Simulated Two-Stage Recovery of Atlantic Meridional Overturning Circulation During the Last Deglaciation - Abrupt Climate Change: Mechanisms, Patterns, and Impacts - page 87

simultaneously contributes to the total NADW reinitiation
during the whole recovery period.
This reconsidered mechanism provides a clearer picture of
how NADW reinitiation could occur after meltwater dis-
charge in the North Atlantic, with some important details
that were lacking in previous proposed mechanisms such as
the nonlocal process [Vellinga and Wood, 2002; Yin et al.,
2006; Krebs and Timmermann, 2007a, 2007b]. The contri-
bution of subsurface warming to the reinitiation of NADW is
not found to be a signi cant positive feedback, it is seen as a

trigger for NADW reinitiation because of its effect on sea ice
cover in each region, so we

nd that the other two processes
are the factors responsible for the two-stage feature of
AMOC recovery in this reconsidered mechanism.

4.4. Nature of Two-Stage Feature

As shown above, the reinitiation process of NADW for-
mation in two main regions of origin is robustly asynchro-
nous, first in the Labrador Sea and then in the GIN seas. So
far, the cause of this asynchronous feature is still a critical
issue for the understanding of AMOC recovery.
First, we should notice the difference between the NADW
formation under an unperturbed background state and the
recovery state after the halting of meltwater discharge. The
variation of NADW formation during an unperturbed state is
dependent on the local ocean surface density

ux. However,
during the period of recovery, there is a large-scale retreat of
extended sea ice cover that directly affects the surface den-
sity flux and allows NADW reinitiation to occur. Another

difference with the unperturbed state is that during the
AMOC recovery period, the North Atlantic is freshened at
greater depths than in other oceans, and the AMOC recovery
process is accompanied by the resumption of salinity advec-
tion in the North Atlantic in the upper layers, enhancing
NADW formation directly. So, owing to the unique local
and nonlocal processes affecting NADW formation during
AMOC recovery, the dynamical ocean environment is very
different from the unperturbed state.
Second, two kinds of processes, which are mentioned in
the reconsidered mechanism, each induce the asynchronous
feature of the NADW reinitiation in its two origins. As
discussed above, the local process is mainly affected by the
extended sea ice cover, which retreats primarily as a result of
heat transport from low latitudes. Since the GIN seas are
located northward of the Labrador Sea, the sea ice retreat
happens in the Labrador Sea before the GIN seas.
Third, it was also found that the ef

Figure 14. Change in Atlantic zonal averaged salinity anomaly
(shading with thin contours) and AMOC stream function (thick
contours) during (a) pre-BA - GLA, (b) REC-GLA, and (c) BA-GLA.

origin ' s surface layer reduces the stratification and starts the
reinitiation of NADW. Here we can use SHF as a represen-
tation of surface density flux because its contribution to the
density flux is dominant [Shin et al., 2003b]. Second is the
nonlocal process shown in Figure 15 (box 1 to box 4 to box 5
to box 6 to box 7 to box 1). The salty/dense water transport
into the upper layers of each region (Figures 11a and 14)
overlies fresher and lighter local water in deeper layers (Fig-
ure 14), directly reduces the stratification, and starts the
reinitiation of NADW. The local deepwater formation from
surface density flux and nonlocal dense water transport

ciency of the nonlocal
dense water transport is mainly controlled by the intensity of
advection within the upper layers and meridional salinity
gradient (Figure 14). After the freshening of the North