Overflow Water Pathways in the North Atlantic: New Observations from the OSNAP Program 

Abstract

<p>As part of the international Overturning in the Subpolar North Atlantic Program (OSNAP), 135 acoustically-tracked deep floats were deployed from 2014 to 2016 to track the spreading pathways of Iceland-Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW). These water masses, which originate in the Nordic Seas, compose the deepest branch of the Atlantic Meridional Overturning Circulation. The OSNAP floats provide the first directly observed, comprehensive Lagrangian view of ISOW and DSOW spreading pathways throughout the subpolar North Atlantic. Contrary to a decades-long expectation for how these deep water masses move equatorward, the collection of OSNAP float trajectories, complemented by model simulations, conclusively reveals that their pathways are (a) not restricted to western boundary currents, and (b) remarkably different from each other in character. The spread of DSOW from the Irminger Sea is primarily via the swift deep boundary currents of the Irminger and Labrador Seas, whereas the spread of ISOW out of the Iceland Basin is slower, more diffusive, and along multiple export pathways. The characterization of these overflow water pathways has important implications for our understanding of the Atlantic Meridional Overturning Circulation (AMOC) and its variability. Finally, reconstructions of AMOC variability from proxy data, involving either the strength of boundary currents and/or the property variability of deep waters, should account for the myriad pathways of DSOW and ISOW, but particularly so for the latter.</p><p> </p>

The interior spreading story of Labrador Sea Water

“…The advective pathways of LSW discussed herein have been previously documented and supported in a multitude of studies using advective tracers, Lagrangian float trajectories, and circulation models (Spall, 1996;Molinari et al, 1998;Bower and Hunt, 2000a;Bower and Hunt, 2000b;Fischer & Schott, 2002;Straneo et al, 2003;Bower et al, 2009;Kieke et al, 2009;Bower et al, 2011;Gary et al, 2011;van Sebille et al, 2011;Gary et al, 2012;Zou and Lozier, 2016;Le Bras et al, 2017;Andres et al, 2018;Bilóand Johns, 2019;Bower et al, 2019;Zhai et al, 2021;Chomiak et al, 2022;Fox et al, 2022;Lozier et al, 2022;Petit et al, 2023). In this study, we present the first, to our knowledge, description of the time-varying spread of two discrete LSW masses.…”

“…Furthermore, by expanding the analysis over the entire North Atlantic rather than just along the boundary, other LSW propagation and advective pathways supported in past and current literature (ex. Spall, 1996;Bower and Hunt, 2000a;Bower and Hunt, 2000b;Bower et al, 2009;Bower et al, 2011;Bilóand Johns, 2019;Bower et al, 2019;Chomiak et al, 2022;Lozier et al, 2022;Petit et al, 2023) are reinforced with these CEOF analyses. The CEOF 1 explains approximately 30% of the salinity variance in both isopycnals, which is a relatively large number considering that it relates to the entire North Atlantic domain; the other modes explain significantly less variance and are not used in the analysis.…”

“…Advances in ocean observations, Lagrangian methodologies, and ocean circulation modeling in recent decades have shown irrefutable evidence of interior Atlantic advective pathways in the equatorward spreading of subpolar water masses (Spall, 1996;Bower and Hunt, 2000a;Bower and Hunt, 2000b;Getzlaff et al, 2006;Bower et al, 2009;Bower et al, 2011;Zou and Lozier, 2016;Bilóand Johns, 2019;Bower et al, 2019;Chomiak et al, 2022;Fox et al, 2022;Lozier et al, 2022). Interior advective pathways have the potential to delay the communication of subpolar water masses to the subtropics, as these signals likely become entrained or rerouted within the Atlantic interior (Chomiak et al, 2022), making this communication slower than our current understanding.…”

Annually Resolved Propagation of CFCs and SF6 in the Global Ocean Over Eight Decades

“…The broad region where a fast arrival time is found in the subpolar North Atlantic reflects the multiple varieties of dense waters being formed here. While their pathways are not necessarily restricted to the western boundary (Lozier, 1997; Lozier et al., 2022), the 10% ventilation time is shorter (10–30 years) in the western North Atlantic, compared to the eastern Atlantic, where CFCs and SF 6 have barely arrived as of 2021 ( t 10 ≃ 80 years). The reconstructed CFC‐11 concentration reveals that the high latitudes in the North Atlantic are already experiencing the roll‐off in CFC‐11 atmospheric history, and even values at 35°N have started plateauing, while CFC‐11 are still steadily increasing further south (Figure 7c).…”

Annually-resolved propagation of CFCs and SF6 in the global ocean over eight decades

“…The broad region where a fast arrival time is found in the subpolar North Atlantic reflects the multiple varieties of dense waters being formed here. While their pathways are not necessarily restricted to the western boundary (Lozier, 1997;Lozier et al, 2022), the 10% ventilation time is shorter (10-30 years) in the western North Atlantic, compared to the eastern Atlantic, where CFCs and SF 6 have barely arrived as of 2021 (t 10 ≃ 80 years).…”