In Dead Water

environment include changes in marine ecosystem productivity, oceanic CO 2 uptake, oceanic oxygen concentrations and shifts in fisheries. Adaptation to MOC-related impacts is very likely to be difficult if the impacts occur abruptly (e.g., on a decadal time-scale). Overall, there is high confidence in predictions of a MOC slowdown during the 21st century, but low confidence in the scale of climate change that would cause an abrupt transition or the associated impacts. However, there is high confidence that the likelihood of large-scale and persistent MOC responses in- creases with the extent and rate of anthropogenic forcing. Dense shelf water cascading is a type of marine current driven exclusively by seawater density contrast. The cascading pro- cess is normally seasonal and triggered by the formation, on the shelf, of dense water by cooling and/or evaporation and its sinking down slope towards deeper offshore areas.

the deep sea, often using and carving submarine canyons. This margin exchange process provides an essential link/exchange between shallow and deep waters and involves water and con- siderable particulate and dissolved loads, especially when oper- ating in a ‘flushing’ pattern. Due to their proximity to land areas, continental shelves are the locus of input, transit and accumulation of land born particu- late substances, including pollutants. Dense shelf water cascad- ing transports these particulate substances for recycling into the deep sea. Any future climate change driven alterations in the temperature regime of the oceans, such as the predicted in- crease in the horizontal layering (‘stratigraphy’) of water mass- es, will have a significant impact in the frequency and intensity of cascading events, and thereby on the biogeochemical bud- gets of shallow waters and the ventilation of deep water areas.

There are a number of places around the world where dense water masses flow ‘over the edge’ of the continental shelf into

Scientists working under the large deep-sea research project HERMES (Hotspot Ecosystem Research on the Margins of Eu-

Figure 21. Coastal regions in the World where dense shelf water cascading ‘flushing’ has been observed. Knowledge and mapping of these processes is still scarce due to uneven research effort. The map shows sites with known dense shelf water cascading phenomena, which often may involve the ‘flushing’ effect (Ivanov et al ., 2004; Durrieu et al ., 2005; Heussner et al , 2006). It is most likely that this phenomenon is also active off the coast of Alaska, Chile, Argentina and West and southern Africa and in parts of the Indian Ocean. Dense shelf water cascading is highly sensitive to increases in temperature, and hence, climate change. Data from Canals et al . (2006).

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