One of the four strongest -AO on record this winter but a different thermal climate from the analog precedent
Fig. 1: The meteorological winter 2020-21 monthly arctic oscillation is among the 4 strongest negative (years) on record.
Introduction: The arctic oscillation (AO) is a measurement of the northern hemisphere circulation pattern centered at the pole and extending south to 20N. The AO is a leading mode of climate variability in the northern hemisphere. A negative arctic oscillation (-AO) is present when anomalous high pressure is dominant in the polar region causing the polar vortex to split into 3 or 4 trough(s) which are displaced southward toward the middle latitudes. -AO patterns are famous for producing cold and snowy climate patterns in the middle latitudes of the northern hemisphere during the winter season. The AO is most dominant during the cold season. The purpose of this article is to identify past -AO patterns present each month of meteorological winter (DEC/JAN/FEB) at moderate-to-strong intensity. The meteorological winter 2020-21 has observed a persistent strong -AO through February forecast to continue in February. If so, the winter 2020-21 -AO pattern will be matched only by 3 past winter seasons (since 1950): 1969-70, 1976-77 and 2009-10.
Data: NOAA/CPC data is used to calculate the analog years, estimate January -AO and project the February -AO. NCDC/PSD provides the analog year climate maps.
Temperature anomalies for the 3 analog winter seasons: Each of the three previous sharp negative AO index for all three months of meteorological winter produced exactly as one would expect: A harsh cold winter climate (Fig. 2-4). The most extreme cold was observed during winter 1976-77 in the Ohio Valley. However, each (analog) winter produced a region of extreme cold. Only winter 1969-70 (Interior West) and 2009-10 (Maine) produced a region of anomalous warmth.
Fig. 2-4: Meteorological winter (DEC/JAN/FEB) temperature anomalies for 1969-70, 1976-77 and 2009-10 when moderate-to-strong -AO was present for each month.
So far, winter 2020-21 is super-warm: Despite the vigorous -AO pattern for winter 2020-21 (so far) the prevailing U.S. temperature anomaly pattern is opposite of the historical strong -AO cold regime (Fig. 5-6). Cold forecasts are indicated for early February but impact and duration on the U.S. is uncertain and likely to leave winter 2020-21 as warmer than normal for most of the U.S. despite the vigorous and persistent -AO.
Fig. 5-6: The preliminary temperature anomalies across the U.S. for December 2020 and January 2021.
Proposed explanation for the winter 2020-21 warmth: A likely contribution to the warmer climate for winter 2020-21 (so far) compared to the bold cold all winter long for the 3 analog years is a more constricted polar ice cap (Fig. 7) leaving more open water in the northern latitudes that slightly suppresses cold air generation and a much warmer than normal middle latitude northern hemisphere ocean surface (Fig. 8) signature increasing the ability of maritime influences to warm the climate via air mass transport from the prevailing mid-latitude westerly flow.
February cold potential: The constricted polar ice cap – most evident in the northern Atlantic and therefore greatest (warm climate) influence on North America and Europe has just the opposite effect on Russia. The ice cap reaches the north coast of Russia early enough in the season and given the much lower probability of maritime warming (in Russia) due to warmer than normal mid-latitude oceans early season cold and snow can more easily develop. Cold air settles over the deep snow cover and can shift cross-polar later in winter if the right air mass trajectory pattern occurs. Therefore, the late winter season cold risk to North America in the modern-day climate is partly attributed to the early winter piling snow and evolving cold bias in Siberia.
Fig. 7: The early 2021 polar ice cap is tied with 2012 for lowest on record.
Fig. 8: The mid-latitude northern hemisphere is unusually warm for the time of season.
Conclusion: Meteorological winter 2020-21 (so far) is very warm across the U.S. despite presence of a moderate-to-strong negative arctic oscillation also projected to last through February. During 3 analog years when -AO was similarly intense the Central/East U.S. experienced historic cold. The explanation for the warmer winter across the U.S. is related in-part to a constricted polar ice cap and warmer mid-latitude ocean surface. These conditions lead to piling snows and gathering cold during early winter in Russia. If a cross-polar flow can emerge during late winter the arctic air travels cross-polar (“Siberia Express”) and into North America. This pattern is possible in February.