As also noted by Holland et al. (2001), model simulations of the variability and sensitivity of the THC appear to depend strongly on model design and complexity. Whether the THC is as sensitive to disruption as some models have indicated is still very much in debate. However, there is observational evidence, the best example being the "Great Salinity Anomaly" (GSA) of 1968-82. During the late 1960s to early 1970s, the upper 100 m of the waters in the Greenland, Iceland and Labrador Seas underwent reductions in salinity of 0.1 to 0.5 psu, with water temperature anomalies of —1 to —2 K. This was associated with positive anomalies in sea ice extent in the Greenland Sea, peaking during 1968-9 and propagating with the salinity anomalies into the Labrador Sea in 1971-2. The salinity anomaly circulated around the Atlantic sub-Arctic gyre, returning to the Greenland Sea in 1981-2 (Dickson et al., 1988).
Observational studies (Walsh and Chapman, 1990; Wohlleben and Weaver, 1995) as well as model evidence (Hakkinen, 1993) suggest that strong northerly winds caused an increase in the sea ice transport through Fram Strait and into the Greenland Sea. The large freshwater flux anomaly associated with this transport was likely enhanced by the relatively large transport of particularly thick ice from north of Greenland. Dickson et al. (1988) estimate the excess freshwater associated with this event to be about 2300 km3, somewhat less than the mean annual Fram Strait ice flux. Hakkinen's (1993) study also suggests an increased oceanic transport of fresh anomalies in the East Siberian Sea, moving across the Arctic and entering the Greenland Sea about four years later. The GSA caused cessation of convection as recorded at weather station "Bravo" (56° N, 51° W) (Lazier, 1980). Other lines of evidence indicate that it was associated with a 30% reduction in the strength of the Gulf Stream system (Greatbatch et al., 1991; Ezer etal., 1995).
The GSA nevertheless remains somewhat of a puzzle. Vinje's (2001) monthly time series (Figure 7.14) shows no obvious indication of a large change in the Fram Strait flux during the late 1960s. But there were no direct observations of the volume flux during this time. Additional GSA-like events have been documented by Belkin et al. (1998). They suggest that some of these events have a local source due to enhanced ice formation in the Labrador Sea/Baffin Bay areas, whereas others, such as the GSA of the late 1960s to early 1970s, are remotely forced by enhanced freshwater export via either the Fram Strait or through the Canadian Arctic Archipelago. As reviewed in Chapter 10, there is convincing evidence of periods with a much weaker or altered North Atlantic THC during the last glacial cycle, the causes of which are still not well understood.
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