Dare i watch the GFS 06Z 10 years is too short to define a climate, you need at least 30 for it to be statistically meaningful. If you took the 2001-2010 average for December in the CET series, it only came out at 4.0C, nearly a degree lower then the 1971-2000 average.
Yet 2011-2020 had an average CET of 6.0C for December, 2 degrees warmer then the decade prior...
The longer term means, including the ones you have posted for Sweden are heading upwards...