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Problems in CMIP5 Modeling of Atlantic Multi-Decadal Variability

Paper Reviewed
Peings, Y., Simpkins, G. and Magnusdottir, G. 2015. Multi-decadal fluctuations of the North Atlantic Ocean and feedback on the winter climate in CMIP5 control simulations. Journal of Geophysical Research: Atmospheres 121: 2571-2592.

In attempting to model Atlantic Multi-decadal Variability or AMV, Peings et al. (2015) encountered a number of problems in simulations derived from the fifth Coupled Model Intercomparison Project (CMIP5). And these problems, as they begin to describe them, were that (1) all but two of the 23 CMIP5 models studied "lack internally generated AMV," that (2) "no consistent feedback of the AMV onto the atmospheric circulation is found among the models," and that (3,4) "most of them exhibit less variance at multi-decadal time scales and less persistence in AMV than do observations."

Continuing to enumerate a number of other problems the CMIP5 models have in portraying Atlantic multi-decadal variability, the three U.S. scientists report that (5) "most of the models exhibit less energy than the observed AMV around the 70-year band," that (6) "most of the CMIP5 models that are analyzed in this study underestimate the amplitude of the AMV," that (7) "most of the models exhibit less multi-decadal variability than any observed AMV index in the 40-70 year range," that (8) "most of the simulated AMV time series exhibit less multi-decadal variability than in observations," that (9) "the amplitude of SST anomalies in the subtropics is considerably smaller than in observations" and that (10,11) most of the CMIP5 models "exhibit less variance at multi-decadal time scales and less persistence in AMV than do observations."

Moving on, Peings et al. additionally note that at decadal to multi-decadal time scales, most of the models simulate positive correlations between the AMV and heat flux anomalies; but they say that (12) "these are of lower amplitude than in observations," while further indicating that (13) "lack of long-term positive SST-heat flux correlations in the mid-North Atlantic limits the ability of the models to simulate a significant influence of the AMV on long-term fluctuations of the atmosphere."

On another sub-topic, the U.S. researchers report that (14) "the absence of consistent lagged correlations when the AMV leads the NAO [North Atlantic Oscillation] is at odds with results from observations," citing in this regard the studies of Omrani et al. (2014) and Peings and Magnusdottir (2014). And they thus conclude that (15) "the CMIP5 models are missing, or at best underestimating, the two-way interaction between the AMV and the NAO on multi-decadal time scales."

In conclusion and summation, Peings et al. thus state that (16) "the amplitude of SST anomalies [in models] is generally smaller than in observations, especially in the subtropical Atlantic," and that (17) "the potential feedback of the ocean that is identified in observations around 5 years after the peak of the AMV is not identified in the models," which fact they suggest is (18) "likely related to the underestimation of AMV amplitude in the models."

Also in this regard, they write that (19) "it is plausible that the state of the art ocean-atmosphere coupled models misrepresent some critical processes to capture this driving role of the ocean on long-term heat flux variability," as well as the fact that (20) "the models lack natural variability at multi-decadal time scales in the North Atlantic," and they further say that (21) "uncertainties in the proportion of internal versus forced AMV in observations prevent us from making firm conclusions."

And in light of these many unpleasant facts, Peings et al. conclude by stating that (22) "the current generation of coupled ocean-atmosphere models may underestimate the unforced AMV and [23] the associated impacts on the wintertime atmospheric circulation," while once again noting that (24) "the internal component of the AMV is too small in the CMIP5 models."

Omrani, N.E., Keenlyside, N.S., Bader, J.R. and Manzini, E. 2014. Stratosphere key for wintertime atmospheric response to warm Atlantic decadal conditions. Climate Dynamics 42: 649-663.

Peings, Y. and Magnusdottir, G. 2014. Forcing of the wintertime atmospheric circulation by the multi-decadal fluctuations of the North Atlantic Ocean. Environmental Research Letters 9: 10.1088/1748-9326/9/3/034018.

Posted 7 October 2016