Major sources of North Atlantic Deep Water in the subpolar North Atlantic from Lagrangian analyses in a high–resolution ocean model

Abstract

Abstract. North Atlantic Deep Water (NADW) is a crucial component of the Atlantic Meridional Overturning Circulation and, therefore, is an important factor of the climate system. In order to estimate the mean relative contributions, sources and pathways of the three different deep water mass components (namely Labrador Sea Water, Northeast Atlantic Deep Water and Denmark Strait Overflow Water) at the southern exit of the Labrador Sea, Lagrangian particle experiments were performed. The particles were seeded according to the strength of the velocity field along the 53° N section and computed 40 years backward in time in the three-dimensional velocity and hydrography field. Water masses were defined within the model output in the central Labrador Sea and the subpolar North Atlantic. The resulting transport pathways, their sources and corresponding transit time scales were inferred. Our experiments show that the majority of NADW passing 53° N is associated with diapycnal mass flux, 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 cross the Greenland–Scotland Ridge within the NADW density class, where about 2/3 pass Denmark Strait, while 1/3 cross the Iceland Scotland Ridge. The NADW exported at 53° N is hence dominated by entrainment through diapycnal mass flux and the 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).…”