Thanks to Major Robb Randall for alerting us to the paper by M. Susan Lozier, Susan Leadbetter, Richard G. Williams, Vassil Roussenov, Mark S. C. Reed, and Nathan J. Moore entitled “The Spatial Pattern and Mechanisms of Heat-Content Change in the North Atlantic” originally published in Science Express on 3 January 2008, Science 8 February 2008, Vol. 319., no. 5864, pp. 800 - 803, DOI: 10.1126/science.1146436
The abstract reads
“The total heat gained by the North Atlantic Ocean over the past 50 years is equivalent to a basinwide increase in the flux of heat across the ocean surface of 0.4 ± 0.05 watts per square meter. We show, however, that this basin has not warmed uniformly: Although the tropics and subtropics have warmed, the subpolar ocean has cooled. These regional differences require local surface heat flux changes (±4 watts per square meter) much larger than the basinwide average. Model investigations show that these regional differences can be explained by large-scale, decadal variability in wind and buoyancy forcing as measured by the North Atlantic Oscillation index. Whether the overall heat gain is due to anthropogenic warming is difficult to confirm because strong natural variability in this ocean basin is potentially masking such input at the present time.”
The authors conclude with the text,
“Lastly, the positive trend in the winter NAO index in the 1990s has been attributed to anthropogenic forcing (Hurrell 1995), implying that the NAO could be the route by which anthropogenic warming is imprinted on the ocean. However, although most climate models show a slight strengthening of the NAO index with anthropogenic forcing, the climate models also underestimate the strength of the recent decadal trend in the NAO, raising doubts as to the viability of the connection between the NAO and anthropogenic forcing in climate models (Gillett et al. 2003; Stephenson et al. 2006). Hence, although the change in ocean heat content over the North Atlantic can be connected to the decadal trend in the NAO, it is premature to conclusively attribute these regional patterns of heat gain to greenhouse warming. Continued long-term monitoring of North Atlantic temperatures is needed to answer the question of whether the basin-average warming is reflecting anthropogenic forcing and/or natural variability. “
This paper illustrates yet another shortcoming of the global climate models that are used to predict the climate system in the coming decades. They cannot accurately simulate the important climate feature of the North Atlantic Oscillation (the NAO). As the authors, themselves write “it is premature to conclusively attribute these regional patterns of heat gain to greenhouse warming.” This shortcoming of the multi-decadal global models applies to other low frequency climate variations, such as ENSO and the North Pacific Decadal Oscillation, which are major factors in the climate that we experience.