Model simulated freshwater transport along the Labrador current east of the Grand Banks of Newfoundland (original) (raw)
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Deep Sea Research Part I: Oceanographic Research Papers, 2010
Historical hydrographic data, spanning the period 1896-2006, are used to examine the annual mean and seasonal variations in the distribution of freshwater along and across the shelf/slope boundary along the Labrador and Newfoundland Shelves and the Grand Banks of Newfoundland. Particular attention is paid to the export of freshwater along the eastern Grand Banks, between Flemish Cap and the Tail of the Grand Banks, as this has long been identified as a preferential region for the loss of mass and freshwater from the boundary. The data are combined into isopycnally averaged long-term annual and monthly mean gridded property fields and the evolving distribution of fresh arctic-origin water is analyzed in fields of salinity anomaly, expressed as departures from the "central water" temperature-salinity relation of the Gulf Stream. The climatology confirms that cold/fresh northern-source waters are advected offshore within the retroflecting Labrador Current along the full length of the boundary between Flemish Cap and the Tail of the Grand Banks. In fact, it is estimated that most of the equatorward baroclinic transport at the boundary must retroflect back toward the north in order to explain the annual mean distribution of salinity in the climatology. While the retroflection of the Labrador Current appears seasonally robust, the freshwater distribution within the retroflection region varies in response to (1) the freshness of the water available for export which is set by the arrival and rapid flushing of the seasonal freshwater pulse at the boundary, (2) seasonal buoyancy forcing at the surface which alters the vertical stratification across the retroflection region, restricting certain isopycnal export pathways, and (3) the density structure along the eastern Grand Banks, which defines the progressive retroflection of the Labrador Current.
Shelf-basin transport of freshwater into the Labrador Sea
Egu General Assembly Conference Abstracts, 2009
Freshwater exiting the Arctic Ocean through the Canadian Arctic Archipelago (CAA) has been shown to affect meridional overturning circulation and thereby the global climate system. However, because of constraints of spatial resolution in most global ocean models, neither the flow of low salinity water through the CAA to the Labrador Sea nor the eddy activity that may transport freshwater from the shelf to areas of open ocean convection can be directly simulated. To address these issues, this study uses a high-resolution iceocean model of the pan-Arctic region with a realistic CAA and forced with realistic atmospheric data. This model resolves conditions in the Arctic Ocean upstream of the Labrador Sea and is coupled to a thermodynamicdynamic sea ice model that responds to the atmospheric forcing. The major shelf-basin exchange of liquid freshwater occurs south of Hamilton Bank, whereas the largest ice flux occurs in the northwest of the basin. Freshwater flux anomalies entering the Labrador Sea through Davis Strait do not immediately affect deep convection. Instead, eddies acting on shorter time scales can move freshwater to locations of active convection and halt the process. Convection is modulated by the position of the ice edge, highlighting the critical need for a coupled ice-ocean model. Finally, the size of eddies and the short duration of events demonstrate the need for high resolution, both spatial and temporal.
Journal of Geophysical Research, 2010
linked to the meridional overturning circulation and the marine ecosystem off northeast North America. Nevertheless, knowledge of its decadal variability is inadequate because of scarcity of current meter data. By using a novel synthesis of satellite altimetry with conductivity-temperaturedepth (CTD) data we assess the Labrador Current variability north of the Hamilton Bank (56 o N) over 1993-2004. Our analysis shows a decline of the surface-to-bottom transport of current by 6.3 ± 1.5 Sv (1 Sv =10 6 m 3 s -1 ) in the 1990s (significant at the 99% confidence level) and a likely partial rebound of 3.2 ± 1.7 Sv in the early 2000s (significant at the 89% confidence level only). The inferred multiyear changes in the Labrador Current transport seem to be primarily barotropic and positively correlated (at the 99% level) with the North Atlantic Oscillation at zero lag implying a fast response of the regional circulation to the atmospheric forcing variability. The results compare favorably with direct current measurements and recent model-based findings on the multi-year variability of the subpolar gyre and its underlying mechanisms. The study demonstrates the feasibility of combining altimetry and CTD data for assessing the climatic variability of the boundary currents.
Seasonal variability of the Labrador Current and shelf circulation off Newfoundland
Journal of Geophysical Research, 2008
1] Three-dimensional finite element models were established for the Newfoundland and Labrador Shelf to investigate climatological monthly mean wind-and density-driven circulation. The model was forced using wind stresses from the National Center for Environmental Prediction-National Center for Atmospheric Research reanalysis data prescribed at the sea surface, large-scale remote forcing determined from a North Atlantic model, monthly mean temperature and salinity climatology, and M 2 tide on the open boundary. The model results were examined against various in situ observations (moored current meter, tide gauge, and vessel-mounted acoustic Doppler current profiler data) and satellite drift measurements and discussed together with literature information. The seasonal mean circulation solutions were investigated in terms of relative importance of wind to density forcing for the Labrador Current. The model results indicate significant seasonal and spatial variations, consistent generally with previous study results and in approximate agreement with observations for the major currents. The region is dominated by the equatorward flowing Labrador Current along the shelf edge and along the Labrador and Newfoundland coasts. The Labrador Current is strong in the fall/winter and weak in the spring/summer. The mean transport of the shelf edge Labrador Current is 7.5 Sv at the Seal Island transect and 5.5 Sv through the Flemish Pass. The seasonal ranges are 4.5 and 5.2 Sv at the two sections, respectively. Density-and wind-driven components are both important in the inshore Labrador Current. The density-driven component dominates the mean component of the shelf edge Labrador Current while the large-scale wind-forcing contributes significantly to its seasonal variability.
Observation of the circulation in the Newfoundland Basin in winter 1997
A hydrographic survey was performed in January-February 1997 to document the winter circulation of the North Atlantic Current system in the Newfoundland Basin, as part of the Fronts and Atlantic Storm Tracks Experiment (FASTEX). Eighty-seven conductivity-temperature-depth (CTD) stations were occupied along a foursection trapezoid, which spanned the ''Northwest Corner'' and the branching of the North Atlantic Current along 35ЊW. Realistic sea surface temperature analyses were produced every 15 days, using all available data collected in this area during the two months of the FASTEX experiment. These maps were combined with sea level anomaly fields from the TOPEX/Poseidon and ERS-2 satellites at the same time intervals to analyze the features of the main currents in the area and their evolution. These combined analyses, providing a coherent overview of the fronts and jets identified along the ship track, and the CTD stations are further used to estimate their transports. The general pattern is a 15 Sv (Sv ϵ 10 6 m 3 s Ϫ1 ) transport by the North Atlantic Current at 47ЊN, 43ЊW, the existence of a recirculating gyre inside the Northwest Corner, and a complex branching of the circulation associated with significant surface fronts. The recirculating gyre forms a closed circulation, in which a very deep warm eddy, 100 km wide, was sampled at the end of February: its mixed layer was 800 m deep and its transport was 27 Sv. Along 35ЊW, three fronts were identified between 45Њ and 52ЊN: the Northern Subarctic Front, the Southern Subarctic Front, and the Mid-Atlantic Front, whose origins are precisely located. The currents associated with these fronts transport 26 Sv toward the east before crossing the Mid-Atlantic Ridge and supplying the eastern part of the North Atlantic basin. An important transport (14 Sv) was calculated near 46ЊN, 37ЊW, which mostly fed the current associated with the Mid-Atlantic Front.
Simulation of three-dimensional circulation and hydrography over the Grand Banks of Newfoundland
Ocean Modelling, 2011
There are few ocean models that both adequately resolve the cross-shelf structure of the Labrador Current and have been sufficiently evaluated against in situ observations at tidal, synoptic and seasonal scales. We present a three-dimensional, high-resolution, prognostic, nonlinear circulation model for the Newfoundland offshore based on the finite volume coastal ocean model (FVCOM). The FVCOM uses unstructured grid in the horizontal and thus allows efficient and effective use of grid resolution to resolve coastal-and shelf-scale features. The model results are evaluated against current meter measurements, vessel-mounted acoustic Doppler current profiler (ADCP) data, and tide-gauge observations. The FVCOM climatological monthly-mean currents over the shelf and slope show good agreement with observations and substantial improvement over those from an earlier finite-element model. The simulated tidal elevations agree well (4 cm of the root-sum-square absolute error for the total tidal height) with observations, and show improvement over previous tidal models over the Labrador Shelf. The hindcasts for the spring to fall of 1999 show reasonable skill in reproducing temperature, salinity and currents. At station 27 the observed temperature and salinity have seasonal ranges of 14°C and 1.5 psu near the surface from April to November; while the root-mean-square (RMS) differences are 2.1°C and 0.3 psu between the model and observations. On the Flemish Cap transect the observed temperature and salinity range from À1.5 to 13.1°C and from 31.3 to 34.9 psu on July 17-20, 1999; while the RMS differences are 1.0°C and 0.2 psu. The model-observation velocity difference ratio is 0.53 on this transect on July 17-18, 1999.
Variability and propagation of Labrador Sea Water in the southern subpolar North Atlantic
Deep Sea Research Part I: Oceanographic Research Papers, 2009
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
Analysis of an 80-Year Integration of a 1/3-Degree Ocean Model of the Subpolar North Atlantic
Journal of Oceanography, 2005
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 eddytransfer 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 σ σ σ σ σ 2 = 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. (10 7 cm 2 s-1) was used in a model of the North Atlantic (England and Holloway, 1998). Since the intensity of the eddy-induced tracer transport is variable in the ocean, there have been attempts to take it into account. The variable eddy-transfer coefficient proposed by Visbeck et al. (1997) (henceforth referred to as VMHS) for the GM scheme outperformed a constant coefficient in simulations with a 1.25°-resolution model of Wright (1997), and a discussion of its effect in an eddy-permitting global ocean model was presented by Gent et al. (2002). Myers and Deacu (2004) modelled the subpolar North Atlantic with an eddy-permitting model (1/3°-resolution) and found an unrealistic drift of the model Labrador Sea salinity. They suggested that enhanced baroclinic eddy activity along the Labrador slope strengthened the countercurrent adjacent to the Labrador Current, which entrained too much high-salinity water of North Atlantic Current origin into the Labrador Sea. Deacu and Myers (2005) considered the inclusion of a VMHS eddy-transfer coefficient and found it had a positive effect on the model results. Improvements to the simulated velocity field included a better representation of the Labrador
Towards an understanding of Labrador Sea salinity drift in eddy-permitting simulations
Ocean Modelling, 2010
Model drift in the Labrador Sea in eddy permitting model simulations is examined using a series of configurations based on the NEMO numerical framework. There are two phases of the drift that we can identify, beginning with an initial rapid 3-year period, associated with the adjustment of the model from its initial conditions followed by an extended model drift/adjustment that continued for at least another decade. The drift controlled the model salinity in the Labrador Sea, overriding the variability. Thus, during this initial period, similar behavior was observed between the inter-annually forced experiments as with perpetual year forcing. The results also did not depend on whether the configuration was global, or regional North Atlantic Ocean. The inclusion of an explicit sea-ice component did not seem to have a significant impact on the interior drift. Clear cut evidence for the drift having an advective nature was shown, based on two separate currents/flow regimes. We find, as expected, the representation of freshwater in the sub-polar gyre's boundary currents important. But this study also points out another, equally important process and pathway: the input of high salinity mode water from the subtropical North Atlantic. The advective regime is dependent on the details of the model, such as the representation of the freshwater transport in the model's East Greenland Current being very sensitive to the strength of the local sea surface salinity restoring (and the underlying field that the model is being restored to).