Lagrangian views of the subpolar North Atlantic Ocean

1 Background and Motivation
1.1 Atlantic Meridional Overturning Circulation
Through the transport of heat and anthropogenic carbon from light northward surface currents to dense southward flows at depth, the Atlantic Meridional Overturning Circulation (AMOC) represents a fundamental control on global climate [1, 2]. The projected weakening of the AMOC in response to greenhouse gas forcing has profound implications for Arctic sea ice decline [3], North Atlantic sea level rise [4], and both European [5] and African climates [6]. Confidence in such projections is severely limited, however, because of the known deficiencies of current generation coupled-climate models [7] and the absence of long-term observations [8].
1.2 Circulation in the subpolar North Atlantic Ocean
The circulation of the subpolar North Atlantic Ocean is dominated by the complex interplay between 3-dimensional gyre and overturning circulations. Here warm, saline water conveyed in the upper branch of the AMOC is partitioned
almost equally between the cyclonic subpolar gyre and currents progressing poleward towards the Arctic Ocean (Fig.1). Constrained by bathymetry, the shoaling of isopycnals (lines of constant density) with distance westwards across the
basin highlights the role of the wind-driven gyre in the densification of surface water masses. At depth, a compensating Deep Western Boundary Current exports cold dense North Atlantic Deep Water (NADW) equatorwards [9]. The NADW
transported in this lower branch of the AMOC evolves from a combination of dense overflow water transformed in the Nordic Seas and intermediate waters sourced from the Labrador and Irminger Seas [10].
In the last decade, observations made by the Overturning in the Subpolar North Atlantic Program (OSNAP) have further complicated this picture, unexpectedly highlighting that water mass transformation in the eastern basin rather
than the Labrador Sea dominates variability in the overturning circulation [11]. This finding has directly challenged the established view, derived from climate models, that Labrador Sea convection is the principal source of subpolar overturning variability [12].