identify patterns in the habitat conditions supporting the species. From W to E the areas were situated between the north side of Eivissa (39°12′N, 1°20′E, in the Balearic Basin) and off Mersin, Turkey (36°15′N, 34°19′E, in the Levantine Sea). Trends identified mainly as a function of longitude from west to east were: (i) higher δ15N, parallel to δ15N shifts in the top 200 m of the water column for particulate organic N (Pantoja et al., 2002). The δ15N trend indicates that the deep trophic web, i.e. A. foliacea at 400–600 m, reflects the δ15N signal of the photic zone; (ii) a similar significant trend of δ13C, related with exploitation of pelagic versus benthic resources by A. foliacea in each area (i.e. by local variability of terrigenous inputs via submarine canyons). More depleted δ13C was found at mid-longitudes (Tyrrhenian Sea and Sicily Channel) linked to higher consumption of macroplankton prey (Pasiphaea spp., euphausiids and mesopelagic fishes). The feeding intensity (gut fullness, F) and prey diversity (J) of A. foliacea were related, according to generalized linear models, with the temperature and salinity of intermediate waters, variables in turn associated with latitude and longitude. Both F and J were higher in areas with greater shrimp density. The optimal ecological habitat of A. foliacea appears to be located in the Tyrrhenian Sea and the Sicily Channel, where we found the highest F, the greatest trophic diversity and A. foliacea in the best biological condition (i.e. with higher hepato-somatic index, HSI). These are also the areas with the highest densities of A. foliacea. In contrast, in the western Mediterranean Sea (Balearic Basin and the southern Balearic Islands), where A. foliacea has low densities, the shrimp showed generally lower values of trophic indicators and biological condition.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号