Simulated Two-Stage Recovery of Atlantic Meridional Overturning Circulation During the Last Deglaciation - Abrupt Climate Change: Mechanisms, Patterns, and Impacts - page 84

4.2. Reinitiation of NADW Formation

The general picture of NADW formation in multiple ori-
gins from an unperturbed state could be described as follows:
through air-sea heat and freshwater exchange, salty and
warm water is transported to the North Atlantic within the
upper ocean layers, which are driven by a meridional steric
gradient; fresh and cold water sinks downward through deep
convection in both NADWorigin regions, i.e., Labrador Sea
and GIN seas, and then moves southward driven by a deep
ocean meridional pressure gradient.
As shown above, the recovery of AMOC is accompanied
by increasing salinity (shown in Figures 6 - 9) and the retreat
of sea ice cover (Figure 3) in the North Atlantic. The change
in salinity is due to net salinity advection from low latitudes
within the upper layers, and the retreat of sea ice cover
significantly affects the local heat - freshwater exchange in the
NADW origins. It seems natural that the evolution of these
two processes should play a significant role during the
AMOC recovery period. On the basis of this preliminary
understanding of NADW formation processes, we investi-
gate the simulated NADW reinitiation process during the
AMOC recovery period of the last deglaciation.
The net salt transport to the Labrador Sea decreases during
meltwater discharge, and it resumes primarily during stage 1
(Figure 11a), consistent with the increase in salinity in this
region (Figure 8a). Before stage 1, the water of the Labrador
Sea in its entire depth is significantly freshened and lightened
through thousands of years of freshwater discharge. During
stage 1, the transport of salty and dense water in upper layers
into this area (Figures 8a and 10a) could directly induce a
reversed vertical density structure (Figure 8a). The reversed
density structure is followed by a resumed convection. As
convection reinitiates, the net heat transport into the upper
layers also increases (Figure 11b). This heat transport comes
from both stored subsurface heat release (Figure 8b) and
resumed warm water advection from low latitudes. This
induces the abrupt retreat of sea ice in the Labrador Sea
(Figure 12a) and a corresponding increase in surface heat
loss (seen as a decrease in SHF) (Figure 12b).
The local oceanic surface heat loss and nonlocal salt water
advection to the Labrador Sea in the upper ocean layers
combine to cause upper layer densification, (Figure 10a) and
thus the abrupt resumption of convection (Figure 12c). Sub-
surface warming also contributes to the resumption of con-
vection, favoring a reversed vertical density structure.
However, the subsequent convection quickly induces the
subsurface heat anomaly to dissipate as it is ventilated (Fig-
ure 8b). These three factors simultaneously force NADW
formation in the Labrador Sea to an enhanced level beyond
its initial intensity (Figure 1, black line).

Figure 11. Net (a) salt and (b) heat transport in upper layers (0 -
800 m) for the Labrador Sea at 50° - 62°N, dashed line, and GIN
seas, 62° - 80°N, dotted line.

The resumption of convection in the Labrador Sea spreads
the dense water to nearly its whole depth and induces a
meridional steric difference in the surface (Figure 13a, black
line) and pressure difference in the deep ocean between the
Labrador Sea and its southern ocean (Figure 13b, black line).
This further induces resumption of northward salt/heat ad-
vection within the upper layers from low latitudes to the
Labrador Sea (Figure 11) and southward deepwater flow
from the Labrador Sea to low latitudes (Figure 1).
In the GIN seas, the reinitiation process of NADW forma-
tion during stage 2 is similar to the Labrador Sea, except the
GIN seas first undergo a preconditioning process during
stage 1. Net salt transport to the GIN seas within the upper
layers resumes during stage 1 (Figure 11a), but net heat
transport in the upper layers remains suppressed during this
stage (Figure 11b). The net salt transport to the GIN seas
induces the densification of its upper layers (Figures 2e and
10b), which causes shallow reinitiation of convection during
this stage (Figure 12c) and is characterized by shallow pen-
etration of freshened water and stored subsurface heat release
(Figure 9). However, at this time, the dynamical environment
is still not sufficient to reinitiate strong deep convection
(Figures 9 and 12c) and NADW formation (Figure 1). The
resumption of net heat transport to the GIN seas in the upper