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In Dead Water

Slowing Down of Thermohaline Circulation and Continental Margin Dense-Water Exchange Mechanisms

A fifth very serious impact of climate change may be on ocean circulation. Palaeo-analogues and model simulations show that the Meridional Overturning Circulation (MOC) can react abruptly and with a hysteresis response, once a certain forcing threshold is crossed. Discussion on the probability of the forcing thresholds being crossed during this century lead to different conclusions depending on the kind of model or analysis (Atmosphere-Ocean General Circulation Models, Earth system models of intermediate complexity or expert elicitations) being used. Potential impacts associated with MOC changes within the marine environment include changes in marine ecosystem productivity, oceanic CO2 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 increases with the extent and rate of anthropogenic forcing.

Figure 20:The Meridional Overturning Circulation plays a crucial role for life in the oceans. If this ocean conveyor belt slows down or changes as a result of melting ice and increasing ocean temperatures, the impacts on marine life may become severe.

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).

Dense shelf water cascading is a type of marine current driven exclusively by seawater density contrast. The cascading process 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.

There are a number of places around the world where dense water masses flow ‘over the edge’ of the continental shelf into 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 considerable particulate and dissolved loads, especially when operating in a ‘flushing’ pattern.

Due to their proximity to land areas, continental shelves are the locus of input, transit and accumulation of land born particulate substances, including pollutants. Dense shelf water cascading 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 increase in the horizontal layering (‘stratigraphy’) of water masses, will have a significant impact in the frequency and intensity of cascading events, and thereby on the biogeochemical budgets of shallow waters and the ventilation of deep water areas.

Figure 22:Climate change models (B, C1–3) predict that the flow of dense shelf water (DSW) into the deep sea (A) will decrease in the next 100 years. (A: Courtesy of GRC Marine Geosciences-University of Barcelona, CEFREM-CNRS/University of Perpignan, and ICM Barcelona-CSIC; B,C: Based on Somot et al, 2006.)

Scientists working under the large deep-sea research project HERMES (Hotspot Ecosystem Research on the Margins of European Seas) – of which UNEP is a partner – documented, three years ago, the occurrence and effects of a dense shelf water cascading phenomenon in the Gulf of Lions (North-western Mediterranean) (Canals et al., 2006). The amount of water transported in 4 months from the Gulf of Lions to the deep Western Mediterranean, via the Cap de Creus canyon, equalled around 12 years of the water input from the river Rhone, or 2 years of input from all rivers draining into the Mediterranean. How this dense shelf water cascading in the Gulf of Lions affects the population of the deep-sea shrimp Aristeus antennatus (marketed as ‘crevette rouge’) was only recently discovered (Company et al., 2008). Initially, the strong currents (up to 80 centimetres per second) associated with intense cascading events displace shrimp populations from the normal fishing grounds, producing a temporary fishery collapse. However, despite this initial negative effect, the food (particulate matter) provided by the currents soon leads to a large increase in recruitment and juveniles of this highly valuable species. This mitigates overexploitation, and results in plentiful landings of large, adult deep-sea shrimp between 3 and 5 years after major cascading events.

A decrease of winter deep water formation in the Gulf of Lions is expected to occur during the twenty-first century according to modelling results using the IPCC-A2 scenario which could obviously decrease the frequency and intensity of dense shelf water cascading events. Without this regenerative mechanism, fishery pressure could quickly deplete the stocks of Aristeus antennatus and other valuable deep-sea living resources in the area. If the predicted reduction of deep water formation in high latitudes as in the Nordic and Arctic regions (Gregory et al., 2006) would affect the frequency of dense shelf water cascading in the margins of the polar regions, the impacts on the biogeochemistry of the global ocean could be considerable.