Warming is not only confined to the surface in December, but also extends vertically aloft. There are numerous feedbacks which contribute to Arctic amplification (i.e., not just sea ice loss). Data from ECMWF ERA5. For more info: doi.org/10.1029/2020...

And here is an equivalent view for only the Southern Hemisphere...

Nice graphs. Is an explanation/mechanism at hand that you are aware of for the lack of december warming in the upper troposphere at 40-50N ? Strat-trop exchange?