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

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 ϵ 10 6 m 3 s Ϫ1) 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.

Causes of Interannual–Decadal Variability in the Meridional Overturning Circulation of the Midlatitude North Atlantic Ocean

“…As discussed in Brandt et al (2007) and Haine et al (2008), there are several possibilities for defining a quantitative measure of convection intensity. Since our interest here is not on the intricate details of LSW formation variability itself, but on elucidating its effect on the MOC, we have used the rather simple diagnostic described by Böning et al (1996), which basically follows the classical account of LSW formation by Clarke and Gascard (1983): the formation rate is defined as the increase in volume during the winter convection phase between December and April, of the water in the density range of the model equivalent of LSW (27.84-27.89 in FLAME; 27.72-27.82 in ORCA); dividing this volume of "new" LSW by a year, gives the annual formation rate (Sv).…”

“…However, whereas a variety of tracer data has illuminated the spreading of the Labrador seawater (LSW) variability signatures along the DWBC to 26°N (Molinari et al 1998) and into the interior subtropical ocean ), inferences of the dynamical effect of this variability on the MOC are presently based on model studies only. Mechanisms that may bear on the response of the basin-scale MOC include a fast exit pathway for a significant fraction of newly formed LSW (Brandt et al 2007); associated with that, a dynamical reaction of the deep boundary current in the Labrador Sea within a year ) and after about 2 yr at the exit of the subpolar basin off the Grand Banks of Newfoundland (Eden and Greatbatch 2003); and a rapid equatorward communication of the MOC signal established there via fast boundary wave processes (Johnson and Marshall 2002;Getzlaff et al 2005).…”

“…An aspect of particular concern is the representation of the (sub-) mesoscale flow features that govern, for example, the exchanges between the deep-water formation sites and the boundary currents (Houghton and Visbeck 2002;Eden and Bön-ing 2002;Brandt et al 2004;Chanut et al 2008) and the equatorward transmission of variability signals along the western boundary (Getzlaff et al 2005). Another aspect is the representation of the outflows of dense waters from the Nordic seas.…”

The Global Subduction/Obduction Rates: Their Interannual and Decadal Variability

“…The advantage of using this concept is that it can be calculated by numerical models through diagnosis of the concentration in the surface layer of a specifically designed tracer. Brandt et al (2007) provided the most updated example along this line of approach. In addition, ventilation rate has been widely used in marine chemistry; thus, this approach can be conveniently compared to the results based on chemical tracer analysis.…”

Effects of the air–sea coupling time frequency on the ocean response during Mediterranean intense events

“…Nevertheless, the common practice is to force ocean models (even those with a high spatial resolution) with atmospheric data with relative coarse spatial and temporal resolutions. The forcing comes generally from analyses or reanalyses of weather forecast models with, at most, a 6-h resolution (Paci et al , 2007Giordani et al 2005;Barnier et al 2006;Brandt et al 2007), partly because of a lack of highresolution atmospheric data over the ocean. Consequently, fine-scale and fast ocean responses are mostly not well represented by these ocean models.…”