Northern hemisphere winter atmospheric climate: modes of natural variability and climate change

• Published in: Climate dynamics Vol. 31; no. 2-3; pp. 195 - 211
• Main Authors: Keeley, Sarah P. E, Collins, Matthew, Thorpe, Alan J
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Berlin/Heidelberg : Springer-Verlag 01.08.2008; Springer-Verlag; Springer; Springer Nature B.V
• Subjects: Anthropogenic factors; Atmosphere; Carbon dioxide; Climate change; Climate system; Climatology; Climatology. Bioclimatology. Climate change; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Exact sciences and technology; External geophysics; Geophysics/Geodesy; Marine; Meteorology; Ocean-atmosphere interaction; Oceanography; Physics of the oceans; Sea-air exchange processes; Surface temperature; Temperature; Variability; Winter; Climate change; Northern Hemisphere Winter; Climate variability; Geopotential height; Empirical orthogonal function; Surface temperature; climate variability; digital simulation; ocean-atmosphere interaction; Dynamical climatology; Ocean-atmosphere model; Spatial distribution; Atmospheric temperature; Winter; Northern Hemisphere; global change; climate change
• ISSN: 0930-7575; 1432-0894
• DOI: 10.1007/s00382-007-0346-6

Under anthropogenic climate change it is possible that the increased radiative forcing and associated changes in mean climate may affect the “dynamical equilibrium” of the climate system; leading to a change in the relative dominance of different modes of natural variability, the characteristics of their patterns or their behavior in the time domain. Here we use multi-century integrations of version three of the Hadley Centre atmosphere model coupled to a mixed layer ocean to examine potential changes in atmosphere-surface ocean modes of variability. After first evaluating the simulated modes of Northern Hemisphere winter surface temperature and geopotential height against observations, we examine their behavior under an idealized equilibrium doubling of atmospheric CO₂. We find no significant changes in the order of dominance, the spatial patterns or the associated time series of the modes. Having established that the dynamic equilibrium is preserved in the model on doubling of CO₂, we go on to examine the temperature pattern of mean climate change in terms of the modes of variability; the motivation being that the pattern of change might be explicable in terms of changes in the amount of time the system resides in a particular mode. In addition, if the two are closely related, we might be able to assess the relative credibility of different spatial patterns of climate change from different models (or model versions) by assessing their representation of variability. Significant shifts do appear to occur in the mean position of residence when examining a truncated set of the leading order modes. However, on examining the complete spectrum of modes, it is found that the mean climate change pattern is close to orthogonal to all of the modes and the large shifts are a manifestation of this orthogonality. The results suggest that care should be exercised in using a truncated set of variability EOFs to evaluate climate change signals.