Background
The authors write that "for climate forecasts to have predictive value, the relevant air-sea interactions must be realistic in climate models." And they therefore state that "the aim of this study is to determine whether AMOC [Atlantic Meridional Overturning Circulation] variability in the (relatively) high-resolution T85 version of the Climate Community System Model, version 3 (CCSM3) (Collins et al., 2006), has a significant impact onto the large-scale atmospheric circulation and to evaluate the degree of realism of such air-sea interactions."

What was done
According to Frankignoul et al., the influence of AMOC variability on atmospheric circulation was "investigated in a control simulation of the CCSM3, where the AMOC evolves from an oscillatory regime into a red noise regime."

What was learned
In this situation, the three researchers report that (1) "an AMOC intensification is followed during winter by a positive North Atlantic Oscillation (NAO)," that (2) "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," that (3) "this alters the low-level atmospheric baroclinicity and shifts the maximum eddy growth northward, affecting the storm track and favoring a positive NAO," that (4) "the AMOC influence is detected in the relation between seasonal upper-ocean heat content or SST [sea surface temperature] anomalies and winter sea level pressure," that (5) "in the oscillatory regime, no direct AMOC influence is detected in winter," that (6) "an upper-ocean heat content anomaly resembling the AMOC footprint precedes a negative NAO," that (7) "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," and that (8) "as the mode has somewhat different patterns when using SST, the wintertime impact of the AMOC lacks robustness in this regime."

However, as they go on to report, "none of the signals compares well with the observed influence of North Atlantic SST anomalies on the NAO [bold and italics added for emphasis]."

What it means
As a result of these facts, Frankignoul et al. state, in the concluding sentence of their paper's abstract, that "although there is some potential climate predictability in CCSM3, it is not realistic [bold and italics added for emphasis]." And as they say in the second to the last sentence in the body of their paper, "although the AMOC influence on the atmosphere that we have documented for CCSM3 raises the hope that some low-frequency NAO variations might be predictable, in particular in the red noise regime, the signal will not be realistic [bold and italics added for emphasis]."

What can we say more? ... Much ado about nothing?

Reference
Collins, W.D., Bitz, C.M., Blackmon, M.L., Bonan, G.B., Bretherton, C.S., Carton, J.A., Chang, P., Doney, S.C., Hack, J.J., Henderson, T.B., Kiehl, J.T., Large, W.G., McKenna, D.S., Santer, B.D. and Smith, R.D. 2006. The Community Climate System Model Version 3 (CCSM3). Journal of Climate 19: 2122-2143.