One of the most controversial issues emerging from the recent Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) is the failure of global climate models to predict a hiatus in warming of global surface temperatures since 1998. Several ideas have been put forward to explain this hiatus, including what the IPCC refers to as ‘unpredictable climate variability’ that is associated with large-scale circulation regimes in the atmosphere and ocean. The most familiar of these regimes is El Niño/La Niña, which are parts of an oscillation in the ocean-atmosphere system. On longer multi-decadal time scales, there is a network of atmospheric and oceanic circulation regimes, including the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation.
A new paper published in a recent online edition of the journal Climate Dynamics suggests that this ‘unpredictable climate variability’ behaves in a more predictable way than previously assumed. The paper’s authors, Marcia Wyatt and Judith Curry, point to the so-called ‘stadium-wave’ signal that propagates like the cheer at sporting events whereby sections of sports fans seated in a stadium stand and sit as a ‘wave’ propagates through the audience. In like manner, the ‘stadium wave’ climate signal propagates across the Northern Hemisphere through a network of ocean, ice, and atmospheric circulation regimes that self-organize into a collective tempo.
Judith “Judy” Curry is a professor and the chair of the School of Earth and Atmospheric Sciences (EAS) in the College of Sciences (CoS). Credit: Rob Felt.
The stadium wave hypothesis provides a plausible explanation for the hiatus in warming and helps explain why climate models did not predict this hiatus. Further, the new hypothesis suggests how long the hiatus might last.
Building upon Wyatt’s Ph.D. thesis at the University of Colorado, Wyatt and Curry identified two key ingredients to the propagation and maintenance of this stadium wave signal: the Atlantic Multidecadal Oscillation (AMO) and sea ice extent in the Eurasian Arctic shelf seas. The AMO sets the signal’s tempo, while the sea ice bridges communication between ocean and atmosphere. The oscillatory nature of the signal can be thought of in terms of ‘braking,’ in which positive and negative feedbacks interact to support reversals of the circulation regimes. As a result, climate regimes — multiple-decade intervals of warming or cooling — evolve in a spatially and temporally ordered manner. While not strictly periodic in occurrence, their repetition is regular — the order of quasi-oscillatory events remains consistent. Wyatt’s thesis found that the stadium wave signal has existed for at least 300 years.
The new study analyzed indices derived from atmospheric, oceanic and sea ice data since 1900. The linear trend was removed from all indices to focus only the multi-decadal component of natural variability. A multivariate statistical technique called Multi-channel Singular Spectrum Analysis (MSSA) was used to identify patterns of variability shared by all indices analyzed, which characterizes the ‘stadium wave.’ The removal of the long-term trend from the data effectively removes the response from long term climate forcing such as anthropogenic greenhouse gases.
The stadium wave periodically enhances or dampens the trend of long-term rising temperatures, which may explain the recent hiatus in rising global surface temperatures.
“The stadium wave signal predicts that the current pause in global warming could extend into the 2030s,” said Wyatt, an independent scientist after having earned her Ph.D. from the University of Colorado in 2012. READ THE REST.