“Great Salinity Anomalies” in the North Atlantic

Abstract

We revisited the “Great Salinity Anomaly” of the 1970s (GSA'70s; Dickson et al., 1988) and documented the newly identified “Great Salinity Anomaly” of the 1980s (hence termed GSA'80s), both propagated around the North Atlantic in a similar fashion. The advective mechanism, initially proposed to explain the observed sequence of low-salinity, low-temperature events during the GSA'70s, apparently holds also for the GSA'80s. The latter was successively observed in the West Greenland Current (1982), the Labrador Current (1983), the Flemish Pass (1984), near the Charlie-Gibbs Fracture Zone (1984–1985), in the Rockall Channel (1985), south of Iceland (1985–1988), in the North Sea (1986–1987), Norwegian Sea (1987–1988), Barents Sea (1988–1989) and Iceland Sea (1989–1990). The advection speed of the GSA'80s seems to be greater than the one of the GSA'70s: The 1980s anomaly reached the Barents Sea 6 to 7 years after peaking in the West Greenland Current, while the 1970s anomaly traveled the same route in 8 to 10 years. These anomalies, however, seem to be of different origin. The GSA'70s was apparently boosted remotely, by a freshwater/sea ice pulse from the Arctic via Fram Strait. Consequently, the GSA'70s was accompanied by a large sea ice extent anomaly in the Greenland and Iceland Seas, which propagated into the Labrador Sea. In contrast, the GSA'80s was likely formed locally, in the Labrador Sea/Baffin Bay mainly because of the extremely severe winters of the early 1980s, but supplemented with a possible contribution of the Arctic freshwater outflow via the Canadian Archipelago (facilitated by strong northerly winds) which would have enhanced stability and ice formation. This anomaly was also associated with a positive sea ice extent anomaly in the Labrador Sea/Baffin Bay which, however, had no upstream precursor in the Greenland Sea. Thus the GSAs are not necessarily caused solely by an increased export of freshwater and sea ice from the Arctic via Fram Strait. These results are corroborated by the early 1990s data when a new fresh, cold anomaly was formed in the Labrador Sea and accompanied by a large positive sea ice extent anomaly. The harsh winters of the early 1990s were, however, confined to the Labrador Sea/Baffin Bay area while the atmospheric and oceanic conditions in the Greenland, Iceland, and Irminger Seas were normal. The Labrador Sea/Baffin Bay area appears therefore to play a key role in formation of GSAs as well as in propagation of the GSAs formed upstream. A likely contribution of the enhanced Canadian Archipelago freshwater outflow to the GSA formation also seems to be significant. Two major modes of the GSA origin are thus identified, remote (generated by an enhanced Arctic Ocean freshwater export via either Fram Strait or the Canadian Archipelago) and local (resulting from severe winters in the Labrador Sea/Baffin Bay). Both modes should be taken into account to model decadal variability of the coupled ocean–ice-atmosphere system in the North Atlantic/Arctic Basin.