Simulated Two-Stage Recovery of Atlantic Meridional Overturning Circulation During the Last Deglaciation - Abrupt Climate Change: Mechanisms, Patterns, and Impacts - page 86

Figure 13. (a) Southward steric gradient (SSH lab , 50° - 62°N; SSH gin , 62° - 80°N; and SSH sa , 30°S - 50°N) and (b) deep
ocean pressure gradient (1500 m, hPa) of the Atlantic Basin. Dashed and dotted lines represent values from the Labrador
Sea and GIN seas, respectively.

and the local freshened water in the deeper layers of two
regions of origin (Figure 14). Another is the contribution of
subsurface warming, which disappears when convection is
restarted (Figures 8b and 9b). These two processes together
induce enhanced NADW formation in each NADW region,
and their effects subside at the end of each corresponding
stage (Figure 1).
According to the above analysis, contributions of salinity
dominate the change of potential density in both NADW
regions (Figure 7), and the subsurface heat anomaly pro-
vides an additional reversed vertical density structure for
convection reinitiation in both stages (Figures 8b and 9b).
The main contribution of net heat transport within the
upper layers is to melt extended sea ice cover during each
stage, which is shown to be of crucial importance in the
reinitiation of deep convection in both NADW regions of
origin (Figures 11b and 12c).

4.3. A Reconsidered Mechanism for NADW Reinitiation

On the basis of the comprehensive analysis of variables
relevant to NADW formation in the DGL-A run, and the
previous hypothesized mechanisms [Vellinga and Wood,
2002; Yin et al . 2006; Krebs and Timmermann, 2007a,
2007b], here we reconsider the mechanisms controlling
NADW reinitiation with some clarified details (Figure 15).
In our reconsidered mechanism, we identify two dominant
processes affecting NADW reinitiation. The first one is the
local process as shown in Figure 15 (box 1 to box 2 to box 3
to box 4 to box 5 to box 6 to box 7 to box 1). This process is
mainly dominated by the resumption of surface density flux
over NADW origins. Resumption of surface density flux,
primarily controlled locally by the retreat in sea ice cover, is
largely initiated by northward heat transport to NADW for-
mation regions (Figures 11b and 12a), then densified NADW