Tracer evidence of the origin and variability of Denmark Strait overflow water

Abstract

The overflow of dense water from the Nordic Seas to the North Atlantic through the Denmark Strait is an important part of the global thermohaline circulation. Denmark Strait Overflow Water (DSOW) has its sources in the Nordic Seas and the Arctic Ocean and is a complex mixture of several water masses. The magnitude and variability of the overflow are significant not only for the local oceanography, but also for the global large-scale circulation. Just as the intensity of the overflow is temporally and geographically variable, so are the hydrographic and hydrochemical characteristics of the overflow shifting. Variations in these properties have two possible sources: (1) changes in the characteristics of water masses and, (2) changes in the water mass composition of the overflow. Changes in atmospheric forcing and convection within the source region for DSOW might change its water mass composition and characteristics, changes that in turn will propagate to the North Atlantic Deep Water. In this chapter, we have synthesised the knowledge of the characterisation and origin of DSOW from historical and recent studies, all using chemical tracers. We are further focusing on the formation and variability of the Denmark Strait Overflow Water as found in the strait or in the nearby Irminger Basin. We are thus ignoring the extensive literature on tracers in the North Atlantic Deep Water further south, as well as those focusing solely on the Arctic Mediterranean. Similarly, results derived solely from classical hydrography are presented elsewhere in this volume (Dickson et al. 2008). The increased number of tracer observations and thus the increased spatial and temporal data coverage has enabled more sophisticated water-mass analysis. Although changes in the water mass composition and hydrochemical characteristics of the DSOW is evident on annual basis, continued monitoring of tracers in the Denmark Strait will enable detection of changes in the source region for DSOW. © 2008 Springer Netherlands.