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reinitiation of convection is limited to the approximately
upper 2000 m. After the first sign of convection reinitiation,
salinity and temperature both increase noticeably in the upper
2000 m, as a result of the resumed upper layer salt/heat
transport from lower latitudes. The deep warm/fresh anoma-
lies in the Labrador Sea soon dissipate, so that the change of
salinity and temperature in Labrador Sea mainly happens
during stage 1.
Similar to the Labrador Sea, a Hovmöller diagram of area
mean salinity and temperature in the GIN seas (Figure 9)
shows that the whole depth recovery of salinity and release of
stored subsurface heat mainly occurs during stage 2. During
stage 1, freshened water in the GIN seas penetrates down-
ward several hundred meters, accompanied by the release of
stored subsurface heat within several hundred meters too.
However, the dynamic environment during stage 1 is still
not sufficient to reinitiate the convection to deeper depths in
this region and delays the reinitiation of NADW formation in
the GIN seas to stage 2. During stage 2, salinity quickly
increases at all depths, ultimately reaching higher levels than
the initial glacial state (Figure 9a) and is accompanied by
subsurface heat release with warming in the upper layers and
cooling in the deeper layers (Figure 9b).
The evolution of potential density at depth in the Labrador
Sea and GIN seas con
rms the main conclusions from the
Hovmöller diagram of salinity and temperature in these two
regions (Figure 10). The increasing of potential density in
Labrador Sea mainly happens during stage 1 within the
upper 1000 m and in the GIN seas during stage 2 within the
entire water column. A key development occurs in the GIN
seas during stage 1, where a very weak strati
cation is
formed followed by the densification of the entire water
column at the onset of stage 2. It is derived by the increasing
of the upper layers potential density and the small change in
the deep layers. This extremely weak stratification provides a
background for subsequent strengthened reinitiation of
convection and NADW formation in the GIN seas during
stage 2. The physical process of extremely weak stratifica-
tion and reinitiation of NADW in the GIN seas is similar to
the
described by Krebs and Timmermann
[2007a, 2007b] and Renold et al. [2009]. Now we will
present the physical process in more detail.
density threshold
Figure 10. Annual mean potential density (PD) of the (a) Labrador Sea (50°
-
62°N, 70°
-
45°W) and (b) GIN seas (62°
-
80°N, 45°W
-
20°E), separated into vertical ocean layers.
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