1. Motivation
The Atlantic Meridional Overturning Circulation (AMOC) consists of a
complex set of currents in the Atlantic Ocean that move warm, saline
water northward and return cold, fresh water southward. Despite this
simple qualitative description, defining the AMOC quantitatively is not
straightforward. Traditionally, oceanographers have defined it in depth
coordinates: locate the depth where the currents shift from net
northward to net southward and sum the meridional velocities above that
depth. If one assumes the Atlantic/Arctic is a closed basin by
considering the Bering Strait throughflow of ~1 Sv
(Woodgate et al., 2018) below the detection level of the AMOC observing
arrays (Cunningham et al., 2007; Lozier et al., 2019), and by neglecting
net mass divergence and precipitation in the North Atlantic on
timescales longer than 10 days (Kanzow et al., 2007), then the depth
where the currents shift from net northward to net southward corresponds
to the depth of maximum overturning, and the sum of the meridional
velocity above it is equal to the maximum in the AMOC stream function.
There is historical precedent for this depth-space definition -
oceanographers have measured the AMOC for decades in the subtropical
North Atlantic, where strong thermal stratification provides enough
baroclinicity in the water column that the warm northward limb of the
AMOC can flow directly over the cold southward limb (Fig. 1). The
longest direct measurements of the AMOC are from a repeat hydrographic
line across 25°N that has been occupied since 1957 (Hall and Bryden,
1982), and the first continuous observations of the AMOC have been made
since 2004 at the RAPID mooring array across 26.5°N (Cunningham et al.,
2007). This latitude was chosen because the oceanic meridional heat
transport (MHT) reaches its maximum in the subtropics (Ganachaud and
Wunsch, 2003), and because much of the Gulf Stream was already being
continuously measured in the Florida Straits by a defunct telephone
cable (Sanford and Larsen, 1985; Barringer and Larsen, 2001).
But this focus on the subtropical North Atlantic has led to a definition
of the AMOC that emphasizes its vertical dependency despite the AMOC
shifting to a horizontal circulation pattern further north (e.g.Zhang and Thomas, 2021). In the subpolar North Atlantic and Nordic Seas,
reduced vertical stratification does not permit opposing currents to
flow directly over one another, and instead, the northward limb of the
AMOC flows along the eastern side of the basin while the southward limb
flows along the western side at similar depths. Here, though the
northward and southward limbs are no longer differentiated in depth,
their densities remain distinct. Thus when the meridional velocities are
zonally summed in density classes, the northward and southward limbs
remain distinct in the streamfunction, even at high latitudes where the
canonical ‘conveyor belt’ lays on its side.