Geoscience Reference
In-Depth Information
Figure 7.23. Conceptual model of the present-day atmospheric and oceanic transports
and its consequences. DM is diapycnal mixing, P A (P P ) is pressure in the Atlantic
(Pacific) and S is salinity (from Stigebrandt, 2000 , by permission of Springer-Verlag).
by evaporation and atmospheric transport have to be compensated by transports in
the ocean.
Figure 7.23 provides a conceptual view of the problem. The net atmospheric
transport of water from the Atlantic to the Pacific (denoted by the curved arrow at
the top) means higher salinities in the Atlantic, promoting deepwater production.
Spatial salinity variations in the upper ocean correspond to density variations, and
hence corresponding variations in sea level (the “steric effect”). Assuming a level of
no motion at 1,200 m depth, the sea level in the fresher North Pacific stands about
0.65 m higher than in the North Atlantic. This leads to a flow of low-salinity water
from the North Pacific through the Bering Strait, through the Arctic Ocean, and
then into the North Atlantic through the Fram Strait. As articulated by M. Steele,
D. Thomas, and Rothrock ( 1996 ), this “back door” through the Arctic Ocean (the
upper conveyor in Figure 7.18 ) works to reduce the salinity difference between the
basins. To close the loop, there must also be a lower ocean conveyor with a net salt
transport from the Atlantic to the Pacific. To summarize, the atmospheric freshwa-
ter transports are continually working to strengthen the salinity difference between
the Atlantic and Pacific, while the ocean circulations are working to destroy the
differences. The Arctic “back door” is a key element of the oceanic freshwater
exchange.
In the limit of an infinitely fast transfer rate of freshwater through the Arctic
Ocean into the Atlantic, the salinity imbalance between the Pacific and Atlantic
would be removed and convection in the north Atlantic might cease (Steele et al.,
1996 ). However, as pointed out by Steele et al., this scenario is unnecessarily
extreme. The deepwater formation regimes of the northern North Atlantic appear to
be “delicately poised” with respect to their ability to sustain convection (Aagaard
Search WWH ::




Custom Search