The influence of intermediate waters on the stability of the eastern North Atlantic (original) (raw)
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Deep Sea Research Part I: Oceanographic Research Papers, 2011
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.
Changes in the vertical structure of the North Atlantic Ocean between glacial and modern times
Quaternary Science Reviews, 1992
The distribution of planktic foraminiferal i5180 over the North Atlantic Ocean has been interpreted by Duplessy et al. (1991, 1992) in terms of the distribution of surface water salinity during the last glacial maximum (LGM) and the Younger Dryas (YD). We present here the implications of this surface salinity distribution for changes in deep convection during these periods. Temperature/salinity/density diagrams of the water column have been constructed using the planktic and benthic foraminifera 61sO values together with sea surface temperature estimates obtained by using micropaleontological transfer functions. The precision bf these reconstructions is limited by the remaining uncertainties in the two basic assumptions upon which the methods are based: --the sea surface temperature reconstruction by CLIMAP are valid within the statistical uncertainties; --the water salinityhSmO relationship is well constrained for surface and intermediate ocean waters. A significant difference between the modern and the reconstructed glacial water structure is observed. During the LGM, the modern relatively warm and salty Norwegian Sea surface water and the North Atlantic Deep Water (NADW) were replaced by low salinity surface water in the northern Atlantic Ocean and the Norwegian Sea and cold deep water (0-1°C). However, surface water salinity in the central North Atlantic Ocean (52-54°N., 25--40°W.) was sufficiently high to permit deep water convection as a source of North Atlantic intermediate and deep waters. Open ocean convection at the northern limit of the subtropical surface waters (around 40°N.) may have contributed to the ventilation of the intermediate waters. The waters deeper than 2000 m were the result of mixing between poorly-oxygenated southern waters and well-ventilated North Atlantic deep waters. A similar reconstruction for the cold YD period indicates that the modern type of circulation, with warm and ventilated NADW, was largely in operation and that the hydrologic pattern was different from that prevailing during the LGM.
Geochemistry, Geophysics, Geosystems, 2016
The Nd isotopic composition (εNd) of seawater and cold-water coral (CWC) samples from the Gulf of Cádiz and the Alboran Sea, at a depth of 280-827 m were investigated in order to constrain mid-depth water mass dynamics within the Gulf of Cádiz over the past 40 ka. εNd of glacial and Holocene CWC from the Alboran Sea and the northern Gulf of Cádiz reveals relatively constant values (-8.6 to -9.0 and -9.5 to -10.4, respectively). Such values are similar to those of the surrounding present-day mid-depth waters from the Mediterranean Outflow Water (MOW; εNd ~ -9.4) and Mediterranean Sea Water (MSW; εNd ~ -9.9). In contrast, glacial εNd values for CWC collected at thermocline depth (550-827 m) in the southern Gulf of Cádiz display a higher average value (-8.9±0.4) compared to the present-day value (-11.7±0.3). This implies a higher relative contribution of water masses of Mediterranean (MSW) or South Atlantic origin (East Antarctic Intermediate Water, EAAIW). Our study has produced the first evidence of significant radiogenic εNd values (~ -8) at 19, 23-24 and 27 ka, which are coeval with increasing iceberg discharges and a weakening of Atlantic Meridional Overturning Circulation (AMOC). Since MOW εNd values remained stable during the last glacial period, it is suggested that these radiogenic εNd values most likely reflect an enhanced northward propagation of glacial EAAIW into the eastern Atlantic Basin.
Deep water changes at the western boundary of the subpolar North Atlantic during 1996 to 2001
Deep Sea Research Part I: Oceanographic Research Papers, 2004
Repeated hydrographic observations between 1996 and 2001 of the deep water mass distribution on four sections in the western Labrador Sea and northwestern North Atlantic at about 56 N, 53 N, 48 N and 43 N show significant changes in the water mass characteristics. These changes are spreading southward mainly with the Deep Western Boundary Current (DWBC). Shallower convection forms a convective water mass known as upper Labrador Sea Water (ULSW). During periods of deep convection in the Labrador Sea, ULSW was described to be formed in the western boundary current region. In the post deep convection period 1996 to 2001 ULSW was formed in the western and central Labrador Sea and spreads mainly westward towards and along the western boundary. At 53 N ULSW moves southward as a part of the deep Labrador Current, also constituting the upper part of the DWBC. In the early 1990s the deep convection produced a large volume of deep Labrador Sea Water (LSW) which filled intermediate layers of the central region of the Labrador Sea. After these years the convection became weaker, with no apparent LSW renewal in 1996, partial mixing down to 1500 m in 1997 and no notable LSW formation between 1998 and 2001. At the southwestern exit of the Labrador Sea at 53 N the deep LSW in 2001 was least in thickness and highest in salinity and temperature compared to the years since 1996. This reflects restratification which resulted in an increase in the density stratification between 1000 and 2000 m in the central Labrador Sea as well as year-to-year transformation of the LSW core. LSW passes 43 N off the Grand Banks about 1 to 2 years after it was first seen at 56 N. At the 48 N and 43 N sections the northward flowing North Atlantic Current (NAC), farther offshore than the DWBC, complicates the property distributions. Saltier and warmer LSW recirculates northward with the NAC at 43 N. Between 1996 and 2001 the Gibbs Fracture Zone Water (GFZW) turned colder and fresher. The Denmark Strait Overflow Water (DSOW)
Structure, transports and transformations of the water masses in the Atlantic Subpolar Gyre
Progress in Oceanography, 2015
We discuss the distributions and transports of the main water masses in the North Atlantic 21 Subpolar Gyre (NASPG) for the mean of the period 2002-2010 (OVIDE sections 2002-2010 every 22 Atlantic Basin to the Irminger Basin (9.4 ± 4.7 Sv) into the contributions of the Central Waters (2.1 35 ± 1.8 Sv), Labrador Sea Water (LSW, 2.4 ± 2.0 Sv), Subarctic Intermediate Water (SAIW, 4.0 ± 0.5 36 Sv) and Iceland-Scotland Overflow Water (ISOW, 0.9 ± 0.9 Sv). Once LSW and ISOW cross over 37 the Reykjanes Ridge, favoured by the strong mixing around it, they leave the Irminger Basin 38 through the deep-to-bottom levels. The results also give insights into the water mass 39 transformations within the NASPG, such as the contribution of the Central Waters and SAIW to the 40 formation of the different varieties of SPMW due to air-sea interaction. 41
Stable isotope and ice-rafted debris records from three core sites in the mid-latitude North Atlantic (IODP Site U1313, MD01-2446, MD03-2699 are combined with records of ODP Sites 1056/1058 and 980 to reconstruct hydrographic conditions during the middle Pleistocene spanning Marine Isotope Stages (MIS) 9-14 (300-540 ka). Core MD03-2699 is the first high-resolution mid-Brunhes record from the North Atlantic's eastern boundary upwelling system covering the complete MIS 11c interval and MIS 13. The array of sites reflect western and eastern basin boundary current as well as north to south transect sampling of subpolar and transitional water masses and allow the reconstruction of transport pathways in the upper limb of the North Atlantic's circulation. Hydrographic conditions in the surface and deep ocean during peak interglacial MIS 9 and 11 were similar among all the sites with relative stable conditions and confirm prolonged warmth during MIS 11c also for the mid-latitudes. Sea surface temperature (SST) reconstructions further reveal that in the mid-latitude North Atlantic MIS 11c is associated with two plateaus, the younger one of Correspondence to: A. Voelker (antje.voelker@lneg.pt) which is slightly warmer. Enhanced subsurface northward heat transport in the eastern boundary current system, especially during early MIS 11c, is denoted by the presence of tropical planktic foraminifer species and raises the question how strongly it impacted the Portuguese upwelling system. Deep water ventilation at the onset of MIS 11c significantly preceded surface water ventilation. Although MIS 13 was generally colder and more variable than the younger interglacials the surface water circulation scheme was the same. The greatest differences between the sites existed during the glacial inceptions and glacials. Then a north -south trending hydrographic front separated the nearshore and offshore waters off Portugal. While offshore waters originated from the North Atlantic Current as indicated by the similarities between the records of IODP Site U1313, ODP Site 980 and MD01-2446, nearshore waters as recorded in core MD03-2699 derived from the Azores Current and thus the subtropical gyre. Except for MIS 12, Azores Current influence seems to be related to eastern boundary system dynamics and not to changes in the Atlantic overturning circulation.
On the intermediate and deep water flows in the South Atlantic Ocean
Journal of Geophysical Research, 1997
A multiparameter analysis is applied on zonal and meridional hydrographic sections obtained for the South Atlantic Ventilation Experiment (SAVE) to determine the spreading and mixing of water masses in the South Atlantic Ocean, focusing our interest on the large-scale flow of intermediate and deep waters. The method utilizes all information from the hydrographic data set including temperature, salinity, dissolved oxygen, and nutrient fields. Mixing proportions are quantified and plotted along the eight sections considered. Results show no evidence of a primats' route of Antarctic Intermediate Water along the western boundary of the South Atlantic. In the eastern basin the eastward extension of the Upper North Atlantic Deep Water (NADW) in the Guinea Basin following the cyclonic subequatorial gyre is confirmed. In the Angola Basin a weak but thick NADW core layer is observed in conjunction with very little presence of Lower Circumpolar Deep Water (LCDW). High LCDW concentrations in Cape Basin are indicative of the communication of this basin to cold water sources in the south. The method is sensitive enough to detect for instance the presence of the Congo River Plume in the Angola Basin or the influence of the Weddell Sea Deep Water in the vicinity of the Romanche and Chain Fracture Zones in the equatorial region. In conjunction with the multiparameter analyses along SAVE sections, an analysis of components of the residual vector R indicates a middepth minimum in the RN/P utilization ratio. Both a suitable explanation for the minimum and the potential consequences for the multiparameter analyses of South Atlantic water mass circulation are still to be found.