Inferring Advective Timescales and Overturning Pathways of the Deep Western Boundary Current in the North Atlantic through Labrador Sea Water Advection

Abstract

As the Earth gains and loses heat at low and high latitudes, respectively, the large-scale ocean and atmosphere circulations are characterized by a net poleward heat flux. In the North Atlantic, warm tropical waters are carried northward by a system of wind-driven near-surface currents, dominated by the Gulf Stream and North Atlantic Current. Upon losing heat to the atmosphere en route, these waters become denser and sink in the Subpolar North Atlantic forming North Atlantic Deep Water (NADW) consisting of Labrador Sea Water (LSW), Iceland-Scotland Overflow Water (ISOW), and Denmark Strait Overflow Water (DSOW). These cold and dense waters are collectively exported southward at depths below 1,000 m, constituting the lower limb of the Atlantic

The interior spreading story of Labrador Sea Water

“…We investigate the advection of the two LSW masses previously defined by Chomiak et al (2022), LSW 1987-1994and LSW 2000 , tracked along the neutral density (g n , kg/m 3 ) isopycnal planes of g n = 27.99 and g n = 27.90, respectively. The scale in the anomaly of the convective imprint along each isopycnal level is nearly identical when looking in both temperature and salinity space (Chomiak et al, 2022) and thus redundant. Therefore, only salinity anomalies are explored here.…”

“…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. Whether or not the Deep Western Boundary Current (DWBC, Figure 1) dictates a primary role in the volume advection of these water masses compared to the contribution of interior advective pathways is still an open question.…”

“…Whether or not the Deep Western Boundary Current (DWBC, Figure 1) dictates a primary role in the volume advection of these water masses compared to the contribution of interior advective pathways is still an open question. Past evidence of LSW spreading through interior pathways, both independent of and branching from the DWBC, have been inferred via advective tracers (van Sebille et al, 2011;Bilóand Johns, 2019;Chomiak et al, 2022) and Lagrangian float trajectories (Bower and Hunt, 2000b;Fischer and Schott, 2002;Getzlaff et al, 2006;Bower et al, 2009;Gary et al, 2011;Gary et al, 2012;Zou and Lozier, 2016;Bower et al, 2019). These observations make it apparent that the entire basin must be considered for quantifying deep water transports, rather than just the western boundary (McCarthy et al, 2015;Volkov et al, 2023), since it is likely these interior advective pathways provide a significant contribution to the lower-limb of the Atlantic Meridional Overturning Circulation (AMOC).…”

“…Talley and McCartney, 1982), a discrete, timevarying distribution of the unique sub-classes of LSW (Yashayaev, 2007;Chomiak et al, 2022) across the entire basin has yet to be explored. Chomiak et al (2022) previously investigated the advection of two unique LSW classes formed during 1987-1994(LSW 1987-1994 ) and 2000(LSW 2000 ) along constant neutral density surfaces from the Labrador Sea to the subtropical North Atlantic using hydrographic transects across the DWBC. LSW 2000-2003 was found to advect on a longer timescale (order of 10-15 years) compared to LSW 1987-1994 (order of 10 years).…”

“…These findings suggested that an interior advective pathway, branching from the western boundary (dashed line in Figure 1), was likely influential. In this follow-up study, following the water mass definitions set in Chomiak et al (2022), we consider the entire North Atlantic and use hydrographic observations to reveal the time-variable spatial distribution of LSW 1987-1994and LSW 2000 along constant isopycnal planes. The observational evidence presented herein confirms the interior presence and time-varying nature of both LSW classes and presents the shape of their equatorward spreading via the DWBC and other pathways.…”