Tropical Influences on North Pacific Sea Surface Temperatures

Source:  CO2 Science

Reference
Lienert, F., Fyfe, J.C. and Merryfield, W.J. 2011. Do climate models capture the tropical influences on North Pacific sea surface temperature variability? Journal of Climate 24: 6203-6209.

Background
The authors write that “climate models are increasingly being used to forecast future climate on time scales of seasons to decades,” and they say that “since the quality of such predictions of the future evolution of the PDO [Pacific Decadal Oscillation] likely depends on the models’ ability to represent observed PDO characteristics, it is important that the PDO in climate models be evaluated,” which they thus set out to do.

What was done
Working with observed monthly-mean SST (sea surface temperature) anomalies they obtained from the Met Office Hadley Centre Sea Ice and Sea Surface Temperature version-1 (Rayner et al., 2003) dataset for 1871-1999, as well as the extended reconstructed SST version-3b dataset (Smith et al., 2008), Lienert et al. assessed the ability of 13 atmosphere-ocean global climate models from the third phase of the Coupled Model Intercomparison Project (CMIP3) – which was conducted in support of the IPCC Fourth Assessment Report (Solomon et al., 2007) – to “reproduce the observed relationship between tropical Pacific forcing associated with ENSO [El Niño Southern Oscillation] and North Pacific SST variability associated with the PDO.”

What was learned
In the words of the three Canadian researchers, they found that (1) “the simulated response to ENSO forcing is generally delayed relative to the observed response,” a tendency that they say “is consistent with model biases toward deeper oceanic mixed layers and weaker air-sea feedbacks,” that (2) “the simulated amplitude of the ENSO-related signal in the North Pacific is overestimated by about 30%,” and that (3) “model power spectra of the PDO signal and its ENSO-forced component are redder than observed because of errors originating in the tropics and extratropics.”

What it means
What are the ramifications of these three important “strikes” against the climate models? There are, according to Lienert et al., three unfortunate implications. First, they say that “because the simulated North Pacific response lags ENSO unrealistically, seasonal forecasts may tend to exhibit insufficient North Pacific responses to developing El Niño and La Niña events in the first few forecast months.” Second, they indicate that “at longer forecast lead times, North Pacific SST anomalies driven by ENSO may tend to be overestimated in models having an overly strong ENSO, as the models drift away from observation-based initial conditions and this bias sets in.” And third, they note that “the relative preponderance of low-frequency variability in the models suggests that climate forecasts may overestimate decadal to multidecadal variability in the North Pacific.”

Therefore, in light of these three egregious fumbles, it is not unreasonable to suggest that the 13 global climate models that participated in the CMIP3 games should probably be “retired.”

References
Rayner, N.A., Parker, D.E., Horton, E.B., Folland, C.K., Alexander, L.V., Rowell, D.P., Kent, E.C. and Kaplan, A. 2003. Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. Journal of Geophysical Research 108: 10.1029/2002JD002670.

Smith, TM., 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.

Solomon, S., Qin, D., Manning, M., Marquis, M., Averyt, K., Tignor, M.B., Miller Jr., H.L. and Chen, Z. (Eds.). 2007.Climate Change 2007: The Physical Science Basis. Cambridge University Press, Cambridge, United Kingdom.

 

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