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Two questions still remain: 1) what are the main tropospheric factors leading to the stronger wavenumber-1 wave-driven IMC and thus the related surface temperature patterns (i.e., cold midlatitude East Asia and warm midlatitude North America) from mid-December to early-January? 2) Besides the downward impact from the SSW, what are the roles of tropospheric factors in contributing to the IMC and its related extreme cold event over North America in February? To tentatively address the remaining questions, we conducted a composite analysis to investigate the historical features of the IMC during winters characterized by boundary forcing mechanisms including La Niña, warm SSTs over the North Pacific and the North Atlantic, low sea ice extent, and the occurrence of displacement-type SSW events, which are the main features found in the 2020/21 winter. Note that with the long-term trend removed, the unique anomalies of the Niño-3.4 index, SSTs over the North Pacific and the North Atlantic, and low sea ice extent shown in Fig. 1 can still be seen. Nevertheless, since the occurrence of extreme cold events may be related to global warming (Dai et al., 2021; Zhang et al., 2021) and many of the defining characteristics of this winter are based on the “history records” (the climatology) without removing the global warming trend, the composite analysis in this section is based on the raw indices without detrending. The years, months, and days to be used in the composites are listed in Table 1. Figures 5d–r illustrate the longitudinal profile of the monthly mean anomalies of meridional mass transport and Plum wave-activity fluxes at 60°N at various isentropic layers composited for winters with various forcing.
Winters Preceded by Low September SIE La Niña Winters Warm Northwestern Pacific Months Warm Northeastern Atlantic Months Central Dates of Displacement-type SSW DEC JAN FEB DEC JAN FEB 2007/08 2008/09
2010/11 2011/12
2012/13 2015/16
2016/17 2017/18
2018/19 2019/20
2020/211988/89
1995/96
1998/99
1999/2000
2007/08
2010/11
2011/12
2020/212014
2015
2018
2019
20202005
2014
2015
2016
2018
2019
2020
20212005
2014
2015
2016
2018
2019
20202014
2015
2016
2017
2019
20202007
2014
2015
2016
2017
2020
20212014
2015
2016
2017
2018
2019
2020
20211980/03/01
1981/03/04
1981/12/04
1984/02/24
1987/01/23
1998/12/15
2000/03/20
2001/12/30
2002/02/17
2004/01/05
2006/01/21
2007/02/24
2008/02/22
2019/01/01
2021/01/04Table 1. The winters, months, and days used for composite analysis in Fig. 5.
It is seen from Figs. 5a and 5b that during the period from December 2020 to January 2021, the equatorward air transport was anomalously strong via the entire longitudinal span of East Asia and is responsible for the extreme cold conditions there in the early winter months. Such strengthened equatorward air transport could be mainly contributed from the low autumn Arctic sea ice extent, as indicated by the significant negative meridional mass flux anomalies across 60°N in 60°–120°E (Fig. 5m). One way of understanding the effect of Arctic sea ice on the enhanced cold air transport route is via the formation of Ural blocking during the first extreme cold event over East Asia (Lu et al., 2021; Zhang et al., 2021), since the Arctic sea ice loss in the Barents and Kara Seas can enhance the Ural blocking through generating a wave train (Luo et al., 2016; Zhang et al., 2021). In addition, the equatorward meridional mass fluxes also tend to be stronger over the Eurasian span in association with Decembers with above-normal SST over the northeastern Pacific and northwestern Atlantic (Figs. 5g and 5j). La Niña tends to be associated with weaker equatorward cold air transport in December and stronger equatorward cold air transport in January (Figs. 5d and 5e). However, these relationships are not statistically significant. In the meantime, the climatological route of cold air via the longitudinal span of North America (90°–150°W) was weakened in December 2021, as indicated by the positive anomalies of meridional mass fluxes below 270 K (Fig. 4a). The weakening of equatorward transport of cold air is within the lower isentropic layers, in contrast with the strengthening of equatorward transport above. A likely explanation could be the lack of cold air mass locally, potentially attributed to the warm SST over the northeastern Pacific (Fig. 5g). We can conclude that the decreased temperature gradient and blockings induced by low autumn Arctic sea ice make the most contributions to the East Asian cold events, whereas the local warming effect of abnormally warm SST over the northeastern Pacific is the main factor leading to the anomalous warmth in North America in December.
We turn now to look at the upper levels. The stronger upward propagated wave fluxes into the upper stratosphere observed in December and January in the 2020/21 winter (Figs. 5a and 5b) appear to be highly consistent with those associated with low sea ice extent, as shown Figs. 5m and 5n. It is also seen that the low SIE-related meridional mass flux anomalies in the stratospheric layers above 400 K tend to be in-phase with the climatological field, leading to a stronger WB in the stratosphere. These results suggest that the sea ice loss in the Barents-Kara Sea during autumn and the following early winter also played an important role in promoting the upward propagation of planetary waves and thus disturbing the stratospheric polar vortex in early winter, as stated in previous studies (Sun et al., 2015; Lu et al., 2021). The warm SST over the northeastern Pacific, as a wave source, can also strengthen the upward propagation of waves in 60°–120°E, but the strengthening is confined within tropospheric levels below 315 K. The pattern of meridional mass flux anomalies as well as the features of wave activities in this early winter strongly resemble the composite pattern that occurred one month before the displacement-type SSW events (Fig. 5p) and provide favorable conditions for the occurrence of the SSW event in the beginning of January.
In February, which is after the displacement-type SSW event, the equatorward mass fluxes at 60°N became anomalously strong over the North American continent (150°–90°W) (Fig. 5c) and were responsible for the extreme cold event there. Comparison with isentropic meridional mass flux anomalies associated with various forcing mechanisms, shown in Figs. 5f, 5i, 5l, and 5o, reveals that the abnormally warm SSTs over both the northeastern Pacific and northwestern Atlantic act to enhance the equatorward cold air mass transport in the region of 120°–90°W. The warm northeastern Pacific SSTs induced an anticyclonic anomaly, thus strengthening the northerlies across the North American continent, according to Zhang et al. (2021). La Niña also tends to make positive, but not statistically significant, contributions to the stronger equatorward mass fluxes at 60°N over the North American sector, which can be explained by their indirect and complicated relationship with extratropical warm SSTs (Matsumura and Kosaka, 2019; Chen et al., 2020) and atmospheric circulation changes over the North Pacific and Atlantic (e.g., the Pacific–North American teleconnection pattern and strength and location of the jet) (Lau and Nath, 2001; Lin and Derome, 2004; Soulard et al., 2019; Mezzina et al., 2020). The downward impact from the displacement-type SSW event, as discussed in previous section, however, is responsible for the strengthening of equatorward cold air mass transport mainly along the west coast of North America (150°–120°W) (Fig. 5r). Though this cold air intrusion is slightly westward, the meridionally oriented Rocky Mountains can guide the polar cold air southward where it can merge into the westerlies, leading to temperature drops over most of the continent. Low Arctic sea ice also tends to induce negative meridional mass fluxes along the west coast of North America, but this effect is not statistically significant (Fig. 5m). Nevertheless, the role of sea ice might be amplified together with SSW events, according to Zhang et al. (2020) who stated that North American cold events tend to occur more frequently following SSW events in the presence of low Barents–Kara Sea sea ice. Therefore, for the North American extreme cold event in February of 2021, the extratropical warm ocean forcing intensifies the equatorward cold air branch over the central and eastern parts of North America. The downward impact from displacement-type SSW events intensifies the equatorward cold air branch over the North American west coast. The autumn sea ice may also play a role, along with the occurrence of SSW.
Winters Preceded by Low September SIE | La Niña Winters | Warm Northwestern Pacific Months | Warm Northeastern Atlantic Months | Central Dates of Displacement-type SSW | |||||
DEC | JAN | FEB | DEC | JAN | FEB | ||||
2007/08 2008/09 2010/11 2011/12 2012/13 2015/16 2016/17 2017/18 2018/19 2019/20 2020/21 | 1988/89 1995/96 1998/99 1999/2000 2007/08 2010/11 2011/12 2020/21 | 2014 2015 2018 2019 2020 | 2005 2014 2015 2016 2018 2019 2020 2021 | 2005 2014 2015 2016 2018 2019 2020 | 2014 2015 2016 2017 2019 2020 | 2007 2014 2015 2016 2017 2020 2021 | 2014 2015 2016 2017 2018 2019 2020 2021 | 1980/03/01 1981/03/04 1981/12/04 1984/02/24 1987/01/23 1998/12/15 2000/03/20 2001/12/30 2002/02/17 2004/01/05 2006/01/21 2007/02/24 2008/02/22 2019/01/01 2021/01/04 |