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Global Warming and the Atlantic Meridional Overturning Circulation
Reference
Zhang, D., Msadek, R., McPhaden, M.J. and Delworth, T. 2011. Multidecadal variability of the North Brazil Current and its connection to the Atlantic meridional overturning circulation. Journal of Geophysical Research 116: 10.1029/2010JC006812.

Background
The authors report that "almost all state-of-the-art climate models project significant slowdown of the AMOC [Atlantic Meridional Overturning Circulation] during this century in response to the increased greenhouse gas concentrations in the atmosphere," citing the studies of Schmittner et al. (2005), Meehl et al. (2007) and Hu et al. (2009). And because the North Brazil Current (NBC) is, as they say, "primarily responsible for the AMOC upper branch return flow which crosses the tropical Atlantic (e.g., Hazeleger and Drijfhout, 2006)," they examined this projection via a new study of the NBC.

What was done
Working with historical hydrographic data they obtained from NOAA's World Ocean Database, Zhang et al. calculated the NBC geostrophic transport time series based on five decades of observations made off the coast of Brazil, while they also assessed the suggested connection between the NBC and AMOC via "a 700-year control simulation of the Geophysical Fluid Dynamics Laboratory's CM2.1 coupled climate model."

What was learned
The four researchers determined that the AMOC's "anticipated slowdown," as they describe it, "has not occurred yet, even though global temperatures have been significantly higher since the 1970s." They note, for example, that "while the AMOC might have been weakened from the 1960s to the early 1970s, it has been strengthening since then to the end of the last century," and they write that "analyses of subsurface temperature and salinity anomalies in the subtropical and subpolar north Atlantic (Zhang, 2008; Wang et al., 2010) also suggest a strengthening of the AMOC from the 1970s to 1990s." In addition, they report that their CM2.1 model results are also "in agreement with observations."

What it means
Could the findings of Zhang et al. mean that the projections of "almost all state-of-the-art climate models" are just plain wrong? They sure could, for real-world observations always win out over theoretical projections if they differ; and so far, at least, that's what the observations are doing -- they're winning.

References
Hazeleger, W. and Drijfhout, S. 2006. Subtropical cells and meridional overturning circulation pathways in the tropical Atlantic. Journal of Geophysical Research 111: 10.1029/2005JC002942.

Hu, A., Meehl, G.A., Han, W. and Yin, J. 2009. Transient response of the MOC and climate to potential melting of the Greenland Ice Sheet in the 21st century. Geophysical Research Letters 36: 10.1029/2009GL037998.

Meehl, G.A., Stocker, T.F., Collins, W.D., Friedlingstein, P., Gaye, A.T., Gregory, J.M., Kitoh, A., Knutti, R., Murphy, J.M., Noda, A., Raper, S.C.B., Watterson, I.G., Weaver, A.J. and Zhao, Z.C. 2007. Global Climate Projections. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. and Miller, H.L. (Eds.) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, New York, USA.

Schmittner, A., Latif, M. and Schneider, B. 2005. Model projections of the North Atlantic thermohaline circulation for the 21st century assessed by observations. Geophysical Research Letters 32: 10.1029/2005GL024368.

Wang, C., Dong, S. and Munoz, E. 2010. Seawater density variations in the North Atlantic and the Atlantic meridional overturning circulation. Climate Dynamics 34: 953-968.

Zhang, R. 2008. Coherent surface-subsurface fingerprint of the Atlantic meridional overturning circulation. Geophysical Research Letters 35: 10.1029/2008GL035463.

Reviewed 29 June 2011