Subpolar Mode Water Research Articles

Water mass transformation in the North Atlantic over 1985-2002 simulated in an eddy-permitting model

Abstract. Water mass transformation in the North Atlantic is examined in an eddy-permitting simulation with the OCCAM ocean general circulation model, forced by realistic surface fluxes over the period 1985-2002. Three Atlantic regions are considered - the subtropics, mid-latitudes, the northeast Atlantic - along with the Labrador Sea. The oceanic boundaries of each region coincide with hydrographic sections occupied in recent years. These regions broadly represent the formation sites of Eighteen Degree Water (EDW), Subtropical Mode Water (STMW), Subpolar Mode Water (SPMW) and Labrador Sea Water (LSW). Water mass budgets are obtained for each region and year. Terms in the budget comprise surface-forced transformation rates, boundary exchanges and unsteadiness. Transformation rates due to "total mixing" are obtained as the difference between net and surface transformation rates. Transports at the boundaries are evaluated alongside recent hydrographic section datasets, while surface-driven and mixing-driven transformation rates are compared with estimates based on air-sea flux datasets and inverse analysis of hydrographic data. In general OCCAM compares well with the observations, although two particular discrepancies are identified: deep overflows at high latitudes too light by around 0.2 kg m-3 and spurious heat gain of up to 100 Wm-2 east of the Grand Banks. Over 1985-2002, there is considerable variability on a range of timescales, in the annual surface-driven and mixing-driven formation rates of all four water masses. In the case of EDW and STMW, surface-driven and mixing-driven formation rates largely cancel. This is not so for SPMW and LSW, leading to regional net formation rates of up to 17 Sv and 15 Sv, respectively. In particular, OCCAM successfully simulates the strong LSW formation event of 1989-1994.

Production regimes in the northeast Atlantic: A study based on Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) chlorophyll and ocean general circulation model mixed layer depth

A 5 year time series of Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) ocean color images (SCHL) is compared with mixed layer depths (MLD) and atmospheric forcings from the Clipper model of the North Atlantic (1998–2002). This comparison is done over the region 16°–22°W, 30°–50°N, where subpolar mode waters are formed and which overlaps the region of the 2001 Programme Océan Multidisciplinaire Méso Echelle (POMME) experiment at sea. Three production regimes are identified on the basis of the seasonal cycling of SCHL and MLD: the well‐known subpolar and subtropical regimes and a midlatitude regime. The midlatitude regime is characterized by a single broad bloom weaker than the subpolar spring bloom and stronger than the subtropical fall bloom, which starts in fall as an entrainment bloom and peaks in spring as a restratification bloom. This specific regime is found between 35°N and 40°N (±2°) in the northeast Atlantic. It corresponds to winter MLDs between Ze (the depth of the euphotic layer) and 2Ze, i.e., it lays between the region where the winter MLD is greater than Sverdrup's critical depth (subpolar regime) and the region where the mixing is never deeper than the well‐lit layer (subtropical regime). The very specific characteristics of the midlatitude regime strengthen the biological carbon pump since production is active in winter within the waters to be subducted. The midlatitude regime also may provide an explanation for the unexpectedly low f ratios sometimes observed during the bloom in the region (North Atlantic Bloom Experiment, POMME). A large interannual variability is observed for the three regimes in terms of the timing and the intensity of the blooms and of the geographical boundaries of the regimes. These variabilities appear to be mainly driven by the synoptic and the low‐frequency atmospheric variabilities. It is also shown that in addition to the northward propagation of the subpolar spring bloom from 41°N (±1.3°) to 50°N, the (fall) entrainment bloom propagates southward over the whole latitudinal range (35°–50°N).

Observed mean and mesoscale upper ocean circulation in the midlatitude northeast Atlantic

Lagrangian instruments and altimetry data are used to investigate the upper (0 to ∼500 m) mean and mesoscale circulations in a region of the midlatitude northeast Atlantic (36°–47°N, 13°–23°W), a formation and subduction area of Subpolar Mode Water. Lagrangian data were collected between September 2000 and September 2001 with isobaric subsurface floats, totaling 34.0 float years, and surface drifting buoys, totaling 27.9 buoy years. The near‐surface (0–100 m) mean circulation is dominated by two weak (∼5 cm/s) eastward currents which veer southward in the domain. In the 100‐ to 500‐m‐deep layer, these eastward mean currents retroflect westward and form two anticyclonic circulation cells, opened to the west and separated by a cyclonic cell. Eddy kinetic energy is decreasing from the northwest to the southeast of the area, from ∼100–200 (cm/s)2 to ∼20–30 (cm/s)2. Long‐lived anticyclones and cyclones display mean coherent displacements, both to the southwest. Westward eddy propagation speeds, close to that of long Rossby waves, increase southward and are consistent with a superposition of beta effect and advection by mean currents. Southward eddy displacements are coherent with the observed mean currents. In the north, anticyclones are seen to consist of Central and Subpolar Mode Water eddies, most of them likely originating from the eastern boundary, whereas in the south, most anticyclonic features result from the influence of “Mediterranean Water Eddies” (meddies) on the upper layers. Cyclonic activity is concentrated in “cyclonic zones,” which are seen to exchange floats and water properties.

A synthesis of the POMME physical data set: One year monitoring of the upper layer

The Programme Océan Multidisciplinaire Méso Echelle (POMME) experiment was designed to describe and quantify the role of mesoscale processes in the subduction of mode waters in the northeast Atlantic. During 1 year (September 2000–October 2001), in situ measurements were conducted over a 8° square area centered on 18°W, 42°N. We present the synthesis of the physical data set collected during this experiment. To improve time and space coverages, these measurements are combined with satellite information from sea surface temperature and altimetry. Daily fields of temperature, salinity, and stream function are produced over a seasonal cycle with a simplified Kalman filter. We analyze the annual cycle of the upper layer. The 1 year mean circulation in the upper 400 m resembles the scheme proposed by Paillet and Mercier (1997). The meridional component of the flow carries cold water southward, thus contributing to cool the POMME area. The annual mean heat budget shows that this advection by the mean current nearly balances the warming by the surface heat fluxes. The mixed layer maximum depth is reached in March. It increases, in zonal averages, from 100 m south of 38°N to more than 270 m north of 45°N and, remarkably, is shaped by the mesoscale. The subpolar mode water formed in the north of the POMME area has a lower density (26.8–27.0) than the mode water formed in the years 1988–1990 that Paillet and Arhan (1996) found in the density range (27.0–27.1). This mode water is continuously advected southward across the 42°N latitude, leading to an annual mean transport of 1.4 106 m3 s−1.

Diagnosing Natural Variability of North Atlantic Water Masses in HadCM3

Abstract A study of thermally driven water mass transformations over 100 yr in the ocean component of the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) is presented. The processes of surface-forced transformations, subduction and mixing, both above and below the winter mixed layer base, are quantified. Subtropical Mode Waters are formed by surface heat fluxes and subducted at more or less the same rate. However, Labrador Seawater and Nordic Seawater classes (the other main subduction classes) are primarily formed by mixing within the mixed layer with very little formation directly from surface heat fluxes. The Subpolar Mode Water classes are dominated by net obduction of water back into the mixed layer from below. Subtropical Mode Water (18°C) variability shows a cycle of formation by surface fluxes, subduction ∼2 yr later, followed by mixing with warmer waters below the winter mixed layer base during the next 3 yr, and finally obduction back into the mixed layer at 21°C, ∼5 yr after the original formation. Surface transformation of Subpolar Mode Waters, ∼12°C, are led by surface transformations of warmer waters by up to 5 yr as water is transferred from the subtropical gyre. They are also led by obduction variability from below the mixed layer, by ∼2 yr. The variability of obduction in Subpolar Mode Waters also appears to be preceded, by 3–5 yr, by variability in subduction of Labrador Sea Waters at ∼6°C. This supports a mechanism in which southward-propagating Labrador seawater anomalies below the subpolar gyre can influence the upper water circulation and obduction into the mixed layer.

The origin of Intermediate and Subpolar Mode Waters crossing the Atlantic equator in OCCAM

The origin of the intermediate waters that cross the equatorial Atlantic as part of the return flow for North Atlantic Deep Water was studied in a high resolution global ocean model using a Lagrangian particle following technique. Most of these waters are subducted in the southeast Indian Ocean. Less than twenty percent comes directly from Drake Passage without looping into the Indian Ocean; the majority being provided by Agulhas leakage. Most of the intermediate waters that subduct in the South Atlantic do not follow the South Atlantic/Indian Ocean supergyre, but remain within the Antarctic Circumpolar Current, and gradually transform into Circumpolar Deep Water by diapycnal mixing.

Transformation of the Warm Waters of the North Atlantic from a Geostrophic Streamfunction Perspective

To obtain a description of the hydrographic state of the North Atlantic Current‐subpolar front (NAC‐SPF) system, historical hydrographic data from the subpolar North Atlantic are projected into a baroclinic streamfunction space, resulting in three-dimensional Gravest Empirical Mode (GEM) fields for temperature and specific volume anomaly, parameterized by pressure, dynamic height, and day of the year. From the specific volume anomaly GEM, the corresponding potential vorticity field is calculated. These fields are constructed for 12 subregions, chosen to follow the mean path of the NAC‐SPF system. Analysis of the seasonal potential vorticity cycle of the GEM fields shows that the main mechanism for the formation of Subpolar Mode Water is winter convection. The GEM fields are also used to obtain the approximate location of formation sites for the different Subpolar Mode Water classes. The evolution of the mean fields for the waters is studied along baroclinic streamlines of the NAC‐SPF system. This shows that cross-frontal mixing, between the cold and fresh subpolar waters and the salty and warm waters coming north from the subtropics via the Gulf Stream, is the dominant mechanism for the light-to-dense transformation process of the NAC‐SPF waters that enter the western subpolar region. On the other hand, a combination of atmospheric cooling, vertical mixing during wintertime convection, and entrainment of the saltier waters found on the northeastern subtropical gyre is the main factor transforming the NAC‐SPF waters that enter the eastern subpolar gyre. This suggests that an influx along the eastern margin of salty water from the European Basin plays a significant role in the transformation of the NAC‐SPF waters that continue their way toward the Nordic seas.

Subpolar Mode Water formation traced by neodymium isotopic composition

The formation of Subpolar Mode Water (SPMW) is documented by combining neodymium isotopic composition (Nd IC) as a conservative tracer with salinity, temperature and O2 concentration. Nd IC reveals a more pronounced eastward extension of the Labrador Current, particularly its Subarctic Intermediate Water component, than the other parameters. Nd IC also suggests that waters flowing through the Denmark Strait do not significantly contribute to the SPMW formation. A two end‐member mixing, involving Subarctic Intermediate Water (and maybe ultimately Labrador Current Water) and North Atlantic Central Water, is proposed to explain SPMW formation. These results provide important insight into the processes controlling the Nd IC in the North Atlantic Subpolar Gyre, which could provide a means for reconstructing the northward extension of the Gulf Stream in the past.

‘Reemergence’ areas of winter sea surface temperature anomalies in the world's oceans

Using datasets of sea surface temperature (SST), surface heat flux, upper ocean thermal data, and climatological temperature and salinity profiles, we try to detect ‘reemergence’ areas of winter SST anomalies in the world's oceans, and describe characteristics of these areas in terms of mixed layer depth (MLD), annual mean heat flux and properties of waters formed in winter mixed layer. Eventually, seven reemergence areas are found: four in the Northern Hemisphere and three in the southern Hemisphere. All areas have a large seasonal variation of MLD, and are the regions where annual mean heat fluxes are relatively small except for two regions in the Northern Hemisphere. In the viewpoint of water properties, it is found that these areas correspond to the mode water formation regions: subtropical mode waters of the Indian Ocean, South Pacific, South Atlantic, North Pacific and North Atlantic, and North Pacific central and North Atlantic subpolar mode waters.

Chlorofluorocarbon Constraints on North Atlantic Ventilation

The North Atlantic Ocean vigorously ventilates the ocean interior. Thermocline and deep water masses are exposed to atmospheric contact there and are sequestered in two principal classes: Subtropical Mode Water (STMW: 26.5 {le} {sigma}{thetas} {le} 26.8) and Subpolar Mode Water (SPMW: 26.9 {le} {sigma}{thetas} {le} 27.8). These ventilation rates and pathways are uncertain, and a powerful way to estimate them is to monitor the penetration of chlorofluorocarbon (CFC) tracers. Here, a CFC dataset of over 44 000 observations, taken between 1982 and 1998, is combined with a non-eddy-resolving ( resolution) general circulation model of the North Atlantic Ocean. The CFC data are assimilated with the model by optimizing the uncertain air{ndash}sea CFC flux. The assimilated CFC fields are then systematically compared with the observations to identify the best fit and hence the most realistic ventilation. Three GCM experiments are performed this way to find the dependence on model thickness diffusivity. Each GCM solution is close to being statistically consistent with the CFC observations and likely sources of error. Lower diffusivity gives the best match to data although some systematic bias in sequestering tropospheric CFC remains. Lower diffusivity, around 150 m2 s{ndash}1, permits a stronger circulation with a more realistic North Atlantic Current. For this experiment, the subduction rate is around 16 Sv (Sv {equiv} 106 m3 s{ndash}1) in the subtropics and eastern subpolar Atlantic (26.35 {le} {sigma}{thetas} {le} 27.13) averaged over 1975{ndash}95. Around 26 Sv is formed in the Labrador and Irminger Seas (27.58 {le} {sigma}{thetas} {le} 27.8). Only about 40{percnt} of the CFC carried into the subpolar interior by this flux remained there in 1998, however. The rest was returned to the subpolar mixed layer after an average period of 6{ndash}8 yr. In contrast, 70{percnt} of the CFC subducted into the subtropical interior remained there in 1998

General circulation and intergyre dynamics in the eastern North Atlantic from a regional primitive equation model

The mean circulation of the eastern North Atlantic is investigated using a regional 0.8° × cos(latitude)‐resolution configuration of the SPEM primitive equation sigma coordinate model, forced by seasonal and monthly surface fluxes. The computational domain is surrounded by three self‐adapting open boundaries which evacuate the outgoing perturbations and laterally control the baroclinic modes in inflow regions, but let the model adjust the barotropic mode to a large extent. The final solution is stable and reproduces most features of the basin's mean circulation well: A realistic Azores Current, the observed paths and transports of subpolar currents, of the branches of the North Atlantic Current, of the intergyre zone modal and intermediate water masses down to about 2000 m. Some unrealistic circulation features, attributed to the modest resolution and to certain limitations of the sigma coordinates, are found below 2000 m. The buoyancy and vorticity balances are investigated in the intergyre zone. The Subpolar Mode Water (SPMW), subducted at a realistic rate, continues its southward journey toward the ocean's interior in accordance with the ventilated thermocline theory, with eddy diapycnal fluxes exerting a moderating effect. The poleward motion observed between 300 and 1000 m is well reproduced north of 45°N and governed by similar dynamics, but is absent south of 45°N within the upper Mediterranean Water.

Constraining a North Atlantic Ocean General Circulation Model with Chlorofluorocarbon Observations

Measurements of anthropogenic tracers such as chlorofluorocarbons and tritium must be quantitatively combined with ocean general circulation models as a component of systematic model development. The authors have developed and tested an inverse method, using a Green’s function, to constrain general circulation models with transient tracer data. Using this method chlorofluorocarbon-11 and -12 (CFC-11 and -12) observations are combined with a North Atlantic configuration of the Miami Isopycnic Coordinate Ocean Model with4 /38 resolution. Systematic differences can be seen between the observed CFC concentrations and prior CFC fields simulated by the model. These differences are reduced by the inversion, which determines the optimal gas transfer across the air‐sea interface, accounting for uncertainties in the tracer observations. After including the effects of unresolved variability in the CFC fields, the model is found to be inconsistent with the observations because the model/data misfit slightly exceeds the error estimates. By excluding observations in waters ventilated north of the Greenland‐Scotland ridge (s 0 , 27.82 kg m23; shallower than about 2000 m), the fit is improved, indicating that the Nordic overflows are poorly represented in the model. Some systematic differences in the model/data residuals remain and are related, in part, to excessively deep model ventilation near Rockall and deficient ventilation in the main thermocline of the eastern subtropical gyre. Nevertheless, there do not appear to be gross errors in the basin-scale model circulation. Analysis of the CFC inventory using the constrained model suggests that the North Atlantic Ocean shallower than about 2000 m was near 20% saturated in the mid-1990s. Overall, this basin is a sink to 22% of the total atmosphere-to-ocean CFC-11 flux—twice the global average value. The average water mass formation rates over the CFC transient are 7.0 and 6.0 Sv (Sv [ 106 m3 s21) for subtropical mode water and subpolar mode water, respectively.

The oxygen isotope composition of water masses in the northern North Atlantic

The ratio of oxygen-18 to oxygen-16 (expressed as per mille deviations from Vienna Standard Mean Ocean Water, δ 18O) 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 δ 18O composition, as are the processes involved in their formation. The isotopically heaviest surface waters occur in the eastern region where values of δ 18O and salinity ( S) lie on an evaporation–precipitation line with slope of 0.6 in δ 18O– 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 δ 18O–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 km 3 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, δ 18O=0.22‰) and Northwest Atlantic Bottom Waters (NWABW, δ 18O=0.13‰) are isotopically distinct reflecting different formation and mixing processes. NEADW lies on the North Atlantic precipitation mixing line in δ 18O–salinity space, whereas NWABW lies between NEADW and LSW on δ 18O–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 δ 18O 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.

Climate Variability Inferred from a Continuously Stratified Model of the Ideal-Fluid Thermocline*

Climate variability in the subtropical gyre interior induced by anomalous surface thermal forcing, Ekman pumping, mixed layer depth variability, and anomalous subpolar water formation is examined, using a continuously stratified model of the ideal-fluid thermocline. Cooling (heating) induces a negative (positive) potential vorticity perturbation in the ventilated thermocline, and the associated density perturbations propagate downstream in the form of second and higher baroclinic modes. The second baroclinic mode resembles the traditional second baroclinic mode because it has a thermal structure with cooling (warming) in the upper thermocline and warming (cooling) in the lower thermocline. Anomalous Ekman pumping can also induce density perturbations that propagate westward in the form of the first baroclinic mode. In addition, if the outcrop lines are nonzonal, there are density perturbations that propagate downstream in the form of the second or third baroclinic modes. Perturbations in the sea surface elevation are mostly confined to the region of anomalous forcing. On the other hand, when the low potential vorticity anomaly in the subpolar mode water spreads into the subtropical basin, both the unventilated and ventilated thermocline move downward. Consequently, temperature at a given depth seems to increase.

Water mass properties and fluxes in the Rockall Trough, 1975–1998

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.

Interannual thermohaline changes in the northern North Atlantic 1991–1996

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.

An inverse model of the eastern North Atlantic general circulation and thermocline ventilation

An inverse model is applied to a set of high-quality hydrographic measurements gathered between 1981 and 1993 in the eastern North Atlantic, between 24°N and 54°N and east of 35°W. The method seeks a density field as close as possible to the hydrographic data and an absolute velocity field in exact geostrophic and hydrostatic balances, satisfying as well as possible the planetary vorticity balance, the Ekman dynamics at the surface and mass, salt and heat conservations in a topto-bottom integrated form. A solution is found that departs only slightly from the hydrographic data and that presents reasonably small constraint residuals. The solution is discussed in terms of the eastern North Atlantic general circulation and thermocline ventilation during the observational period.The North Atlantic Current appears to be composed of several branches, and its influence extends down to more than 2500 m. Two-thirds of its total transport recirculates northward across 54°N. The Azores Current appears as an almost zonal current around 33°N that, while recirculating southward, is fed from the north by the southward recirculation of the North Atlantic Current. The total eastward transport of these two currents above the Mid-Atlantic Ridge amounts to 58 ± 11 Sv, far more than predicted using the Sverdrup relation, and more than most of the previous estimates. It is shown that the interaction between the deep circulation and the bottom topography allows such a transport to be compatible with the planetary vorticity balance.Using the modeled circulation, thermocline ventilation is thoroughly studied. The subduction rate patterns show where the lightest varieties (27.0 < σθ < 27.4) of Subpolar Mode Water (SPMW) are mainly formed and subducted. A total of 11.9 ± 0.5 Sv of SPMW formation is estimated in the modeled area, while the total subduction transport amounts to 7.7 ± 0.5 Sv, of which 2.5 ± 0.4 Sv is SPMW subduction. Eddy diffusion along the isopycnals is estimated as playing no important role in SPMW subduction and in the first stages of its subsequent southward transport, but seems to be the dominant process by which that water crosses the Azores Front south of 36°N. The Lagrangian, annual buoyancy budget of the winter mixed layer is finally diagnosed from the model fields, and it is confirmed that SPMW is formed where the mixed-layer loses buoyancy, and subducts as soon as the mixed layer gains buoyancy.

Carbon dioxide characteristics of water masses in the northern North Atlantic Ocean

Abstract Accurate measurements of total dissolved carbon dioxide (TCO 2 ) together with other tracers (Θ, S , O 2 , and nutrients) have been carried out during a hydrographic survey of a zonal section between Ireland and Greenland and a number of sections in the Iceland Basin. The resulting data, with > 1300 TCO 2 data points, have allowed the determination of the TCO 2 signature of the water masses in the northern North Atlantic Ocean in terms of seven water types. The structure of TCO 2 and dissolved phosphate and nitrate appears to be well correlated with the apparent oxygen utilization. This indicates that ageing of the water types, by mineralization of organic matter, is more important for the observed TCO 2 structure than differences in the preformed TCO 2 when the water types were formed. The overturning of water in the “oceanic conveyor belt” will support meridional TCO 2 transport. This transport was estimated at 0.16 · 10 −1 mol s −1 (−0.06 Gt a −1 C) from the differences of TCO 2 content between the northward flowing Surface and Sub-Polar Mode water and the southward flowing deep Iceland-Scotland and Denmark Strait Overflow Waters.

Oceanic Ventilation in the Eastern North Atlantic

Abstract Ventilation of the eastern North Atlantic by subpolar mode water is illustrated by recent hydrographic section and analyzed using a large-scale stationary thermocline model including a surface mixed layer. The distributions of potential vorticity along the hydrographic lines and the geostrophic transports across them confirm the circulation pattern of the mode water that was previously deduced from older data, that is, arrival from the west at intergyre latitudes and southward subduction of the lightest part at about 42°N. The model approach emphasizes the large-scale advective-thermodynarnic equilibrium, which controls the shape of the thick (>500 m) winter mixed layer observed in the eastern basin. Following recent theories, the process of ventilation is interpreted in terms of exchanges between the mixed layer and the stratified thermocline: Mode water is formed where thermocline water is entrained into a thick mixed layer and is subducted where mixed layer water is injected into the thermocli...

Shallow Pycnoclines and Mode Water Subduction in the Eastern North Atlantic

Abstract A Lagrangian one-dimensional model is used to study the subduction of Subpolar Mode Water in the eastern North Atlantic and to analyze recently observed hydrological features related to this process. Considering a southward moving column of mode water, subduction occurs when the upper part of the column starts gaining buoyancy in an annual budget. The seasonal pycnocline on top of the column can no longer be completely eroded in winter, and it is shown how its remnant forms a shallow “secondary” pycnocline at about 200 m depth, which isolates the lower part of the fluid column from the atmosphere. This mechanism for the subduction of Subpolar Mode Water induces a strong meridional gradient in the winter mixed layer depth. The various components of the buoyancy input are thoroughly studied. Horizontal advection in the shallow Ekman layer and vertical advection along the column are shown to modify significantly both the thermal and haline contents of the column, with magnitudes comparable to the ai...