October 13, 2006

Overturning Ocean Hype

Filed under: Climate Changes, Gulf Stream

You may remember a major study regarding the greenhouse debate that surfaced last Christmas season. Harry Bryden and two associates at the UK’s National Oceanography Centre had analyzed five decades of data regarding the ocean circulation of the North Atlantic. They concluded in Nature magazine that “The comparison suggests that the Atlantic meridional overturning circulation has slowed by about 30 percent between 1957 and 2004.” The greenhouse crusade went wild, the media produced widespread coverage of Bryden’s findings, and the public was warned that the oceanic response to the build up of greenhouse gases could produce catastrophic results, particularly for European countries.

The story was straight out of “The Day After Tomorrow.” We were all told that the meridional circulation of the Atlantic carries warm upper waters into the mid-to-high latitudes and returns cold deep water southward across the Equator. We all learned about the “thermohaline circulation” that is a critical component in the energy balance of the earth-atmosphere system. The Bryden et al. findings could only heighten fears that human activities were having a profound impact on air-sea interactions, and if you recall, this could only lead to climate disasters – the entire story was straight out of a movie set.

We at World Climate Report were skeptical and questioned immediately why a 30 percent reduction in the thermohaline circulation had not produced noticeable cooling effects in Europe, after all, a complete shutdown of the circulation is expected to cause a cooling of 4°C in Europe, according to some computer models. We pointed out that the literature on ocean circulation contained evidence that the thermohaline circulation may be strengthening, exactly opposite what Bryden et al. claimed to have found.

Well, the issue has surfaced again with the recent publication of two important papers in Geophysical Research Letters. The first article is by Christopher Meinen and two associates at the Atlantic Oceanographic and Meteorological Laboratory in Miami. They measured the Meridional Overturning Circulation (MOC) from September 2004 through September 2005 using a line of inverted echo sounders, bottom pressure sensors, and a deep current meter east of Abaco Island, Bahamas, at 26.5° N. Their “picket fence” allowed them to measure the transport of water in millions of cubic meters per second (Sv) over that time period. Meinen et al. concluded in their last sentence “The 1-year-mean southward transport of 39 Sv is statistically indistinguishable from the 40 Sv estimate obtained at the same location by current meter mooring arrays in the late 1980s and early 1990s.” There was no evidence whatsoever of any 30 percent reduction in the strength of the thermohaline circulation.

German scientist Friedrich Schott along with three other countrymen also published a paper on this issue in Geophysical Research Letters. They measured the Deep Western Boundary Current east of the Grand Banks over the period 1999–2005 by moored current-meter stations and shipboard current profiling sections. They compared their observations with the data collected in the same area in 1993-1995. They conclude that “Although the water mass characteristics show interannual to decadal variations at
those locations,” “there is no sign of any MOC ‘slowdown’ trend over the past decade, contrary to some recent suggestions [Bryden et al., 2005].”

Here we see the greenhouse debate in a nutshell. Bryden claimed to have found a 30 percent slowdown in the thermohaline circulation, the results are published in the very prestigious Nature magazine, and the story was carried breathlessly by the media in outlets around the world. “The sky is falling” does make for an interesting story afterall, and there is no question that a 30 percent slowdown in the thermohaline circulation would be a major geophysical trend. Less than a year later, two different research teams present convincing evidence that no slowdown is occurring whatsoever. Of course, their research is published in Geophysical Research Letters with no media coverage at all.

Keep reading World Climate Report!

References

Bryden, H.L, H.R. Longworth, and S.A. Cunningham, 2005. Slowing of the Atlantic meridional overturning circulation at 25° N. Nature, 438, 655-657.

Meinen, C. S., M. O. Baringer, and S. L. Garzoli, 2006. Variability in Deep Western Boundary Current transports: Preliminary results from 26.5° N in the Atlantic. Geophysical Research Letters, 33, L17610, doi:10.1029/2006GL026965.

Schott, F. A., J. Fischer, M. Dengler, and R. Zantopp, 2006. Variability of the Deep Western Boundary Current east of the Grand Banks. Geophysical Research Letters, 33, L21S07, doi:10.1029/2006GL026563.




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