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New Mann Paper Pins Pause Partly On Atlantic Variability

02.05.2014
02.05.2014 11:35 Age: 4 yrs

Click to enlarge. Simulated vs. Observed NH mean temperatures. (a) Instrumental annual [HadCRUT4—solid black, GISTEMP—dashed black) NH mean temperatures (AD 1850-2012) along with model-based estimates of forced component using EBM (blue), CMIP5-GISS (green) and CMIP5-Full (cyan). Courtesy of Geophysical Research Letters, American Geophysical Union and the authors.

Click to enlarge. EBM simulated series (blue) along with an ensemble of five differerent realisations of the estimated internal variability contribution to global warming. Courtesy of Geophysical Research Letters, American Geophysical Union and the authors.

Click to enlarge. See caption below.

Click to enlarge. See caption below.

Click to enlarge. The above three graphs show time series of estimated unforced NH mean variability and associated multidecadal “AMO” components (smooth curves) based on Differenced-AMO (gray) vs Detrended-AMO (Black) approaches applied to the observed NH mean record. Top is EBM simulation. Middle is CMIP5-GISS model. Bottom is CMIP5-Full model. The blue lines are the Detrended AMMO approach applied to the simulated forced series. Courtesy of Geophysical Research Letters, American Geophysical Union and the authors.

 

The recent slowdown in the warming rate of the Northern Hemisphere may be a result of internal variability of the Atlantic Multidecadal Oscillation a natural phenomenon related to sea surface temperatures, according to a new paper from Michael Mann and others just published in Geophysical Research Letters.

Mann, from Pennsylvania State University in the US, explained the research in a recent news release which we reported here. "Some researchers have in the past attributed a portion of Northern Hemispheric warming to a warm phase of the AMO," he said. "The true AMO signal, instead, appears likely to have been in a cooling phase in recent decades, offsetting some of the anthropogenic warming temporarily."

Problem

According to Mann, the problem with the earlier estimates of the influence of the AMO stems from having defined the AMO as the low frequency component that is left after statistically accounting for the long-term temperature trends, referred to as detrending. Initial investigations into the multidecadal climate oscillation in the North Atlantic were hampered by the short length of the instrumental climate record which was only about a century long.

Mann and his colleagues took a different approach in defining the AMO. They compared observed temperature variation with a variety of historic model simulations to create a model for internal variability of the AMO that minimizes the influence of external forcing including greenhouse gases and aerosols. They call this the differenced-AMO because the internal variability comes from the difference between observations and the models' estimates of the forced component of North Atlantic temperature change.  They found that their results for the most recent decade fall within expected multidecadal variability.

They also constructed plausible synthetic Northern Hemispheric mean temperature histories against which to test the differenced-AMO approaches.  Because the researchers know the true AMO signal for their synthetic data from the beginning, they could demonstrate that the differenced-AMO approach yielded the correct signal.  They also tested the detrended-AMO approach and found that it did not come up with the known internal variability.

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Conclusion

Abstract

Citation

On forced temperature changes, internal variability, and the AMO by Michael E. Mann, Byron A. Steinman and Sonya K. Miller. Article first published online: 1 MAY 2014 in Geophysical Research Letters. DOI: 10.1002/2014GL059233

Read the abstract and get the paper here

Source

Quotes from the paper and images courtesy of Geophysical Research Letters, American Geophysical Union and the authors. An earlier report from Penn State on this research which we published here based on a news release from Penn State here.