Major sources of North Atlantic Deep Water in the subpolar North Atlantic from Lagrangian analyses in an eddy-rich ocean model

Abstract

Abstract. The North Atlantic Deep Water (NADW) is a crucial component of the Atlantic meridional overturning circulation and is therefore an important factor of the climate system. In order to estimate the mean relative contributions, sources, and pathways of the NADW at the southern exit of the Labrador Sea, a Lagrangian particle experiment is performed. The particles were seeded according to the strength of the velocity field along the 53∘ N section and traced 40 years backward in time in the three-dimensional velocity and hydrography field. The resulting transport pathways, their sources and corresponding transit timescales were inferred. Our experiment shows that, of the 30.1 Sv of NADW passing 53∘ N on average, the majority of this water is associated with a diapycnal mass flux without contact to the atmosphere, accounting for 14.3 Sv (48 %), where 6.2 Sv originate from the Labrador Sea, compared to 4.7 Sv from the Irminger Sea. The second-largest contribution originates from the mixed layer with 7.2 Sv (24 %), where the Labrador Sea contribution (5.9 Sv) dominates over the Irminger Sea contribution (1.0 Sv). Another 5.7 Sv (19 %) of NADW crosses the Greenland–Scotland Ridge within the NADW density class, where about two-thirds pass the Denmark Strait, while one-third crosses the Iceland–Scotland Ridge. The NADW exported at 53∘ N is hence dominated by entrainment through the diapycnal mass flux and mixed-layer origin in the Labrador Sea.

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

“…Finally, we show that the largest density changes (greater than 0.1 kg m −3 in Figure 6b) are mainly localized along the WGC between Eirik Ridge and Cape Desolation, which is where the Irminger Rings are shed and recirculation takes place (Cuny et al., 2002), instead of the interior of the basin. This agrees well with previous studies revealing the importance of density changes along the boundary current for the dynamic of the subpolar gyre (Menary et al., 2020; Spall, 2004; Straneo, 2006), as well as with a recent study (Fröhle et al., 2022) showing that uNADW formation in the Labrador Sea on decadal time scales is mainly a result of diapycnal mass fluxes over this area rather than by mixed layer formation over the interior of the basin.…”

“…In this set of experiments, nearly all the particles released at the LC section are advected from the WGC section, and more than 60% of the particles reach the destination section after only 1 year. This time scale is consistent with previous studies that document a transit time along the boundary current of the Labrador Sea of 1–2 years (Bower et al., 2009; Feucher et al., 2019; Georgiou et al., 2020) and with Lagrangian experiments that tracked backward in time overflow water from the Denmark Strait to 53°N in VIKING20X‐JRA‐OMIP (Fröhle et al., 2022).…”

Pathways and Timescales of Connectivity Around the Antarctic Continental Shelf

“…It is common practice in many Lagrangian studies of advective timescales (e.g., Drake et al., 2018; Tamsitt et al., 2021), and pathways (e.g., Blanke et al., 1999; Fröhle et al., 2022; Rühs et al., 2022; Schmidt et al., 2021; Tamsitt et al., 2017; Vallès‐Casanova et al., 2022; van Sebille et al., 2014), to tag particles with a volume transport (van Sebille et al., 2018). Indeed Lagrangian particle experiments conducted with Parcels have previously been used to estimate volume transports, even when the integration scheme is not strictly volume conserving (Fröhle et al., 2022; Rühs et al., 2022; Schmidt et al., 2021; Vallès‐Casanova et al., 2022), because the trajectories calculated by explicit time‐stepping in the absence of diffusion are similar to trajectories calculated by analytical methods (van Sebille et al., 2018). We follow this convention in our analyses and weight each particle in our simulation with the transport through the corresponding model grid cell in which it was released.…”

Mixing and Geometry in the North Atlantic Meridional Overturning Circulation

“…In summary, this viewpoint puts into value techniques coming from dynamical systems theory that provides a simple framework to understand the chaotic transport of water masses across the global ocean circulation. These techniques are different from previous efforts in this direction based on first and last passage time distributions (Primeau, 2005), direct statistics on trajectories (Rousselet et al, 2021) or other seeding strategies (Fröhle et al, 2022). Our study starts from available velocity products provided by qualified models.…”