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1.
《Ocean Modelling》2008,20(3-4):138-160
We analyze the water mass transformation in coarse (1°) and high (1/6°) resolution ocean simulations with the identical configuration of the CLIPPER model and interannual ERA15 forcing function. Climatological characteristics of surface water mass transformation in the two experiments are quite different. The high resolution experiment exhibits a stronger surface transformation in equatorial and tropical regions, in the Gulf Stream area and in the location of the formation of Subtropical Mode Water (STMW), associated with high levels of eddy kinetic energy. The coarse resolution experiment shows a better representation of the transformation rates corresponding to the densest subpolar mode waters and Labrador Sea Water (LSW). This is explained by the differences in lateral mixing procedures between high and coarse resolution experiments. The high resolution 1/6° run is eddy-resolving only in the tropics and mid-latitudes. In these areas eddies are found to enhance the process of water mass transformation compared to the isopycnal diffusion used to parameterized the eddies in the 1° model. Despite its 1/6° resolution, the high resolution model does not adequately represent eddies in the subpolar gyre and Labrador Sea. In these areas the high resolution model fails to correctly simulate water mass transformation because the lateral mixing (provided through the bi-harmonic sub-gridscale parameterization) of newly ventilated waters with surrounding waters is not efficient enough. In contrast in the coarse 1° resolution model, the strong lateral mixing and the unrealistically broad boundary currents imposed by the high diffusivity required for numerical stability mixes newly formed LSW waters with the warmer and saltier waters of the rim current. Finally, it results in a more effective representation of the surface water mass transformation in high latitudes in the 1° model. A possible impact of the increased lateral diffusion in high resolution experiment on the representation of re-stratification in the Labrador Sea was studied in sensitivity experiments with different lateral diffusion coefficients compared to the regional eddy-resolving 1/15° simulation in the subpolar North Atlantic. If the eddies are not resolved in subpolar latitudes (as in the case of 1/6° model), the GM90 parameterization with the coefficient close to 800 m2 s−1 provides the closest agreement with the solution of eddy-resolving 1/15° model.  相似文献   

2.
The ratio of oxygen-18 to oxygen-16 (expressed as per mille deviations from Vienna Standard Mean Ocean Water, δ18O) is reported for seawater samples collected from seven full-depth CTD casts in the northern North Atlantic between 20° and 41°W, 52° and 60°N. Water masses in the study region are distinguished by their δ18O composition, as are the processes involved in their formation. The isotopically heaviest surface waters occur in the eastern region where values of δ18O and salinity (S) lie on an evaporation–precipitation line with slope of 0.6 in δ18O–S space. Surface isotopic values become progressively lighter to the west of the region due to the addition of 18O-depleted precipitation. This appears to be mainly the meteoric water outflow from the Arctic rather than local precipitation. Surface samples near the southwest of the survey area (close to the Charlie Gibbs Fracture Zone) show a deviation in δ18O–S space from the precipitation mixing line due to the influence of sea ice meltwater. We speculate that this is the effect of the sea ice meltwater efflux from the Labrador Sea. Subpolar Mode Water (SPMW) is modified en route to the Labrador Sea where it forms Labrador Sea Water (LSW). LSW lies to the right (saline) side of the precipitation mixing line, indicating that there is a positive net sea ice formation from its source waters. We estimate that a sea ice deficit of ≈250 km3 is incorporated annually into LSW. This ice forms further north from the Labrador Sea, but its effect is transferred to the Labrador Sea via, e.g. the East Greenland Current. East Greenland Current waters are relatively fresh due to dilution with a large amount of meteoric water, but also contain waters that have had a significant amount of sea ice formed from them. The Northeast Atlantic Deep Water (NEADW, δ18O=0.22‰) and Northwest Atlantic Bottom Waters (NWABW, δ18O=0.13‰) are isotopically distinct reflecting different formation and mixing processes. NEADW lies on the North Atlantic precipitation mixing line in δ18O–salinity space, whereas NWABW lies between NEADW and LSW on δ18O–salinity plots. The offset of NWABW relative to the North Atlantic precipitation mixing line is partially due to entrainment of LSW by the Denmark Strait overflow water during its overflow of the Denmark Strait sill. In the eastern basin, lower deep water (LDW, modified Antarctic bottom water) is identified as far north as 55°N. This LDW has δ18O of 0.13‰, making it quite distinct from NEADW. It is also warmer than NWABW, despite having a similar isotopic composition to this latter water mass.  相似文献   

3.
Previous work had examined an ocean model of the subpolar gyre of the North Atlantic Ocean that used the Gent and McWilliams parameterization with a variable eddy-transfer coefficient, and showed significant improvements to the model’s circulation and hydrography. This note examines an extended (80-year-long) integration of the same model and focuses on the adjustment of the intermediate and deep waters as well as on model stability. It is shown that the model is able to retain a good representation of the water masses, especially in the Labrador Sea, through the full integration. Labrador Sea Water dispersal is well simulated by the model in the western basin, with a good correspondence between the model and observational salinities on the σ2 = 36.95 isopycnal surface. Labrador Sea Water dispersal to the eastern basin is not nearly as well represented, as this water mass has trouble passing over the Mid-Atlantic Ridge in the model. The variable eddy-transfer coefficient significantly improves the model representation of the Cold Intermediate Layer on the Labrador shelf by reducing spurious diapycnal mixing. Finally, the evidence in this note suggests that open boundary conditions do not generate significant model drift, even for integrations approaching a century in length.  相似文献   

4.
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   

5.
The Irminger Gyre: Circulation, convection, and interannual variability   总被引：2，自引：0，他引：2  
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6.
Three eddy-permitting (1/4°) versions and one eddy-resolving (1/12°) version of the OCCAM ocean model are used to simulate the World Ocean circulation since 1985. The first eddy-permitting simulation has been used extensively in previous studies, and provides a point of reference. A second, improved, eddy-permitting simulation is forced in the same manner as the eddy-resolving simulation, with a dataset based on a blend of NCEP re-analysis and satellite data. The third eddy-permitting simulation is forced with a different dataset, based on the ERA-40 re-analysis data. Inter-comparison of these simulations in the North Atlantic clarifies the relative importance of resolution and choice of forcing dataset, for simulating the mean state and recent variability of the basin-scale circulation in that region. Differences between the first and second eddy-permitting simulations additionally reveal an erroneous influence of sea ice on surface salinity, dense water formation, and the meridional overturning circulation. Simulations are further evaluated in terms of long-term mean ocean heat transport at selected latitudes (for which hydrographic estimates are available) and sea surface temperature errors (relative to observations). By these criteria, closest agreement with observations is obtained for the eddy-resolving simulation. In this simulation, there is also a weak decadal variation in mid-latitudes, with heat transport strongest, by around 0.2 PW, in the mid-1990s. In two of the eddy-permitting simulations, by contrast, heat transport weakens through the study period, by up to 0.4 PW in mid-latitudes. The most notable changes of heat transport in all simulations are linked to a weakening of the subpolar gyre, rather than changes in the meridional overturning circulation. It is concluded that recent changes in the structure of mid-latitude heat transport in the North Atlantic are more accurately represented if eddies are explicitly resolved.  相似文献   

7.
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   

8.
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   

9.
Labrador Sea convection was most intense and reached the greatest depths in the early 1990s, followed by weaker, shallower, and more variable convection after 1995. The Simple Ocean Data Assimilation (SODA) version 2.0.2/2.0.4 assimilation model is used to explore convective activity in the North Atlantic Ocean for the period from 1992 to 2007. Hydrographic conditions, which are relatively well observed during this period, are used to compare modeled and observed winter mixed-layer depths and water mass anomalies in relation to Deep Western Boundary Current transports and meridional overturning circulation (MOC) changes at the exit of the subpolar basin. The assimilation differs markedly from local observations in the March mixed-layer depth, which represents deep convection and water mass transformation. However, mean MOC rates at the exit of the subpolar gyre, forced by stratification in the mid-latitudes, are similar to estimates based on observations and show no significant decrease during the 1992–2007 period. SODA reproduces the deep Labrador Sea Water formation in the western North Atlantic without any clear indication of significant formation in the Irminger Sea while the lighter upper Labrador Sea Water density range is reached in the Irminger Sea in the 1990s, in agreement with existing assumptions of deep convection in the Irminger Sea and also supported by computed lag correlations with the Labrador Sea. Deep Water transformation mainly takes place in the eastern North Atlantic. The introduction of CFC-11 into the SODA model as a tracer reproduces the mean and multiyear variations of observed distributions.  相似文献   

10.
In order to confirm the results of the authors’ previous work, which found that the existence of disturbances smaller than meso-scale eddies is important in large-scale mixing process between the Oyashio and Kuroshio waters in the intermediate layer, the results of an eddy-resolving model experiment are analyzed and compared with those of an eddy-permitting model. The intermediate salinity minimum given in the initial condition weakens as integration advances in the eddy-permitting model, while it recovers rapidly and is maintained thereafter in the eddy-resolving model, initialized from the unrealistic salinity distribution of the former. Filament-like fine structures in temperature and salinity develop actively in the latter, which are much smaller in horizontal width than meso-scale eddies, suggesting the importance of such disturbances in the large-scale mixing. The mixing ratio of the Oyashio water defined by the original Oyashio and Kuroshio waters shows that its value is generally higher in the intermediate lower sub-layer than in the intermediate upper sub-layer in the Mixed Water Region, and the salinity minimum exists between layers with low and high values of the mixing ratio with its strong vertical gradient. The eddy transports of the Oyashio and Kuroshio waters in an isopycnal layer are divided into four components, usual isopycnal mixing of temperature and salinity being dominant, followed by the component associated with the thickness flux. The southward eddy transport of the Oyashio water and the northward eddy transport of the Kuroshio water are not symmetric to each other because the thickness-flux-associated components are in the same direction (southward).  相似文献   

11.
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   

12.
Transient tracer data (tritium, CFC11 and CFC12) from the southern, central and northwestern Weddell Sea collected during Polarstern cruises ANT III-3, ANT V-2/3/4 and during Andenes cruise NARE 85 are presented and discussed in the context of hydrographic observations. A kinematic, time-dependent, multi-box model is used to estimate mean residence times and formation rates of several water masses observed in the Weddell Sea.Ice Shelf Water is marked by higher tritium and lower CFC concentrations compared to surface waters. The tracer signature of Ice Shelf Water can only be explained by assuming that its source water mass, Western Shelf Water, has characteristics different from those of surface waters. Using the transient nature of tritium and the CFCs, the mean residence time of Western Shelf Water on the shelf is estimated to be approximately 5 years. Ice Shelf Water is renewed on a time scale of about 14 years from Western Shelf Water by interaction of this water mass with glacial ice underneath the Filchner-Ronne Ice shelf. The Ice Shelf Water signature can be traced across the sill of the Filchner Depression and down the continental slope of the southern Weddell Sea. On the continental slope, new Weddell Sea Bottom Water is formed by entrainment of Weddell Deep Water and Weddell Sea Deep Water into the Ice Shelf Water plume. In the northwestern Weddell Sea, new Weddell Sea Bottom Water is observed in two narrow, deep boundary currents flowing along the base of the continental slope. Classically defined Weddell Sea Bottom Water (θ ≤ −0.7°C) and Weddell Sea Deep Water (−0.7°C ≤ θ ≤ 0°C) are ventilated from the deeper of these boundary currents by lateral spreading and mixing. Model-based estimates yield a total formation rate of 3.5Sv for new Weddell Sea Bottom Water (θ = −1.0°C) and a formation rate of at least 11Sv for Antarctic Bottom Water (θ = −0.5°C).  相似文献   

13.
The south-western Atlantic region, 30–46°S, 35–55°W is noted for the confluence of the warm Brazil Current and the cold Falkland Current, as well as the eddies created from the two systems. Previous studies of the Brazil-Falkland Current confluence have concentrated on the formation of eddies south of 35°S. However, in situ measurements and TOPEX/POSEIDON imagery of the region reveal eddies developing as far north as 32°S. These features are not evident in the coincident AVHRR (Advanced Very High Resolution Radiometer) imagery. In this study, the origin of the eddies identified are discussed with reference to the contemporary in situ measurements of temperature and salinity. Using the combination of satellite imagery and in situ data, it can be determined that the surface waters of the eddies north of 36°S are influenced by water from an estuarine source. TOPEX imagery enables the movements and variability of the regions water masses: estuarine outflow, Falkland and Brazil currents, to be monitored synoptically. The combination of satellite imagery and the higher resolution of in situ AMT measurements enables these water masses to be better identified.  相似文献   

14.
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15.
Observations of the Labrador Sea eddy field   总被引：2，自引：0，他引：2  
This paper is an observational study of small-scale coherent eddies in the Labrador Sea, a region of dense water formation thought to be of considerable importance to the North Atlantic overturning circulation. Numerical studies of deep convection emphasize coherent eddies as a mechanism for the lateral transport of heat, yet their small size has hindered observational progress. A large part of this paper is therefore devoted to developing new methods for identifying and describing coherent eddies in two observational platforms, current meter moorings and satellite altimetry. Details of the current and water mass structure of individual eddy events, as they are swept past by an advecting flow, can then be extracted from the mooring data. A transition is seen during mid-1997, with long-lived boundary current eddies dominating the central Labrador Sea year-round after this time, and convectively formed eddies similar to those seen in deep convection modeling studies apparent prior to this time. The TOPEX / Poseidon altimeter covers the Labrador Sea with a loose “net” of observations, through which coherent eddies can seem to appear and disappear. By concentrating on locating and describing anomalous events in individual altimeter tracks, a portrait of the spatial and temporal variability of the underlying eddy field can be constructed. The altimeter results reveal an annual “pulsation” of energy and of coherent eddies originating during the late fall at a particular location in the boundary current, pinpointing the time and place of the boundary current-type eddy formation. The interannual variability seen at the mooring is reproduced, but the mooring site is found to be within a localized region of greatly enhanced eddy activity. Notably lacking in both the annual cycle and interannual variability is a clear relationship between the eddies or eddy energy and the intensity of wintertime cooling. These eddy observations, as well as hydrographic evidence, suggest an active role for boundary current dynamics in shaping the energetics and water mass properties of the interior region.  相似文献   

16.
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   

17.
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18.
The variability in the Caribbean Sea is investigated using high resolution (1/15°) general circulation model experiments. For the first time in this region, simulations were carried out with a 2-way nested configuration of the NEMO primitive equation model. A coarse North Atlantic grid (1/3°) reproduces the main features of the North Atlantic and Equatorial circulation capable of influencing ocean dynamics in the Caribbean Sea. This numerical study highlights strong dynamical differences among basins and modifies the view that dynamics are homogeneous over the whole Caribbean Basin. The Caribbean mean flow is shown to organize in two intense jets flowing westward along the northern and southern boundaries of the Venezuela Basin, which merge in the center of the Colombia Basin. Diagnostics of model outputs show that width, depth and strength of baroclinic eddies increase westward from the Lesser Antilles to the Colombia Basin. The widening and strengthening to the west is consistent with altimetry data and drifter observations. Although influenced by the circulation in the Colombia Basin, the variability in the Cayman Basin (which also presents a westward growth from the Chibcha Channel) is deeper and less energetic than the variability in the Colombia/Venezuela Basins. Main frequency peaks for the mesoscale variability present a westward shift, from roughly 50 days near the Lesser Antilles to 100 days in the Cayman Basin, which is associated with growth and merging of eddies.  相似文献   

19.
The variability of two modes of Labrador Sea Water (LSW) (upper and deep Labrador Sea Water) and their respective spreading in the interior North Atlantic Ocean are investigated by means of repeated ship surveys carried out along the zonal WOCE line A2/AR19 located at 43–48°N (1993–2007) and along the GOOS line at about 48–51°N (1997–2002). Hydrographic section data are complemented by temperature, salinity, and velocity time series recorded by two moorings. They have been deployed at the western flank of the Mid-Atlantic Ridge (MAR) in the Newfoundland Basin during 1996–2004. The analysis of hydrographic anomalies at various longitudes points to a gradual eastward propagation of LSW-related signals, which happens on time scales of 3–6 years from the formation region towards the MAR. Interactions of the North Atlantic Current (NAC) with the Deep Western Boundary Current (DWBC) close to Flemish Cap point to the NAC being the main distributor of the different types of LSW into the interior of the Newfoundland Basin. Comparisons between the ship data and the mooring records revealed that the mooring sites are located in a region affected by highly variable flow. The mooring time series demonstrate an elevated level of variability with eddy activity and variability associated with the NAC considerably influencing the LSW signals in this region. Hydrographic data taken from Argo profiles from the vicinity of the mooring sites turned out to mimic quite well the temporal evolution captured by the moorings. There is some indication of occasional southward flow in the LSW layer near the MAR. If this can be considered as a hint to an interior LSW-route, it is at least of minor importance in comparison to the DWBC. It acts as an important supplier for the interior North Atlantic, distributing older and recently formed LSW modes southward along the MAR.  相似文献   

20.
The impact of bio-optical heating on the properties of the upper Labrador Sea water was investigated by considering changes in light attenuation in water associated with the seasonal change of chlorophyll distribution. The time- and depth-dependent attenuation coefficients were obtained from remotely sensed SeaWiFS ocean-colour data. Sea-surface temperature (SST) and mixed-layer depth (MLD) were computed from a three-dimensional ocean circulation model. The model was integrated from 1999 to 2003 with 6-hourly atmospheric forcing. The changes in SST and MLD attributable to bio-optical heating were determined by comparing the model results using the observed attenuation coefficients (chlorophyll) to those using a weak and constant attenuation (clear water). The model results show that bio-optical heating is controlled mainly by chlorophyll concentration and MLD. The increase in SST is around 1 °C in most parts of the Labrador Sea and the shelves, and up to 2.7 °C in areas of shallow MLD and high chlorophyll concentrations (the Grand Banks and Northeastern Newfoundland Shelf). The increase is much higher than that found in previous studies, which was typically a fraction of a degree. Bio-optical heating also can enhance the stratification of the upper ocean and reduce the MLD by 20–50%.  相似文献   

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