Background
The authors write that "a large fraction of the meridional transport of heat, momentum and moisture in mid-latitudes is carried by extratropical cyclones (Peixoto and Oort, 1992), which makes them fundamental in determining the equilibrium state of the climate system." And, therefore, it is very important that these particular cyclonic systems are modeled correctly.

What was done
Zappa et al. say they "inspected the ability of CMIP5 models to capture the observed behavior of the North Atlantic extratropical cyclones," which they did by evaluating "the number, intensity and spatial distribution of North Atlantic extratropical cyclones across a wide range of climate models" by comparing their simulations "against four recent reanalyses including ERA-Interim (1980-2009)."

What was learned
Unfortunately, the three UK researchers report that (1) "the majority of CMIP5 models still have a too-zonal storm track, so that too many North Atlantic cyclones propagate toward Europe and too few propagate toward the Norwegian Sea area," that (2) "a group of models also tends to place the storm track too far south in the central Atlantic," that (3) "some CMIP5 models tend to underestimate the total number of North Atlantic extratropical cyclones," and that (4) "CMIP5 models tend to underestimate the intensity of cyclones in both DJF [December, January and February] and JJA [June, July and August]." In addition, they note that "the tendency to place the North Atlantic storm track too far south is also found in the recent CMIP5 study by Chang et al. (2012)."

What it means
In the words of Zappa et al. in their paper's concluding paragraph, "the tendency of CMIP5 models to have weak cyclones and a too-zonal North Atlantic storm track in DJF is certainly a source of concern for interpreting their future projections."

References
Chang, E.K.M., Guo, Y. and Xia, X. 2012. CMIP5 multi-model ensemble projection of storm track change under global warming. Journal of Geophysical Research 117: 10.1029/2012JD018578.

Peixoto, J. and Oort, A. 1992. Physics of Climate. American Institute of Physics. College Park, Maryland, USA.