Tuesday, August 6, 2013

New paper finds the world's most commonly used climate model produces predictions "not realistic"

A paper published today in the Journal of Climate finds that the world's most commonly used climate model [CCSM3] is unable to reproduce the observed sea surface temperature changes influencing two of the most important natural oscillations, the North Atlantic Oscillation [NAO] and Atlantic meridional overturning circulation [AMOC], which in turn influence global climate change. The authors conclude, "Hence, although there is some potential climate predictability in CCSM3, it is not realistic.

Note: CCSM3, and it's even worse successor CCSM5, are the most commonly used climate models of the IPCC. GIGO

The influence of the AMOC variability on the atmosphere in CCSM3

Claude Frankignoul and Guillaume Gastineau
LOCEAN, University Pierre and Marie Curie, Paris, France
Young-Oh Kwon
Woods Hole Oceanographic Institution, USA

The influence of the Atlantic meridional overturning circulation (AMOC) variability on the atmospheric circulation is investigated in a control simulation of the NCAR Community Climate System Model 3, where the AMOC evolves from an oscillatory regime into a red-noise regime. In the latter, an AMOC intensification is followed during winter by a positive NAO. The atmospheric response is robust and controlled by AMOC-driven SST anomalies, which shift the heat release to the atmosphere northward near the Gulf Stream/North Atlantic Current. This alters the low-level atmospheric baroclinicity and shifts the maximum eddy growth northward, affecting the storm track and favoring a positive NAO. The AMOC influence is detected in the relation between seasonal upper ocean heat content or SST anomalies and winter sea level pressure. In the oscillatory regime, no direct AMOC influence is detected in winter. However, an upper ocean heat content anomaly resembling the AMOC footprint precedes a negative NAO. This opposite NAO polarity seems due to the southward shift of the Gulf Stream during AMOC intensification, displacing the maximum baroclinicity southward near the jet exit. As the mode has somewhat different patterns when using SST, the wintertime impact of the AMOC lacks robustness in this regime. However, none of the [climate model] signals compares well with the observed influence of North Atlantic SST [Sea Surface Temperature] anomalies on the NAO because SST is dominated in CCSM3 by the meridional shifts of the Gulf Stream/North Atlantic Current that co-vary with the AMOC. Hence, although there is some potential climate predictability in CCSM3, it is not realistic.

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