Geoscience Reference
In-Depth Information
the Atlantic Basin (Figures 2 and 3), surface air temperature
(SAT) (Figure 4), and zonal mean salinity, temperature, and
potential density (Figure 5).
As shown in Figure 2, sea surface salinity (SSS) and sea
surface density (SSD) of the whole North Atlantic Basin
and SST of south of Greenland signi
cantly increase during
stage 1. However, during stage 2 the increase in SSS, SST,
and SSD primarily happens in the GIN seas. SSS and SSD do
increase somewhat in the GIN seas during stage 1 but not
enough to induce robust NADW formation, as shown in
Figure 1. The reason why the increased SSS and SSD in the
GIN seas during stage 1 did not induce the robust reinitiation
of NADW will be addressed in section 4.
The changes in sea ice cover (annual mean sea ice concen-
tration), surface heat
flux (SHF) (downward) and maximum
mixed layer depths provide a clearer picture of the two-stage
feature in the Labrador Sea and GIN seas (Figure 3). Because
of the coarse resolution of the model used here and other
similar models, the simulated deepwater formation regions
are broader than the Labrador Sea and GIN seas. For simple
presentation, here we refer to them as the Labrador Sea and
GIN seas to present the surrounding changes in general. The
Irminger Sea also has similar significant changes to the GIN
seas during stage 2. Because of its small area and complex
structure (shown in Figure 3), here and in following sections
Figure 6. T-S diagram of the North Atlantic upper layer water (0 -
800 m) for GLA (solid line, mean within 20 - 19 ka), pre-BA
(dashed line, 14.67 ka), REC (dotted line, 14.5 ka) and BA (dash-
dotted line, 14.32 ka). Latitude bands are shown with different
markers (circle for the tropical North Atlantic, square for the sub-
tropical North Atlantic, diamond for the subpolar North Atlantic
which includes the Labrador Sea, and triangle for the GIN seas).
Latitude interval is 5°.
Sea is activated from the turned off state to the enhanced state
(above initialized volumes) within 150 years. This has the
result that while NADW formation in the GIN seas has not
fully recovered by the end of stage 1, the total volume of
NADW formation in the North Atlantic (AMOC intensity)
reaches its original glacial level at the end of stage 1 (REC).
Thereafter, the continuing recovery of NADW formation in the
GIN seas in stage 2 pushes the AMOC intensity to an even
higher level (about 20 Sv) within 170 years. Continuous
increasing of AMOC intensity results in a robust AMOC
overshoot phenomenon on a time scale of hundreds of years
and a mean state transition of AMOC from 12.5 Sv at the Last
Glacial Maximum to about 17.5 Sv at interglacial state (stable
AMOC intensity during BA, similar to its modern simulated
value of about 17 Sv in CCSM3 according to Renold et al.
[2009]). On the basis of the precondition from stage 1, it is
evident that the rapid recovery and subsequent overshoot of the
AMOC can largely be attributed to the speed and magnitude of
enhanced NADW formation in the GIN seas during stage 2.
The two-stage feature of AMOC recovery can also be
confirmed by changes in associated sea surface variables in
Figure 7. T-S diagram of the Labrador Sea (dashed line, 50° - 62°N)
and GIN seas (solid line, 62° - 80°N) within upper layers (0 - 800 m).
GLA/pre-BA/REC/BA are shown with different symbols.
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