The UKCIP has released an update on its assessment of the Atlantic Meridional Overturning Circulation, often referred to as the THC, based on recent findings. The summary is available here.
Based on the most recent work from the RAPID programme, (Cunningham et. al., Science, August 2007), which was the source of the data presented in the Bryden et. al. paper of 2005 which suggested a 30% slowdown in modern times, the first year’s data from the moorings it has placed confirm the idea that the Bryden results were an anomaly based on short-term variability; in other words, the AMOC appears to have a ‘pulse’, rather than being a ‘smoothly flowing stream’.
The press release also cites Jungclaus et. al. (2006) for North Atlantic freshening from glacial outflow estimates, and bases its conclusions on the outputs of model runs included in the AR4 and completed since, with the newer data in place. These are the current conclusions:
What do these findings mean?
The RAPID observational programme has not been running long enough to
provide conclusive results in terms of understanding and predicting the behaviour
of the AMOC. Some points to note in relation to these preliminary observations
and related predictions are below:
• It is now known that the AMOC is quite variable from day-to-day. It is possible
that the apparent 30% slow down reported by Bryden et al., 2005, is an
artefact of this variability: further statistical analysis and data are needed to
investigate this. It is not known how variable the AMOC is from year-to-year.
• A 30% slow down in the AMOC could be expected to result in a cooling of 1°C
in the British Isles and 2°C in Scandinavia. Such a cooling has not been
recorded in the Central England Temperature record over recent years.
• These results do not, in themselves, change the overall projection of the UK
climate throughout the 21st century. The UKCIP02 scenarios are based on the
Hadley Centre climate model that includes a dynamic AMOC which weakens
over the 21st century and in which warming due to increased greenhouse gas
concentration exceeds any AMOC-related cooling.
• Models developed within the RAPID programme and all other comprehensive
climate models (including those used by the IPCC) suggest a shutdown of the
AMOC within the 21st century is very unlikely (< 10% possibility, IPCC, 2007).
So, no imminent ice age for the British Isles. Shame: I always liked those cold, snowy days of Winter…