Background
The authors write that North Pacific Decadal Variability (NPDV) "is a key component in predictability studies of both regional and global climate change," and they say that "two patterns of climate variability in the North Pacific generally characterize NPDV." These two "dominant modes," as they refer to them, are the Pacific Decadal Oscillation (PDO; Mantua et al., 1997) and the recently identified North Pacific Gyre Oscillation (NPGO; Di Lorenzo et al., 2008); and they emphasize that given the links between both the PDO and the NPGO with global climate, the accurate characterization and the degree of predictability of these two modes in coupled climate models is an important "open question in climate dynamics" that needs to be addressed.

What was done
Furtado et al. investigated this situation by comparing the output from the 24 coupled climate models used in the IPCC AR4 with observational analyses of sea level pressure (SLP) and sea surface temperature (SST), based on SLP data from the National Centers for Environmental Prediction (NCEP)-National Center for Atmospheric Research (NCAR) Reanalysis Project (Kistler et al., 2001), and SST data from the National Oceanic and Atmospheric Administration (NOAA) Extended Reconstruction SST dataset, version 3 (Smith et al., 2008), both of which datasets contain monthly mean values from 1950-2008 gridded onto a global 2.5° x 2.5° latitude-longitude grid for SLP and a 2° x 2° grid for SST.

What was learned
The four U.S. scientists report that model-derived "temporal and spatial statistics of the North Pacific Ocean modes exhibit significant discrepancies from observations in their twentieth-century climate, most visibly for the second mode, which has significantly more low-frequency power and higher variance than in observations." They also find that the two dominant modes of North Pacific oceanic variability "do not exhibit significant changes in their spatial and temporal characteristics under greenhouse warming," stating that "the ability of the models to capture the dynamics associated with the leading North Pacific oceanic modes, including their link to corresponding atmospheric forcing patterns and to tropical variability, is questionable."

But there are even more "issues with the models," in the words of Furtado et al., who report that "in contrast with observations, the atmospheric teleconnection excited by the El Niño-Southern Oscillation in the models does not project strongly on the AL [Aleutian low]-PDO coupled mode because of the displacement of the center of action of the AL in most models." In addition, they note that "most models fail to show the observational connection between El Niño Modoki-central Pacific warming and NPO [North Pacific Oscillation] variability in the North Pacific." In fact, they state that "the atmospheric teleconnections associated with El Niño Modoki in some models have a significant projection on, and excite the AL-PDO coupled mode instead."

What it means
Furtado et al. conclude that "for implications on future climate change, the coupled climate models show no consensus on projected future changes in frequency of either the first or second leading pattern of North Pacific SST anomalies," and they say that "the lack of a consensus in changes in either mode also affects confidence in projected changes in the overlying atmospheric circulation." In addition, they note that the lack of consensus they find "mirrors parallel findings in changes in ENSO behavior conducted by van Oldenborgh et al. (2005), Guilyardi (2006) and Merryfield (2006)," and they state that these significant issues "most certainly impact global climate change predictions." And, we would add, they impact them in a highly negative way.

References
Di Lorenzo, E., Schneider, N., Cobb, K.M., Franks, P.J.S., Chhak, K., Miller, A.J., McWilliams, J.C., Bograd, S.J., Arango, H., S.J., Curchitser, E., Powell, T.M. and Rivière, P. 2008. North Pacific Gyre Oscillation links ocean climate and ecosystem change. Geophysical Research Letters 35: 10.1029/2007GL032838.

Guilyardi, E. 2006. El Niño mean state-seasonal cycle interactions in a multi-model ensemble. Climate Dynamics 26: 329-348.

Kistler, R., Kalnay, E., Collins, W., Saha, S., White, G., Woollen, J., Chelliah, M., Ebisuzaki, W., Kanamitsu, M., Kousky, V., van den Dool, H., Jenne, R. and Fiorino M. 2001. The NCEP-NCAR 50-year reanalysis: Monthly means CD-ROM and documentation. Bulletin of the American Meteorological Society 82: 247-267.

Mantua, N.J., Hare, S.R., Zhang, Y., Wallace, J.M. and Francis, R. 1997. A Pacific interdecadal climate oscillation with impacts on salmon production. Bulletin of the American Meteorological Society 78: 1069-1079.

Merryfield, W.J. 2006. Changes to ENSO under CO2 doubling in a multimodel ensemble. Journal of Climate 19: 4009-4027.

Smith, T.M., Reynolds, R.W., Peterson, T.C. and Lawrimore, J. 2008. Improvements to NOAA's historical merged land-ocean surface temperature analysis (1880-2006). Journal of Climate 21: 2283-2296.

van Oldenborgh, G.J., Philip, S.Y. and Collins, M. 2005. El Niño in a changing climate: A multi-model study. Ocean Science 1: 81-95.