The multivariate ENSO index (MEI) for NOV/DEC 2020 was -1.2. This value represents moderate intensity La Nina. The MEI is an excellent ENSO diagnostic as the index measures the reaction of the atmosphere to the cooling waters of the equatorial East Pacific. During the mature phase of La Nina, typically occurring in DEC/JAN for La Nina events since the middle 1990’s a mature phase La Nina MEI of <-1.0 was typically followed by sustained La Nina whereas MEI >-1.0 were more likely to produce La Nina dissipation and possibly El Nino later in the year.

The arctic oscillation was -1.7 in December and (traditionally) representative of blocking high pressure in the polar latitudes while a split in the polar vortex southward causes widespread mid-latitude chill. But! December 2020 was mild across most of North America and Europe and only Central Eurasia was (very) cold (about as far away from a maritime influence as possible).