The Remarkably Strong Arctic Stratospheric Polar Vortex of Winter 2020: Links to Record‐Breaking Arctic Oscillation and Ozone Loss

Abstract

The Northern Hemisphere (NH) polar winter stratosphere of 2019/2020 featured an exceptionally strong and cold stratospheric polar vortex. Wave activity from the troposphere during December-February was unusually low, which allowed the polar vortex to remain relatively undisturbed. Several transient wave pulses nonetheless served to help create a reflective configuration of the stratospheric circulation by disturbing the vortex in the upper stratosphere. Subsequently, multiple downward wave coupling events took place, which aided in dynamically cooling and strengthening the polar vortex. The persistent strength of the stratospheric polar vortex was accompanied by an unprecedentedly positive phase of the Arctic Oscillation in the troposphere during January-March, which was consistent with large portions of observed surface temperature and precipitation anomalies during the season. Similarly, conditions within the strong polar vortex were ripe for allowing substantial ozone loss: The undisturbed vortex was a strong transport barrier, and temperatures were low enough to form polar stratospheric clouds for over 4 months into late March. Total column ozone amounts in the NH polar cap decreased and were the lowest ever observed in the February-April period. The unique confluence of conditions and multiple broken records makes the 2019/2020 winter and early spring a particularly extreme example of two-way coupling between the troposphere and stratosphere. Plain Language Summary Wintertime westerly winds in the polar stratosphere (from ∼15-50 km), known as the stratospheric polar vortex, were extraordinarily strong during the Northern Hemisphere winter of 2019/2020. The exceptional strength of the stratospheric polar vortex had consequences for winter and early spring weather near the surface and for stratospheric ozone depletion. Typically atmospheric waves generated in the troposphere spread outward and upward into the stratosphere where they can disturb and weaken the polar vortex, but tropospheric wave activity was unusually weak during the 2019/2020 winter. In addition, an unusual configuration of the stratospheric polar vortex developed that reflected waves traveling upward from the troposphere back downward. These unique conditions allowed the vortex to remain strong and cold for several months. During January-March 2020, the strong stratospheric polar vortex was closely linked to a near-surface circulation pattern that resembles the positive phase of the so-called "Arctic Oscillation" (AO). This positive AO pattern was also of record strength and influenced the regional distributions of temperatures and precipitation during the late winter and early spring. Cold and stable conditions within the polar vortex also allowed strong ozone depletion to take place, leading to lower ozone levels than ever before seen above the Arctic in spring.

Seasonal Forecasts of the Exceptional Northern Hemisphere Winter of 2020

“…These results are consistent with Lawrence et al. (2020), who found that low vertically propagating tropospheric wave activity was present during the winter.…”

“…Similar, albeit weaker, results are found when using Z500 in December and JFM SPV forecasts (not shown). These results are consistent with Lawrence et al (2020), who found that low vertically propagating tropospheric wave activity was present during the winter.…”

Record‐Low Arctic Stratospheric Ozone in 2020: MLS Observations of Chemical Processes and Comparisons With Previous Extreme Winters

“…Cumulative ozone loss did not match or surpass that in 2011 only because a major final warming in early March 2016 halted chemical processing and dispersed processed air from the vortex (Johansson et al., 2019; Manney & Lawrence, 2016). In 2019/2020, lower stratospheric temperatures were persistently below the threshold for chemical processing earlier than in any other year observed by MLS and remained low approximately as late as in 2011 (Lawrence et al, 2020, describe stratospheric vortex meteorology in 2019/2020).…”

Stratospheric drivers of extreme events at the Earth’s surface

“…A record-strong NH polar vortex in early 2020 was associated with a series of successive storms that hit the UK and Northern Europe, and caused extensive damage 33 (Fig. 3) and unprecedented warmth over Eurasia 34 . Wave reflection events can contribute to cold air outbreaks over central Canada and North America 35,36 , such as, e.g., in December 2017 37 (Fig.…”

“…This transition is associated with an NAO-like shift in surface climate 38 and may influence Arctic sea ice conditions into autumn 39 . The timing of the final vortex breakdown can have significant implications for stratospheric ozone chemistry, as a vortex that stays strong well into spring as sunlight returns to the pole provides ideal conditions for rapid ozone loss, as observed in spring 2020 34 . Stratospheric ozone minima in spring have been suggested to be followed by anomalous cold over subtropical Asia and southern Europe, and anomalous warmth over northern Asia 40,41 .…”