Atlantic and Pacific oscillations lost in the noise

If the Atlantic Multidecadal or Pacific Decadal oscillations existed, there would be evidence for their existence across the suite of current state-of-the-art climate model simulations.

Recently, meteorologists report that the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO) do not appear to exist. The discovery could have implications for both the validity of previous studies attributing past trends to these hypothetical natural oscillations and for the prospects of decade-scale climate predictability.

The discovery is based on observational data and climate model simulations, that shows there was no reliable proof for decadal or longer-term internal oscillatory signals that could be separated from climatic noise— arbitrary year to year variation. The apparent main swaying is the well-known El Niño/Southern Oscillation (ENSO).

Scientists noted, “A distinct — 40 to 50-year timescale — a spectral peak that appears in global surface temperature observations appears to reflect the response of the climate system to a combination of anthropogenic and natural forcing rather than any intrinsic internal oscillation.”

“if the Atlantic Multidecadal or Pacific Decadal oscillations existed, there would be evidence for their existence across the suite of current state-of-the-art climate model simulations.”

Michael E. Mann, distinguished professor of atmospheric science at Penn State, said, “Given the current sophistication of climate models as seen in their ability to capture the El Niño/Southern Oscillation, we would expect to see consistent evidence for oscillations across a suite of climate models. We found no such evidence.”

Scientists used the MTM-SVD method to take a gander at observational and long term “control” simulation generated global surface temperature data. The observational record returns over 150 years. The control simulations, which have no outside drivers applied to the models, are from the latest worldwide atmosphere model intercomparison projects (CMIPS).

Mann said, “We found a tendency in the control models for oscillations in the three to seven-year ENSO band. However, we found no other signals, no Pacific or Atlantic climate variability on decadal or longer timescales that could be characterized as a true oscillation. Such variability was essentially indistinguishable from random noise.”

Utilizing the ‘forced’ suite of CMIPS simulations where the climate models are driven with external factors, for example, volcanoes and human increase in pollution, the specialists demonstrated that the evident 40 to 50-year spectral peak once in a while connected with the AMO is an artifact of the slowdown in warming from the 1950s to 1970s. 

This warming was because of the development of sulfur “aerosol” pollutants that cool the Earth’s surface. The passage of the Clean Air Acts during the 1970s evacuated the cooling impact, and greenhouse gas warming increasingly dominated. The slowdown and subsequent acceleration of warming masquerade as an apparent “oscillation.”

Study co-author Byron A. Steinman, associate professor of earth and environmental science at the University of Minnesota Duluth said“Our study provides another line of evidence that purported decadal and longer timescale internal oscillation in climate that has been identified through analysis of observational data is mostly a result of external influences like greenhouse gas and aerosol emissions by humans.”

The paper is presented in the journal Nature Communications.

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