Abstract
AbstractThe 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.
Highlights
The subpolar North Atlantic (SPNA) is a region of high importance for the global climate (e.g., Buckley & Marshall, 2016; Lozier et al, 2019; Schott & Brandt, 2007) and in particular for the Atlantic Meridional Overturning Circulation (AMOC)
We use Lagrangian particles launched at 53°N and traced backward in time using the output of an ocean eddy-permitting model (MOM) to investigate the upstream pathways of the water masses that exit the Labrador Sea
Building on the insights gained from the idealized Lagrangian study of Georgiou et al (2020), we explore the relative importance of the different pathways that water masses follow prior to exiting the Labrador Sea at 53°N by investigating the water mass transformation along these pathways, the impact on the overturning in the basin, and the residence and export timescales associated with the pathways in this more realistic ocean model
Summary
The subpolar North Atlantic (SPNA) is a region of high importance for the global climate (e.g., Buckley & Marshall, 2016; Lozier et al, 2019; Schott & Brandt, 2007) and in particular for the Atlantic Meridional Overturning Circulation (AMOC). More recent studies revealed that deep convection occurs in the southern Irminger Sea and that this results in a water mass with similar properties similar to LSW (de Jong et al, 2012; Le Bras et al, 2020; Pickart et al, 2003; Piron et al, 2016; Våge et al, 2008). The processes that govern the transport along these various export routes of dense water masses formed in the Labrador Sea have yet to be clarified; in particular, quantification of the magnitude of the transports, the typical water mass properties, and travel times associated with each pathway are still lacking. The recent idealized study of Georgiou et al (2020) indicates that this is the case, but the idealizations did not allow for a reliable quantification of timescales This complexity clearly makes it difficult to capture a potential link between formation rates of dense waters and AMOC variability further downstream. A discussion on the three-dimensional view of the export of convected waters is provided in Section 6, followed by the summary and conclusions of our results (Section 7)
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