Lagrangian Views of the Pathways of the Atlantic Meridional Overturning Circulation

Abstract

The Lagrangian method—where current location and intensity are determined by tracking the movement of flow along its path—is the oldest technique for measuring the ocean circulation. For centuries, mariners used compilations of ship drift data to map out the location and intensity of surface currents along major shipping routes of the global ocean. In the mid‐20th century, technological advances in electronic navigation allowed oceanographers to continuously track freely drifting surface buoys throughout the ice‐free oceans and begin to construct basin‐scale, and eventually global‐scale, maps of the surface circulation. At about the same time, development of acoustic methods to track neutrally buoyant floats below the surface led to important new discoveries regarding the deep circulation. Since then, Lagrangian observing and modeling techniques have been used to explore the structure of the general circulation and its variability throughout the global ocean, but especially in the Atlantic Ocean. In this review, Lagrangian studies that focus on pathways of the upper and lower limbs of the Atlantic Meridional Overturning Circulation (AMOC), both observational and numerical, have been gathered together to illustrate aspects of the AMOC that are uniquely captured by this technique. These include the importance of horizontal recirculation gyres and interior (as opposed to boundary) pathways, the connectivity (or lack thereof) of the AMOC across latitudes, and the role of mesoscale eddies in some regions as the primary AMOC transport mechanism. There remain vast areas of the deep ocean where there are no direct observations of the pathways of the AMOC.

Topological Constraints by the Greenland–Scotland Ridge on AMOC and Climate

“…Because of the zonal barrier in ridge, however, a substantial part of the poleward flow is steered along the GSR following constant f/H contours (where f is the Coriolis parameter and H is the depth), seen as an enhanced westward barotropic flow (at the GSR latitudes). This is consistent with observations and models showing that a substantial part of the NAC is steered by the complex bottom topography and recirculates south of the GSR (Bower et al 2019;Stärz et al 2017).…”

The Relationship Between U.S. East Coast Sea Level and the Atlantic Meridional Overturning Circulation: A Review

“…Below this upper cell is a lower cell of Antarctic Bottom Water that originates from sources at high southern latitudes (Buckley & Marshall, ; Kuhlbrodt et al, ). See Buckley and Marshall () and other reviews in this special issue, particularly Bower et al (), for a more comprehensive description of AMOC structure and variability.…”

Tracking water masses using passive-tracer transport in NEMO v3.4 with NEMOTAM: application to North Atlantic Deep Water and North Atlantic Subtropical Mode Water

“…However, the effect of turbulence on water mass pathways has only been observed on a large scale since the widespread adoption of costeffective Lagrangian profiling floats, which have been observed to follow chaotic trajectories (e.g. Fischer and Schott, 2002;Fratantoni et al, 2013;Bower et al, 2019). Despite their number (typically several tens of floats for dedicated Published by Copernicus Publications on behalf of the European Geosciences Union.…”

“…Leaman and Harris, 1990;Molinari et al, 1992) is that the main southward pathway is the Deep Western Boundary Current (DWBC). However, new data from floats and modelled Lagrangian drifters (Bower et al, 2009(Bower et al, , 2011 suggest that this is not the complete story. Gary et al (2011) investigated alternative export pathways in a hierarchy of model resolutions and found a second, slower pathway driven by a series of deep, eddy-driven anticyclonic gyres.…”