Hydrographic conditions were highly variable in the Greenland Gyre during the past few decades. High tracer concentrations in the deep water obtained in the 1970s and early 1980s were associated with low salinities and temperatures and indicate high deep water formation rates. Constant CFC and decay-corrected tritium concentrations in the 1980s and early 1990s were associated with increasing salinities and temperatures in the deep water and reflect a decreased surface contribution and larger relative contributions of EBDW and NSDW to the deep water in the Greenland Sea. However, the transient tracer observations in the 1980s and early 1990s indicate, that water with higher transient tracer concentrations continued to contribute to the deep water formation during this period, although at a lower rate. This water can only originate from the overlying water column of the Greenland Sea. It may probably have been intermediate water. The decrease in deep water formation rates in the 1980s and early 1990s was very likely not the first event of this kind, the warming of the deep water in the late 1950s suggests a similar event.
Although the intermediate water is inhomogeneous in respect to all parameters, the mean temperature showed only small variations between 1952 and 1980. Between 1981 and 1994 however, the layer between 200m and 2000m depth warmed continuously, while the average salinity showed only small variations. This warming is equivalent to a net heating rate of about 5 W/m2. The heat accumulated over a period of 13 years could be removed by 2 winters of intensive cooling. The density decrease in the intermediate water can be explained by sinking or mixing of the most dense part of the intermediate water into the deep water below 2000m. This scenario is consistent with the evolution of all parameters in the deep and intermediate water.
The time series available for this study does not include the seasonal cycles and therefore generally does not allow statements about the surface waters. However, the possible comparison of the surface salinities between 1991 and 1994 shows a steady increase over this period and the reduced salinity difference between the surface and the intermediate water destratifies the water column and makes more rigurous convection more likely.
The combination of hydrographic and transient tracers is useful to determine long-term trends of deep water formation rates and processes. If annual high-quality time series data are available, it is possible to detect climatically important changes that occur on time scales of several years to several decades.