Search

This paper extends A. Bracco and J. Pedlosky’s investigation of the eddy-formation mechanism in the eastern Labrador Sea by including a more realistic depiction of the boundary current. The quasigeostrophic model consists of a meridional, coastally trapped current with three vertical layers. The current configuration and topographic domain are chosen to match, as closely as possible, the observations of the boundary current and the varying topographic slope along the West Greenland coast. The role played by the bottom-intensified component of the boundary current on the formation of the Labrador Sea Irminger Rings is explored. Consistent with the earlier study, a short, localized bottom-trapped wave is responsible for most of the perturbation energy growth. However, for the instability to occur in the three-layer model, the deepest component of the boundary current must be sufficiently strong, highlighting the importance of the near-bottom flow. The model is able to reproduce important features of the observed vortices in the eastern Labrador Sea, including the polarity, radius, rate of formation, and vertical structure. At the time of formation, the eddies have a surface signature as well as a strong circulation at depth, possibly allowing for the transport of both surface and near-bottom water from the boundary current into the interior basin. This work also supports the idea that changes in the current structure could be responsible for the observed interannual variability in the number of Irminger Rings formed. [ABSTRACT FROM AUTHOR]

The effect of increasing horizontal resolution is examined to assess the response of the Atlantic meridional overturning circulation (AMOC) to freshwater perturbations. Versions of a global climate model with horizontal resolutions ranging from 1.8° (latitude) × 3.6° (longitude) to 0.2° × 0.4° are used to determine if the AMOC response to freshwater forcing is robust to increasing resolution. In the preindustrial equilibrium climate, the representation of western boundary currents and meridional heat transport are improved with resolution. Freshwater forcings similar to the final drainage of proglacial Lakes Agassiz and Ojibway are applied evenly over the Labrador Sea and exclusively along the western boundary. The duration and maximum amplitude of model responses to freshwater forcing showed little sensitivity to increasing resolution. An evaluation with tracers of the forcing impact on different regions of North Atlantic Deep Water formation revealed the possibility that increases in Labrador Sea deep convection at higher resolution mitigate the effect of stronger boundary currents and enhanced mixing. With increasing resolution, there is less cooling in the subpolar west Atlantic, more cooling in the subpolar east Atlantic, and greater variability in the deep ocean response to the boundary forcing. While differences exist, the coarse-resolution model response remains robust at finer horizontal resolutions. [ABSTRACT FROM AUTHOR]

Labrador Sea Water (LSW), a dense water mass formed by convection in the subpolar North Atlantic, is an important constituent of the meridional overturning circulation. Understanding how the water mass enters the deep western boundary current (DWBC), one of the primary pathways by which it exits the subpolar gyre, can shed light on the continuity between climate conditions in the formation region and their downstream signal. Using the trajectories of (profiling) autonomous Lagrangian circulation explorer [(P)ALACE] floats, operating between 1996 and 2002, three processes are evaluated for their role in the entry of Labrador Sea Water in the DWBC: 1) LSW is formed directly in the DWBC, 2) eddies flux LSW laterally from the interior Labrador Sea to the DWBC, and 3) a horizontally divergent mean flow advects LSW from the interior to the DWBC. A comparison of the heat flux associated with each of these three mechanisms suggests that all three contribute to the transformation of the boundary current as it transits the Labrador Sea. The formation of LSW directly in the DWBC and the eddy heat flux between the interior Labrador Sea and the DWBC may play leading roles in setting the interannual variability of the exported water mass. [ABSTRACT FROM AUTHOR]

The overturning and horizontal circulations of the Labrador Sea are deduced from a composite vertical section across the basin. The data come from the late-spring/early-summer occupations of the World Ocean Circulation Experiment (WOCE) AR7W line, during the years 1990–97. This time period was chosen because it corresponded to intense wintertime convection—the deepest and densest in the historical record—suggesting that the North Atlantic meridional overturning circulation (MOC) would be maximally impacted. The composite geostrophic velocity section was referenced using a mean lateral velocity profile from float data and then subsequently adjusted to balance mass. The analysis was done in depth space to determine the net sinking that results from convection and in density space to determine the diapycnal mass flux (i.e., the transformation of light water to Labrador Sea Water). The mean overturning cell is calculated to be 1 Sv (1 Sv ≡ 106 m3 s-1), as compared with a horizontal gyre of 18 Sv. The total water mass transformation is 2 Sv. These values are consistent with recent modeling results. The diagnosed heat flux of 37.6 TW is found to result predominantly from the horizontal circulation, both in depth space and density space. These results suggest that the North Atlantic MOC is not largely impacted by deep convection in the Labrador Sea. [ABSTRACT FROM AUTHOR]

The depth of winter convection in the central Labrador Sea is strongly influenced by the prevailing stratification in late summer. For this late summer stratification salinity is as important as temperature, and in the upper water layers salinity even dominates. To analyze the source of the spring and summer freshening in the central region, seasonal freshwater cycles have been constructed for the interior Labrador Sea, the West Greenland Current, and the Labrador Current. It is shown that none of the local freshwater sources is responsible for the spring–summer freshening in the interior, which appears to occur in two separate events in April to May and July to September. Comparing the timing and volume estimates of the seasonal freshwater cycles of the boundary currents with the central Labrador Sea helps in understanding the origin of the interior freshwater signals. The first smaller pulse cannot be attributed clearly to either of the boundary currents. The second one is about three times stronger and supplies 60% of the seasonal summer freshwater. Transport estimates and calculated mixing properties provide evidence that its source is the West Greenland Current. The finding implies a connection also on interannual time scales between Labrador Sea surface salinity and freshwater sources in the West Greenland Current and farther upstream in the East Greenland Current. The freshwater input from the West Greenland Current thus also is the likely pathway for the known modulation of Labrador Sea Water mass formation by freshwater export from the Arctic (via the East Greenland Current), which implies some predictability on longer time scales. [ABSTRACT FROM AUTHOR]

A model of the subpolar North Atlantic Ocean is used to study different aspects of ventilation and water mass transformation during a year with moderate convection intensity in the Labrador Sea. The model realistically describes the salient features of the observed hydrographic structure and current system, including boundary currents and recirculations. Ventilation and transformation rates are defined and compared. The transformation rate of Labrador Sea Water (LSW), defined in analogy to several observational studies, is 6.3 Sv (Sv ≡ 106 m3 s-1) in the model. Using an idealized ventilation tracer, mimicking analyses based on chlorofluorocarbon inventories, an LSW ventilation rate of 10 Sv is found. Differences between both rates are particularly significant for those water masses that are partially transformed into denser water masses during winter. The main export route of the ventilated LSW is the deep Labrador Current (LC). Backward calculation of particle trajectories demonstrates that about one-half of the LSW leaving the Labrador Sea within the deep LC originates in the mixed layer during that same year. Near the offshore flank of the deep LC at about 55°W, the transformation of LSW begins in January and is at a maximum in February/March. While the export of transformed LSW out of the central Labrador Sea continues for several months, LSW generated near the boundary current is exported more rapidly, with maximum transport rates during March/April within the deep LC. [ABSTRACT FROM AUTHOR]

The seasonal and interannual variations in the export of Labrador Sea Water (LSW), and in the heat and freshwater transport through the central Labrador Sea, are examined for two different periods: from 1964 to 1974, using Ocean Weather Station Bravo data, and from 1996 to 2000, using data collected from profiling floats. A typical seasonal cycle involves a 300-m thickening of LSW (convection) followed by an equivalent thinning (restratification). Restratification is characterized by a drift of properties toward boundary current values that is indicative of a vigorous lateral exchange. The net result is a convergence of heat and salt, between 200 and 700 m, that balances the net surface heat loss to the atmosphere and partially offsets the surface freshwater accumulation due to surface, lateral exchange. Interannual variations in the export of LSW can be explained by taking into account changes in the central Labrador Sea–boundary current density gradient, which governs the lateral exchange. Interannual variations in how much heat is converged into the region, on the other hand, mostly reflect changes in the temperature of LSW. This only partly explains, however, the increased convergence of heat that occurs during the late 1990s. In years in which convection does not occur, restratification trends continue throughout the entire year, albeit at a reduced rate. [ABSTRACT FROM AUTHOR]

An idealized model is used to study the restratification of the Labrador Sea after deep convection, with emphasis on the role of boundary current eddies shed near the west coast of Greenland. The boundary current eddies carry warm, buoyant Irminger Current water into the Labrador Sea interior. For a realistic end-of-winter state, it is shown that these Irminger Current eddies are efficient in restratifying the convected water mass in the interior of the Labrador Sea. In addition, it is demonstrated that Irminger Current eddies can balance a significant portion of the atmospheric heat loss and thus play an important role for the watermass transformation in the Labrador Sea. [ABSTRACT FROM AUTHOR]