Fig. 20.9 CFC-12 concentrations (pmol kg ') in DSOW (> 27.85 kg m 3) from six cruises from 1993 to 2002 versus latitude. The data are from the following cruises, with project names in parenthesis; 1993 (crosses); RVAranda (Nordic-WOCE), 1994 (dots); RV Johan Hjort(Nordic-WOCE), 1997 (squares); RV Aranda (VEINS), 1998 (circles); RV Valdivia (VEINS), 1999 (diamonds); RV Marion Dufresne (GINS), 2002 (stars); IB Oden. The Denmark Strait sill is located at approximately 66.2° N

downstream the sill, and at about 63° N the plume has a uniform tracer concentration of approximately 2.2 pmol kg-1 in 1997. Since most data presented here do not extend far enough south, it is difficult to draw any conclusions on the temporal variability in the age of DSOW. Additionally, a mixing model must be employed for such a calculation due to the nearly constant atmospheric CFC-12 concentrations since the early 1990s. When the plume descends into the Irminger Basin, the CFC concentration indicates a relatively low age for DSOW (the TTD mean age is ~30 years in 1997, but only about 20 years in 1998). The other overflow water mass, Iceland-Scotland Overflow Water, is considerably older.

20.5 Open Questions for the Denmark Strait Overflow

With this article we have tried to synthesise the current knowledge of the changing sources and characteristics of DSOW the last 30 years, as determined from relatively sparse and short time records of tracers (longer records are available for hydrographic and hydrochemical data, but such changes are beyond the scope of this work). We have shown that the water mass composition of the Denmark Strait Overflow seems to be changing over time, and that the characteristics of the source water masses might be changing as well. Both these processes will result in changing characteristics of the overflow, and it is not a trivial problem to distinguish between these two processes. Variations in the overflow are mainly and most likely due to variations in climatological factors such as freshwater input, wind forcing and temperature. Such variations are often found as a result of the North Atlantic Oscillation (NAO), but with the signal of climate change superimposed. Variations in the forcing are reflected in the DSOW characteristic and composition. Thus, monitoring of the DSOW characteristics has the potential to be an index of the conditions in the Nordic Seas. This is further discussed by Dickson et al. (2008).

The changing sources of the deep overflows over the Greenland-Scotland Ridge are, in our opinion, an important piece of information to understand the dynamics of the Nordic Seas region, as well as those of the North Atlantic. Even though these changes, to some extent, can be monitored by hydrography and current measurements, we have shown that the inclusion of a set of tracers provides additional information on the source water masses, and their pathways and transport times to the overflow, something that complements the physical measurements substantially.

Our suggestion is to monitor tracers in the overflow during hydrographic surveys on a regular basis. New approaches to regular sampling of water in the overflow are very interesting, and we would like to promote efforts in the direction of automated sampling arrays (e.g., moorings) as recently initiated, to complement hydrographic surveys by research vessels.

It is a bold endeavour to suggest sampling and measurement strategies for the future. Nonetheless, a few suggestions, based on the experience gained from a decade of tracer measurements in the Denmark Strait region, are presented in the following.

The first suggestion regards sampling strategies. The temporal variability at the sill of the Denmark Strait is large on short timescales, and one-time surveys on the sill will most likely never be able to representatively sample the overflow water, at least it is difficult to really know whether the conditions at the sill were representative or not. Rather, we suggest that the priority sampling is done at positions sufficiently far north and south of the sill to filter out most of the short time variability. We further suggest that sampling is concentrated on routinely repeated sections, which have a history of measurements to facilitate comparison (see Fig. 20.10).

Fig. 20.10 Map of the Denmark Strait with the position of a few historical sampling locations; GEOSECS 1972, TTO-NAS 1981, WOCE A24N 1997, a VEINS standard section repeated several times and one section sampled from RRS James Clark Ross 1999 (ARCICE) north of the sill

Fig. 20.10 Map of the Denmark Strait with the position of a few historical sampling locations; GEOSECS 1972, TTO-NAS 1981, WOCE A24N 1997, a VEINS standard section repeated several times and one section sampled from RRS James Clark Ross 1999 (ARCICE) north of the sill

South of the sill, it is reasonable to sample close to the TTO section, for three reasons: (1) There exists a significant historical record from this section, including TTO-NAS in 1981 and it is close to one GEOSECS station from 1972, as well as current moorings and more recent tracer measurements discussed above such as the WOCE section A24N and the standard VEINS section repeated several times, albeit not all the way across the basin. (2) The location is far enough downstream for the sill for DSOW to have homogenised sufficient to represent an "end-product". (3) The section extends not only over the Greenland shelf, but also over the Reykjanes Ridge, thus capturing the inflow of water important to the mixing south of the sill.

For the sampling upstream, there is not the same history of tracer measurements. Again, it is important that the section extends across the basin and up on both the Icelandic and Greenland shelf. Such a section was sampled by an ARCICE survey on the RRS James Clark Ross in 1999 (e.g., Messias et al. 2007). This section will be able to represent all the water masses transported with the East Greenland Current, as well as water masses formed locally in the Iceland Sea. The Icelandic standard section Kogur, located south of the ARCICE section, has the benefits of being a standard section, although it suffers from not reaching the Greenland shelf. However an extension of the Kogur section would be well-suited section for tracer measurements. Additionally, a section at the sill certainly has many benefits, and should also be sampled along with the two sections suggested above if possible.

We have here shown data and results obtained from a wide variety of tracers, and that the observed tracers have shifted over time. Also in the future it is likely that "new" tracers will be added and other disappear as when their transient signal decline, as in the case of cessation in CFC increase in the atmosphere. One example of an additional tracer with currently increasing source function is HCFC-22 that could prove to be complementary to the CFC measurements. Therefore, it is sensible to continue monitoring tracers such as CFCs, SF6, 129I, tritium, 137C and 90Sr in the overflow to connect to the historical records, and at the same time be alert to new tracers that might develop with time.

We also would like to stress the importance of including measurements of parameters such as oxygen and nutrients that are shown very valuable for water mass analysis (Tanhua et al. 2005b). In addition to water mass analysis, there is a scientific interest measuring the flux of nutrients and oxygen, as well as the carbonate system (i.e., anthropogenic carbon) across the sills. For these flux calculations hydrochemical measurements of high quality are very important, ideally calibrated against certified reference materials.

20.6 Conclusions

The understanding of the composition and variability of Denmark Strait Overflow Water (DSOW) has evolved considerably since the early 1990s, and part of this knowledge stems from tracer observations. Already in the 1980s, the general opinion on what was the main source of DSOW changed from the Norwegian Sea Deep Water to intermediate waters. Most pre-1990 studies, however, pointed out the Iceland Sea as the main source region of DSOW while it since 1990, has been realised that DSOW is a rather complex mixture of a large set of water masses formed by different processes and in different regions. This change of view is, at least partially, an effect of the introduction of new methods and parameters and of higher temporal and spatial data resolution, and might not reflect an actual change in water mass composition. However, inter-annual comparison indicates moderate variability in recent years although decadal variability might be considerably higher. The development and use of new methods and tracer compounds have been fundamental in understanding the water mass composition, and its variability. Examples of new tracers include the radioactive isotope 129I and the SF6 released in the Greenland Sea. The former has both a site-specific and temporal source implying its large potential while the later tagged one specific water mass, which has been followed into the overflow.

Since the 1990s, tracer data suggest that the bulk of the overflow has been supplied by the East Greenland Current with water from the Arctic Ocean, the Fram Strait and the Greenland Sea. The denser part of the overflow has two main sources: the Arctic Ocean and the Greenland Sea, of which the Arctic Ocean dominated during the last decade although it seemed to vary considerably (Fig. 20.8). For the shallower layers, modified Atlantic Water and Arctic intermediate water from the Nordic Seas were the dominating contributors, although the highly variable influence of Polar waters is important by making DSOW fresher. The contribution from the central Iceland Sea was minor, in general around 5%, except for in 1999 when it contributed to about one third of the less dense DSOW fraction. This extreme in fresh (i.e. low-saline) water is clear in the time series presented by Dickson et al. (2008)

It has been suggested by Rudels et al. (2003) that during periods of modest convection, the regional circulation affects the contribution to DSOW in a way that the denser layers will be more influenced by the Greenland Sea and less by the Arctic Ocean while the Atlantic layer will instead have a larger portion that has passed through the Arctic Ocean and a smaller that has been recirculated already in the Fram Strait. The supply of water from the Iceland Sea on the other hand seems to vary on shorter timescales. Throughout the short period of detailed, tracer-based, studies on the composition of DSOW, water formation processes in both the Arctic Ocean and the Greenland Sea, together with the transformation of Atlantic water in the Arctic Ocean and the Fram Strait, are of large importance for the DSOW. The properties of the water masses show temporal variability, in particular those of the more locally produced water masses, such as water from the Iceland Sea and Polar waters since they are more directly affected by changes in, for example, wind fields. The tracer-based water mass studies further suggest that a change in the production of one water mass, for example, caused by the shift in convection intensity in the Greenland Sea, may, at least initially, be compensated by a change in the supply of another water mass. As a result, the volume of the overflow can stay relatively constant, whereas, at the same time, the properties of the overflow may change significantly, which would affect the entrainment downstream the Denmark Strait and the further circulation. The notion of relatively constant strength of the Denmark Strait Overflow over decadal timescales is indeed supported by observations (e.g., Ross 1984; Dickson and Brown 1994; Dickson et al. 2008), as well as by models (e.g., Käse 2006), even though Macrander et al. (2005) found the variation in the transport to be about 30% over a 4-year period. Large-scale changes in forcing will likely affect more than one of the regions or processes of water mass formation. Thus, such changes may have long-term effects on the overflow, even if the overflow appears to be robust due to its origin in more than one process and region.

Acknowledgements We thank all investigators that under several decades with great effort and the uttermost care have collected tracer data relevant to the Denmark Strait Overflow. We particularly thank the authors of a as yet unpublished manuscript (Jeansson et al. 2008) for letting us use some of their results in this work. We gratefully acknowledge Peter Jones for letting us use some unpublished CFC data and for valuable comments on the manuscript. During the preparation of this manuscript, T.T. was supported by the Deutsche Forschungsgemeinschaft (DFG) through SFB460.


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