Ventilation and Transformation of Labrador Sea Water and Its Rapid Export in the Deep Labrador Current

Abstract

Abstract 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.