The nature of the THC is an area of intense study. No complete explanations can be offered as to why deepwater forms in the North Atlantic and not in the North Pacific and how the Arctic maintains or modulates this state. The issue is a complex one and only the major points are outlined here. Weaver et al. (1999) and Stigebrandt (2000) provide further reading.
One observation bearing on the problem is the more stable stratification in the North Pacific as compared to the North Atlantic. In the North Pacific, surface water salinities are on average 32.8 psu with deeper waters at 34.6 to 34.7 psu. By contrast, in the North Atlantic, surface waters are 34.9 psu, with deeper waters between 34.9 and 35.0 psu. Even if North Pacific waters are cooled to their freezing point, they are so fresh that they can sink to only a few hundred meters, while those in the North Atlantic can sink to great depths.
Regarding how the stratification difference between the two basins is maintained, Warren (1983) highlights that there is nearly twice as much evaporation over the North Atlantic as over the North Pacific (1030 mm yr-1 versus 550 mm yr-1). The higher evaporation in the North Atlantic is associated with higher SSTs, in part due to the greater northward advection of warm, subtropical waters by the Gulf Stream. It has also been proposed that, because of the narrower width of the Atlantic as compared to the Pacific, the Atlantic will be more strongly influenced by incursions of cold, dry continental air that favor evaporation and heat loss at the sea surface. There is also a corresponding net transport of water vapor from the Atlantic to the Pacific. The idea is that the low-elevation Isthmus of Panama allows a transport of moisture from the Atlantic to the Pacific via the trade winds, whereas the high-elevation Rocky Mountains inhibit transport from the Pacific to the Atlantic by the westerly flow. For a steady state, imbalances in net precipitation between different ocean basins brought about by atmospheric transport have to be compensated by transports in the ocean.
Figure 7.15 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 1200 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. As articulated by Steele et al. (1996), this "back door" through the Arctic Ocean (the upper conveyor in Figure 7.15)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 freshwater 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 deepwa-ter formation regimes of the northern North Atlantic appear to be "delicately poised"
Figure7.15 Conceptual model of the present-day atmospheric and oceanic transports and their consequences. DM is diapycnal mixing, PA (PP) is pressure in the Atlantic (Pacific) and S is salinity (from Stige-brandt, 2000, by permission of Springer-Verlag).
with respect to their ability to sustain convection (Aagaard and Carmack, 1989). Density is largely a function of salinity at low temperatures. Consequently, only a moderate freshening, such as from a change in the Fram Strait outflow, could cause freshwater capping, leading to a reduction or cessation of deepwater formation, an idea first proposed by Weyl (1968). Because of the differential compressibility of seawater, with cold water being more compressible (the thermobaric effect), it is likely that within some critical range, a small amount of salinity stratification actually serves to promote convection (Aagaard and Carmack, 1989). However, too much freshwater caps the convection.
The study of Holland etal. (2001), based on aglobal coupled ice-ocean-atmosphere model, suggests that realistic variations in the Fram Strait ice export can lead to decadal-scale variability in the THC. The basis of this interaction is that reduced ice export leads to less ice melting in the North Atlantic. This destabilizes the ocean column, causing more deepwater formation. An increase in the THC results, leading to anomalously high ocean heat transport, which reduces ice growth in the GIN seas. This warms and freshens the surface, reducing the original anomalous high ocean density, acting as a negative feedback on the system. The sensitivity of the THC to freshwater export to the North Atlantic finds further support from simulations of coupled ice-ocean models (e.g., Hakkinen, 1993, 1999).
An interesting facet of the THC debate is that the Arctic "back door" can be influenced by changes within the Arctic itself. The Fram Strait outflow is sensitive to the regional atmospheric circulation. Freshwater inputs to the Arctic Ocean from river runoff are larger than the Bering Strait inflow. Changes in the atmospheric circulation, river discharge, as well as in net precipitation over the Arctic Ocean itself, will also ultimately be seen in the Fram Strait outflow. The Holland etal. (2001) study also indicates that variability in the THC affects the atmospheric state, altering air temperature, precipitation and runoff.
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