2. Scientific Gain
When compressing the three-dimensional North Atlantic circulation into a
two-dimensional streamfunction, the goal is to make the data more
manageable and easily visualized, while retaining its essential
components. In this section, we present evidence that the AMOC
streamfunction in density coordinates retains more essential information
than its counterpart in depth coordinates.
The depth space streamfunction (Fig. 2a), presents the AMOC as one large
overturning cell covering all depths and all latitudes. In contrast, the
density space streamfunction (Fig. 2b and 2c) identifies three distinct
overturning cells that are important to the large-scale North Atlantic
circulation:
- a light overturning cell in the subtropical North Atlantic (30.50-34.8
kg/m3 and 0°-40°N) that depicts the formation of
Subtropical Mode Water (STMW)
- an intermediate overturning cell spanning all latitudes but with a
peak in the subpolar gyre (35.50-37.02 kg/m3,
20°S-60°N) that depicts the formation of Labrador Sea Water (LSW) in
the Labrador and Irminger Seas
- a dense overturning cell in the Greenland, Iceland, and Norwegian
(GIN) Seas (37.02-37.20 kg/m3, 60°N-75°N) that
depicts the formation of the densest water masses north of the
Greenland-Scotland Ridge.
In depth coordinates, the STMW cell is only apparent in the upper 100 m
between 10°N-20°N, and the incredible amount of water mass
transformation (4 kg/m3 between the northward and
southward limbs) in this cell is lost (Fig. 2b). This is a critical
omission because the STMW cell corresponds almost exactly to the peak
oceanic MHT from 0°-40°N (Ganachaud and Wunsch, 2003), implying that
this cell is indeed important to the MHT, one of the most
societally-relevant aspects of the AMOC. Similarly, the strength of the
GIN cell (4 Sv) in depth coordinates is only a fraction of its strength
in density coordinates (6 Sv), and its importance to forming the densest
water masses that spill over the Greenland-Scotland Ridge and fill the
deep North Atlantic is not conveyed in depth coordinates.
The AMOC streamfunction in density coordinates also produces a more
continuous streamfunction that correctly positions the AMOC maximum in
the subpolar North Atlantic, where the majority of the southward limb
waters are formed. In contrast, the depth-space streamfunction
artificially shifts the maximum of the LSW cell into the subtropical
gyre and away from the regions of deep-water formation in the subpolar
North Atlantic and Nordic Seas. This southward shift of the maximum AMOC
is due to the inability of the depth-space AMOC to capture the
horizontal circulation. For example, consider that in depth coordinates,
the southward flow of cold, fresh waters in the Labrador Current is
negated by the northward flow of warm, saline water in the North
Atlantic Current. When these two currents meet near the Grand Banks of
Newfoundland, the cold, fresh water subducts under the warm, saline
waters and the two limbs start to project back onto the vertical
dimension. But this process yields a sharply discontinuous AMOC
streamfunction in depth coordinates north of 35°N (Fig. 2a). Instead,
summing the meridional velocity fields in density classes rather than
depth levels highlights the water mass transformation that occurs as the
water circulates cyclonically around the subpolar North Atlantic
(Desbruyères et al., 2019), and produces a more continuous AMOC
streamfunction between the subtropical and subpolar North Atlantic (Fig.
2b).
The AMOC streamfunction in density coordinates also differentiates
between overturning cells that are confined to one gyre and the
overturning cell that crosses gyre boundaries. This differentiation
becomes essential when assessing forcing mechanisms of AMOC variability.
For example, the mechanisms driving the AMOC at subtropical and subpolar
latitudes of the North Atlantic are different and time scale dependent
(e.g. Jackson et al., 2022). In essence, while wind and buoyancy forcing
are both considered important at higher latitudes on
interannual-to-decadal scales, in the subtropics wind forcing alone can
explain a substantial portion of the variability (Yang, 2015; Kostov et
al., 2021), especially on seasonal-to-interannual timescales (Moat et
al., 2020). Opposing wind stress variability induced by the NAO in the
subpolar and subtropical ocean can lead to opposing decadal AMOC
variations, which indeed breaks the notion of a single metric diagnosing
the basin-scale overturning cell (Lozier et al., 2010). It is thus
imperative to represent the AMOC in density space to gain correct
insights into its latitudinal-dependent mechanisms.