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The Atlantic Meridional Overturning Circulation

Paper Reviewed
Srokosz, M.A. and Bryden, H.L. 2015. Review Summary. Observing the Atlantic Meridional Overturning Circulation yields a decade of inevitable surprises. Science 348: 10.1126/science.1255575.

In the Review Summary of their study, Srokosz and Bryden (2015) note that the climate-model-inspired Intergovernmental Panel on Climate Change report of 2001 suggested that the Atlantic Meridional Overturning Circulation (AMOC) could weaken over the 21st century; and this intriguing possibility led to the deployment of an ocean-current-observing system across the Atlantic Ocean at 26.5°N in the spring of 2004, which had achieved a full decade of AMOC measurements by the time of the two researchers' recent study of them. And what did the system's findings reveal about the AMOC? The two UK researchers report that they revealed a number of unexpected surprises.

Surprise No.1 was the fact that the range of AMOC variability found in the first year of its new and improved measurements -- 4 to 35 Sv (Sverdrups, each representing a million cubic meters per second, which is the standard unit for ocean circulation) -- was larger (by a factor of 3.875) than the 15 to 23 Sv range that had been determined previously from five sets of ship-based observations that had been conducted over a period of 50 years.

Surprise No. 2 was the fact that "the amplitude of the seasonal cycle, with a minimum in the spring and a maximum in the autumn, was much larger (~6.7 Sv) than anticipated."

Surprise No. 3 was the fact that "the driving mechanism of wind stress in the eastern Atlantic was unexpected."

Surprises No. 4 & 5 were the facts that [4] "the 30% decline in the AMOC during 2009-2010 was totally unexpected" and that [5] it "exceeded the range of interannual variability found in climate models used for the IPCC assessments."

Surprise No. 6 was the fact that "over the period of the 26.5°N observations, the AMOC has been declining at a rate of about 0.5 Sv per year," which Srokosz and Bryden say was fully "10 times as fast as predicted by climate models."

And thus we see that turn-of-the-century climate model projections of AMOC behavior have turned out to have been far different from what was actually observed over this period of much-improved AMOC measurements.

Posted 26 May 2016