Rationale

Justification for indicator selection

Relevance
Sea surface temperature (SST) is closely linked to one of the strongest drivers of climate in western Europe: the ocean circulation known as the Atlantic Meridional Overturning Circulation (MOC). This circulation (also known as the great conveyor belt) carries warm upper waters north in the Gulf Stream and returns cold deep waters south. It is widely accepted that the MOC is an important driver of low frequency variations in sea surface temperature on the time scale of several decades (Griffies et al., 1997). It is also widely accepted that the NAO index (a proxy of atmospheric circulation variability) plays a key role in forcing variations in MOC as well as the northward extent of the Gulf Stream (Frankignoul and Kestenare, 2005; De Coetlogon et al., 2006). At present, changes in sea surface temperatures of the global ocean and the regional seas of Europe are consistent with the changes in atmospheric temperature (Levitus et al., 2000; Rayner et al., 2006).
The sensitivity of the MOC to greenhouse warming, however, remains a subject of much scientific debate. Observations indicate that there has indeed been a freshening of the North Atlantic since 1965 due to increased freshwater inputs from rivers, precipitation and melting glaciers (Curry and Mauritzen, 2005), and thus possibly a weakening of the Atlantic MOC. The freshening calculated by these authors occurred mainly before 1970 and does not yet appear to have substantially altered the MOC and its northward heat transport. Uncertainties regarding the rates of future climate warming and glacial melting limit the predictability of the impact on ocean circulation, but do not exclude the possibility of a weakening of the MOC. Recent observations, have, however, shown that the variability of the MOC is large. The yearlong average MOC is 18.7 ± 5.6 Sverdrup (Note: Sverdrup = 10⁶ m³s^-1) but with large variability ranging from 4.4 to 35.3 Sverdrup (Cunningham et al., 2007). A recent study has shown that the variability of the MOC may be predictable on decadal time scales, and the study predicts that North Atlantic and European sea surface temperatures will fall slightly in the next decade as natural climate variability off-sets the projected anthropogenic warming (Keenlyside et al., 2008). The plausibility of the Keenlyside et al., 2008 projections are, however, also subject to intense debate in the scientific community (see e.g. http://www.realclimate.org).
One of the most visible ramifications of increased temperature of the ocean is the reduced area of sea ice coverage in the Arctic polar region and there is an accumulating body of evidence suggesting that many marine ecosystems are responding both physically and biologically to changes in regional climate caused predominantly by the warming of air and SST, as shown in the following sections.

Scientific references

• No rationale references available