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Natural Climate Variability on Decade-to-Century Time Scales
Over the past two decades there have been many studies of the nature and causes of interannual variability of sea-ice cover in the Arctic Ocean and marginal seas. Observational studies have been carried out by Walsh and Johnson (1979), Mysak and Manak (1989), Parkinson (1991), and others, whereas model simulations of atmospherically forced interannual variability of sea-ice extent and concentration have been performed by, for example, Walsh et al. (1985) and Fleming and Semtner (1991). Mysak and Manak (1989) also noted that the low-pass filtered areal sea-ice anomalies in the Barents and Greenland seas contain well-defined decadal-scale fluctuations, and they hypothesized that some of these might be related to the Great Salinity Anomaly (GSA; see Dickson et al., 1988), a widespread freshening of the surface waters in the subpolar gyre of the North Atlantic during the 1960s and 1970s.
In Mysak et al. (1990), hereafter referred to as M3, the decadal-scale fluctuations of ice cover in the Greenland and Iceland seas and attendant salinity anomalies in the Iceland sea (defined here as the region between Jan Mayen Island (near 71°N, 8°W) and Iceland) were further analyzed for the period 1901 to 1984, using updated Walsh and Johnson (1979) sea-ice concentration and Danish Meteorological Institute ice-limit data (Sear, 1988). In an attempt to formulate a comprehensive theory of the origin, evolution, and decay of these anomalies in the Greenland and Iceland seas, M3 linked a number of hydrological, oceanic, and atmospheric processes in the Arctic in the form of a negative feedback loop (Kellogg, 1983), which could give rise to self-sustained climatic oscillations in the Arctic for periods of 15 to 20 years. Under this scenario, GSA-like events could be regarded as cyclic rather than isolated, and as part and parcel of a sequence of complex air-ice-sea interactions.
In this paper a modified (and simplified) form of the feedback loop proposed by M3 is described, and evidence is presented for the occurrence and possible origin of another GSA-like event during the 1980s, which was predicted by M3. Because of the close relationship between salinity and sea-ice anomalies, we shall hereafter refer to these events as GISAs—great ice and salinity anomalies. Some results of a recent cross-correlation analysis (Mysak and Power, 1992), which show that sea-ice anomalies in the western Arctic consistently lead those in the Greenland Sea by 2 to 3 years, are also presented below. The question of whether GISAs have occurred prior to the 1960s is briefly addressed, and some possible links of GISAs with lower-latitude interdecadal climate fluctuations are described. (Within this context, we regard the 1960s-1970s GSA as also a GISA.)
NEGATIVE FEEDBACK LOOP FOR AN INTERDECADAL ARCTIC CLIMATE CYCLE
Figure 1 shows a six-component feedback loop of an interdecadal Arctic climate cycle that represents a modified
Negative feedback loop linking Iceland Sea-Irminger Basin cyclogenesis, northern Canadian river runoff, sea-ice extent, and salinity and convective overturning in the Iceland Sea (from Mysak and Power, 1992). This is a modified (and simplified) version of the ten-component negative feedback loop originally proposed by Mysak et al. (1990) to account for interdecadal Arctic climate oscillations with periods of about 15 to 20 years.
(and simplified) version of the ten-component loop in M3. The plus (minus) appearing between two boxes indicates that an increase in the first would cause an increase (decrease) in the second. Since the number of negative signs around the loop is odd, it represents a reversing or negative feedback loop (Kellogg, 1983). Therefore, in the absence of other strongly damping factors, a perturbation transferred from any one component to the next can theoretically result in a reversal of the sign of the initial perturbation. It has been estimated by M3 that the period of the climate cycle, which is twice the loop circuit time, is about 15 to 20 years.
A key feature of the loop in Figure 1 is that large negative salinity (and positive ice) anomalies in the Iceland Sea are hypothesized to be due, at least in part, to prior large runoffs from North America into the western Arctic (see the box at top of the loop). This concept of a remote forcing for anomalies like the GSA contrasts with earlier hypotheses that suggest that such anomalies are due mainly to local or neighboring atmospheric effects (Dickson et al., 1975; Pollard and Pu, 1985; Serreze et al., 1992; Walsh and Chapman, 1990a; see also the discussion in Mysak and Power, 1991). As noted in M3 (see their Figure 17), during the mid-1960s there were indeed anomalously large runoffs from North America into the Arctic and subarctic, which may have been associated with the climatic jump at that time (see