Background
Climate models are forever being tested and reworked with the goal of developing ever more accurate representations of how the real world's climate system operates, so as to be able to make ever more accurate projections of earth's climatic future.

What was done
In describing their latest foray into this important endeavor, in the words of the authors, "the seasonal prediction skill for the Northern Hemisphere winter is assessed using retrospective predictions (1982-2010) from the ECMWF System 4 (Sys4) and [the] National Center for Environmental Prediction (NCEP) CFS version 2 (CFSv2) coupled atmosphere-ocean seasonal climate prediction systems."

What was learned
Once again quoting Kim et al., they report that: (1)"for the Sys4, a cold bias is found across the equatorial Pacific," (2) "the CFSv2 has [a] strong warm bias from the cold tongue region of the Pacific to the equatorial central Pacific and [a] cold bias in broad areas of the North Pacific and the North Atlantic," (3) "a cold bias over large regions of the Southern Hemisphere is a common property of both reforecasts," (4) "with respect to precipitation, the Sys4 produced excesses along the Intertropical Convergence Zone, the equatorial Indian Ocean and the western Pacific," (5) "in the CFSv2, a strong wet bias is found along the South Pacific Convergence Zone and the southern Indian Ocean, as well as in the western Pacific," (6) "a dry bias is found for both modeling systems over South America and northern Australia and wet bias[es] in East Asia and the equatorial Atlantic," and (7) "both models have difficulty in forecasting the North Atlantic Oscillation and the year-to-year winter temperature variability over North America and northern Europe."

What it means
It would appear that with all the things climate models have been able to accomplish over the past several decades, even the best of them still have numerous significant deficiencies that have yet to be overcome.