Variability in the Greenland Sea as revealed by a repeated high spatial resolution conductivity-temperature-depth survey (original) (raw)

1993, Journal of Geophysical Research

This paper deals with the mesoscale variability of water masses in the Greenland Sea and its implications for bottom water formation. The results are based on two conductivity-temperature-depth surveys performed in successive years. The same section in the central Greenland Basin was sampled in 1989 and 1990; an additional transect across Fram Strait was carried out in 1990. The transects extended from shelf to shelf with a station spacing of 18 km throughout. The data set reveals a surprisingly strong horizontal variability of space scales between 20 and 60 km in the entire Greenland Sea, not only in the frontal zones. The area investigated is subdivided into four hydrographical regimes for which mesoscale variability is discussed in detail. A noteworthy result is the major change of the deep-sea thermal structure within 1 year. The classical pattern with upward doming cold waters in the central basin was found in 1989 but was replaced by a "capped" structure with a warm intermediate layer in 1990. The implications of the observed changes are discussed with respect to deep water formation. A mechanism, based on differential compressibility, is proposed which is able to introduce negative heat input selectively into the bottom layer. It is shown that the fine structure of temperature profiles observed in summer can be used as a tracer for the occurrence of deep convection during the preceding winter. Convective depths are concluded of about 2200 m for 1989 and of only about 250 m for 1990. INTRODUCTION Since a general view of the hydrographic processes in the Greenland Sea was established around the beginning of our century [Helland-Hansen and Nansen, 1909], considerable efforts have been made to quantify proposed hydrographic processes inherent to this area of outstanding water mass composition, as well as to modify the initially general picture of current regimes and water mass formations. The better and the more precise the prevailing conditions in the Greenland Sea became known, the more complex the mechanisms of water mass exchanges and of transport processes appeared. Most interactions turned out to be more intricate than had been previously thought. For example, the Polar Water (PW) in the western Greenland Sea contains in certain regions or from time to time exceptionally cold and salty constituents of unknown origin [Aagaard and Coachman, 1968; Bourke et al., 1987]; even large circulation patterns in the Greenland Sea seem to vary [Quadfasel and Meincke, 1987], and bottom waters in the central Greenland Basin show changes of still unresolved time scales [Aagaard, 1968; Clarke et al., 1990; Meincke et al., 1992]. Most variations of seasonal patterns have been inferred from comparisons between different years, sometimes separated by a decade, and large uncertainties still exist about the spatial extent and spatial variability of water mass structures. This situation is partly caused by the specific difficulties of field work in the area which include restricted accessibility of parts of the region due to ice cover, the short period of favorable working conditions during the summer season, the large extent of the area, and the extreme measurement precision required to distinguish between some of the water masses. As even best available conductivity-temperature