Done this cruise last year and weather was great.
We have taken several Atlantic trips and had brilliant weather every time - in the 70's (except for one morning of rain off the Cape Verde islands).
However, our cruises were all on the southern Atlantic route, which it seems as if yours cruise will be too.
If your ship is docking at all in Boston, New York or any of the eastern seaboard states you could be taking the northern route - which is much much rougher, with not such good weather.
The key to packing for these cruises is layering. Thin layers which can be dispensed with as the need arises.
Incidentally we were cruising around Italy and Spain before a transatlantic to Miami in November/December and we were sunbathing in high 70's temperatures. Perhaps we were lucky, I don't know. Enjoy your trip.
We cruised in the Caribbean in January and the weather was perfect, beautiful sunny days, hope this is helpful to you, enjoy. J .
Sorry I cant answer your question, but I have booked on the same cruise, maybe I will see you in December 0.
We went on a cruise from southampton in november last year and done transatlantic crossing to barbados the weather was great that was the best part of the cruise it was so relaxing and not rocky at all
good luck and enjoy