There has been an overwhelming popular demand for us to weigh in on recent reports in the Times Britain faces big chill as ocean current slows and CNN Changes in Gulf Stream could chill Europe (note the interesting shift in geographical perspective!).
At the heart of the story was a statement at the recent EGU meeting by Peter Wadhams from Cambridge University, that convection in a normally active area of the Greenland Sea was much reduced last winter. Specifically, in an area where a dozen or so convective ‘chimneys’ form, only two small chimneys were seen. (Unfortunately, I can’t seem to be able to find a relevant abstract of Dr. Wadhams talk, and so I have to rely on the Times’ news reports for the specifics).
Convective chimneys in the seas bounded by Greenland, Iceland and Norway occur when intense cooling of the ocean, usually associated with a low-pressure system passing through, breaks down the normally stable ocean layers and causes the now colder, denser water to convect and mix down to a relatively deep layer. This area of the world is one of only a few places where the underlying ocean column is marginally stable enough that this process can occur in the open ocean and lead to convective chimneys going down 2000 to 3000 meters. The deep water masses formed in this way are then exported out of the area in deep currents that eventually make up “North Atlantic Deep Water” (which also contains contributions from the Labrador Sea and entrainment of other water masses). This process is part of what is called the ‘thermohaline’ or ‘overturning’ circulation and is associated with a significant amount of heat transport into the North Atlantic, which indeed keeps Britain and the rest of the North Atlantic region 3 to 6 degrees C warmer than they otherwise would be. The figure gives two model estimates for the impact of this circulation (Stocker, 2002).
This heat transport is often associated with the Gulf Stream in the media and among the public. However, my pedantic side obliges me to point out that the Gulf Stream is a predominantly wind-driven western boundary current that moves up from the Gulf of Mexico along the US coast to Cape Hatteras, at which point it heads off into the central Atlantic (see also this letter by Carl Wunsch). It then turns into the North Atlantic Drift which is really the flow of water responsible for the anomalous northward heat transport in the Atlantic. There is good evidence from past climates, theoretical studies and climate models that large changes, a slowing down or even a complete collapse, in the North Atlantic Drift and the thermohaline circulation can happen. Indeed climate models generally (though not exclusively) forecast a slowdown in this circulation by 2100. This occurs mainly as a function of increased rainfall in the region which strengthens the ocean layering and reduces the amount of convection in the region. It is probably futile to insist on it at this point, but a collapse of the overturning circulation is not the same as a collapse or reversal of the Gulf Stream (which as I mentioned above is predominantly wind-driven).
Getting back to the statement by Peter Wadhams though, how does this relatively small-scale observation get translated into headlines forecasting changes in the Gulf Stream and chilly times ahead for Europe? The major problem is that the background story and the climate model results are now very well known, and any scientific result that appears to project onto this storyline therefore gets a lot of attention.
However, it is a long way from the Greenland Sea to the Gulf Stream and some important points did not get a mention in the news stories. Firstly, we know that there is a great deal of decadal variability in how much and where deep convection takes place. Indeed, it was reported by Schlosser et al (1991), that based on CFC measurements, very little convection had occured in the Greenland Sea over the previous 7 years. Subsequently, convection was renewed. Similarly, convection in the Labrador Sea (the other main component) has also oscillated, possibly out of phase with the convection in Greenland. Studies by Dickson et al (1999, 2002) showed that properties of the deep water overflowing the Denmark Strait (between the Greenland Sea and rest of the Atlantic) appear to be related to patterns of variability like the North Atlantic Osillation, and this may help explain some of the variabilty.
To be sure, there are some long term trends that are becoming discernable. There is a freshening of the North Atlantic visible since the 1950s. Long continuous records of temperature and salinity at Ocean Weather Station M in the Norwegian Sea indicate that the deep water has also warmed noticeably. However, monitoring networks are now starting to be put in place (Osterhus et al, 2005) and better integrated data will be available in the future. It is important to bear in mind that while the changes being seen are indeed significant given the accuracy of modern oceanography, the magnitude of the changes (a few hundredths of a salinity unit) are very much smaller (maybe two orders of magnitude) than the kinds of changes inferred from the paleo data or seen in climate models. Thus while continued monitoring of this key climatic area is clearly warranted, the imminent chilling of the Europe is a ways off yet.