Direct and Indirect Pathways of Convected Water Masses and Their impacts on the Overturning Dynamics of the Labrador Sea

Abstract

The dense waters formed by wintertime convection in the Labrador Sea play a key role in setting the properties of the deep Atlantic Ocean. To understand how variability in their production might affect the Atlantic Meridional Overturning Circulation (AMOC) variability, it is essential to determine pathways and associated timescales of their export. In this study, we analyze the trajectories of Argo floats and of Lagrangian particles launched at 53°N in the boundary current and traced backward in time in a high-resolution model, to identify and quantify the importance of upstream pathways. We find that 85% of the transport carried by the particles at 53°N originates from Cape Farewell, and it is split between a direct route that follows the boundary current and an indirect route involving boundary-interior exchanges. Although both routes contribute roughly equally to the maximum overturning, the indirect route governs its signal in denser layers. This indirect route has two branches: part of the convected water is exported rapidly on the Labrador side of the basin and part follows a longer route toward Greenland and is then carried with the boundary current. Export timescales of these two branches typically differ by 2.5 years. This study thus shows that boundary-interior exchanges are important for the pathways and the properties of water masses arriving at 53°N. It reveals a complex three-dimensional view of the convected water export, with implications for the arrival time of signals of variability therein at 53°N and thus for our understanding of the AMOC.

The Changing Behavior of the West Greenland Current System in a Very High‐Resolution Model

“…This is also consistent with tracer and virtual Lagrangian float studies that show most of the freshwater coming from the Greenland icesheet reaches the northern Labrador Sea and/or the Labrador Current rather than the interior convective patch (Gillard et al, 2016;Luo et al, 2016). More recently, by employing virtual particles in an eddy-permitting model, Georgiou et al (2021) shows that the low-density water transported out of the Labrador Sea are predominantly via the circulating boundary currents, instead of the offshore exchange from the WGC. This may be why higher resolution model studies generally do not find a significant impact of Greenland freshwater on LSW formation as of yet (Böning et al, 2016).…”

Propagation and Transformation of Upper North Atlantic Deep Water From the Subpolar Gyre to 26.5°N

“…These two (indirect and direct) routes and the associated difference in residence time scale are consistent with Georgiou et al. (2021) which showed that the indirect route governs the transformation within the denser layers.…”

“…Thus, the buoyancy loss over the Labrador Sea contributes to both the net volume of newly formed uNADW for particles remaining less than 1 year in the basin and to a further densification of uNADW within the layer for particles remaining more than 1 year in the basin. These two (indirect and direct) routes and the associated difference in residence time scale are consistent with Georgiou et al (2021) which showed that the indirect route governs the transformation within the denser layers.…”

Labrador sea water spreading and the Atlantic meridional overturning circulation

“…Brandt et al [43], using a 1 year run of an eddy resolving model, found that there is about equal transport of upper and deep LSW in the Labrador Current, and that about half of the waters in both density classes were ventilated within 1 year [43]. More recently, MacGilchrist et al [44] and Georgiou et al [45] used Lagrangian model analyses to identify where deep waters are subducted in the Labrador Sea [44,45] the location of exchange with the boundary is important. They find that waters that enter the boundary current from the interior near western Greenland take about 2.5 years longer to exit the Labrador Sea than those entering the boundary current on the Canadian side.…”