In discussing the observed temporal changes, we arbitrarly separate the water column into deep (below 2000m), intermediate (200m - 2000m), and near surface (above 200m) waters. The choice of 2000m as the upper boundary of the deep layer is primarily based on tracer profiles, in particular on the location of the base of the exponentially decreasing tracer profiles (Figs. 2d, g, and h). Furthermore, 2000m is the approximate sill depth of the ridge separating the Greenland from the Norwegian Sea (Bourke et al. [1993]). The upper 200m are strongly affected by exchange with the atmosphere and show seasonal effects.
In the following, we have chosen not to discuss specific water masses, such as Greenland Sea Deep Water (GSDW), because these are defined in terms of specific temperature and salinity characteristics. One purpose of this contribution is to point out that these characteristics are variable in time. For example, GSDW has been defined as water with temperatures between -1.26 C and -1.29 C, and salinities between 34.889 and 34.892 by Swift et al. [1983], temperatures below -1.0 C, and salinities between 34.88 and 34.90 by Aagaard et al. [1985], temperatures of -1.28 C and salinities of 34.891 by Smethie et al. [1986], temperatures of -1.242 C and salinities of 34.895 by Swift and Koltermann [1988], and temperatures between -1.2 C and -1.9 C, and salinities between 34.890 and 34.905 by Measures and Edmond [1992]. By the end of 1994, the water found in the center of the Greenland gyre below 2000m depth had a temperature of -1.149 C and a salinity of 34.899, i.e., can no longer be identified as GSDW according to most of the above definitions.