https://link.springer.com/article/10.1007%2Fs00376-021-1229-1#change-history
Abstract
Three striking and impactful extreme cold weather events successively occurred across East Asia and North America during the mid-winter of 2020/21. These events open a new window to detect possible underlying physical processes. The analysis here indicates that the occurrences of the three events resulted from integrated effects of a concurrence of anomalous thermal conditions in three oceans and interactive Arctic-lower latitude atmospheric circulation processes, which were linked and influenced by one major sudden stratospheric warming (SSW). The North Atlantic warm blob initiated an increased poleward transient eddy heat flux, reducing the Barents-Kara seas sea ice over a warmed ocean and disrupting the stratospheric polar vortex (SPV) to induce the major SSW. The Rossby wave trains excited by the North Atlantic warm blob and the tropical Pacific La Nina interacted with the Arctic tropospheric circulation anomalies or the tropospheric polar vortex to provide dynamic settings, steering cold polar air outbreaks. The long memory of the retreated sea ice with the underlying warm ocean and the amplified tropospheric blocking highs from the midlatitudes to the Arctic intermittently fueled the increased transient eddy heat flux to sustain the SSW over a long time period. The displaced or split SPV centers associated with the SSW played crucial roles in substantially intensifying the tropospheric circulation anomalies and moving the jet stream to the far south to cause cold air outbreaks to a rarely observed extreme state. The results have significant implications for increasing prediction skill and improving policy decision making to enhance resilience in “One Health, One Future”.
https://link.springer.com/content/pdf/10.1007/s00376-021-1229-1.pdf
4. Driving mechanisms—Integrated effects of multiple processes
It has been a perplexing problem to answer what causes the occurrence of the extreme cold events in the context of the accelerating warming climate (Huang et al., 2017b). The majority of prevailing research on the topic focuses on the emer- gence of anomalous thermodynamic forcing associated with Arctic warming amplification and sea ice decrease. The cent- ral piece of the debate about the problem results from the inconsistence and statistical insignificance in research results about atmospheric circulation responses to these anomalous forcing. In addition, Arctic forcing may interact with tropical/extratropical ocean forcing to further complicate the problem. Therefore, in this study, we first examine the ocean environment conditions and then the atmospheric circulation, as well as possible associations between them.
4.1. Arctic sea ice and tropical/extratropical ocean forcing
As an outstanding indicator of Arctic warming amplification, sea ice decrease adds additional surface thermodynamic for- cing to the overlying atmospheric circulation. When looking at sea ice data since 1979, we found that the sea ice extent in the winter of 2020/21 was considerably smaller than its climatology (Fig. 3a). Specifically, the sea ice area in the Barents–Kara seas reached its lowest value on record, particularly in the month of December 2020. On the North Pacific Arc- tic side (i.e., the Bering–Chukchi–Beaufort seas), the sea ice area also shows the second lowest value over the past 42 years. Considering the nature of the poleward intrusion of the North Atlantic and North Pacific warm water into these two ocean areas and absorbed heat energy through open water during the prior summer season, the greater retreat of sea ice cover in these areas would lead to a larger increase in turbulent heat fluxes and upwelling longwave radiation to the atmosphere.
At the same time, large SST anomalies occurred from the tropical Pacific Ocean to the North Atlantic Arctic in winter 2020/21 (Fig. 3b). One of the most prominent phenomena was a strong La Niña with a cold tongue of SST anomalies ran- ging from the eastern to the central tropical Pacific Ocean. This La Niña was developed from September 2020 throughout March 2021 (https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml). Large warm SST anomalies also appeared outside the Niño regions (0°–10°S, 90°W–80°W; 5°N–5°S, 160°E–90°W), extending from the western tropical Pacific to the northeastern North Pacific. A warm blob with a maximum SST anomaly of 3.5°C was present in the Gulf of Alaska and off the west coast of North America.
Other notable ocean thermal anomalies are the warm blob off the east coast of North America and the cold anomaly near the southern tip of Greenland in the North Atlantic Ocean. A warm SST anomaly also occurred from the Norwegian Sea to the Barents–Kara seas, in correspondence to the substantially retreated sea ice there. When examining the temporal evolution of their intensities, we found that all of these North Atlantic and Arctic SST anomalies were at their strongest state in December 2020 and then gradually weakened at a slow pace in the following two months.
4.2. Tropospheric circulation, Rossby waves, and jet streams
The tropospheric atmospheric circulation clearly exhibited high GHT anomalies over the Arctic, wave patterns across the North Atlantic and the Eurasian continent, and a southward shift and intensification of the jet stream over East Asia at 300 hPa associated with the occurrence and development of the first East Asia cold event (Figs. 4a1–a6). Initially, an anomal- ous high center occurred off the east coast of North America with a ridge extending into the Nordic Seas on 25 December 2020, in concert with the warm SST blob in the same location (Fig. 3b). The warm blob could have served as a source of wave activity and excited Rossby wave train propagation, which can be observed on 26 December. As a result, anomalous high and low centers emerged from the northwestern North Atlantic to the Barents–Kara seas. The initially forced ridge and the subsequently developed wave train would enhance poleward transient eddy heat and moisture fluxes into the Arctic, lead- ing to the decrease and minimum of sea ice area in the Barents–Kara seas in December 2020 as mentioned above. During this time period, the East Asian trough became stronger and exhibited a negative GHT anomaly, which can be associated with the increased baroclinicity due to the cold and warm SST anomalies between the Sea of Okhotsk and the rest of the west- ern North Pacific Ocean (Fig. 3b). Meanwhile, the jet stream was located around 40°N over the Japan Sea.
Following the enhanced transient eddy heat influx, the decreased sea ice cover over the warm ocean, and the resultant increase in the surface and lower-tropospheric air temperatures over the Barents–Kara seas (not shown), the high GHT anom- alies over the Barents–Kara seas intensified and extended over a large area of the Arctic, as explained by the quasi-geo- strophic (QG) theory (Holton, 2004). The anomalous low center over the East Greenland Sea accordingly moved southeast- ward to the area of the United Kingdom. As a consequence, a zonally aligned wave train developed in the midlatitudes from the North Atlantic to East Asia during the period of 27–30 December. The wave train anchored and amplified the fluctu- ation of the atmospheric circulation, enhancing blocking highs over eastern Europe and western Siberia (i.e., to the west and east sides of the Ural Mountains). Meanwhile, the high Arctic GHT anomaly developed further to the east and then shifted southeastward to the Laptev Sea coastal area. The combination of the western Siberia ridge and the southeastward-shifted Arctic GHT anomaly, together with the rapidly deepened East Asian trough and intensified jet stream over the Japan Sea, strengthened the meridionally oriented circulation over East Asia, triggering a cold air outbreak (Figs. 1 and 2).
The interactions between the Arctic and midlatitude circulations also played an essential role in the occurrence of the second cold event in East Asia. The jet stream was also located relatively to the south, between 30–40°N, from East Asia to the North Pacific. However, their spatial structures, temporal evolutions, and the way in which they interacted demon- strated some obvious differences (Figs. 4b1–b6). Although there were blocking highs over the North Atlantic and Ural Moun- tains areas since the beginning of the event (2–5 January 2021), the formation and intensification of the meridionally orient- ated circulation over East Asia was predominately initiated and shaped by the strong positive GHT anomaly over the Kara and Laptev seas and the negative GHT anomaly over East Asia. The southeastward shift of the Arctic anomalous high, the substantial deepening of the East Asian anomalous low (i.e., the East Asian trough), and the intensified jet stream over East Asia from 4–7 January provided an outstanding dynamic setting driving cold polar air to plunge southward. Note that the spa- tial distribution of the high and low GHT anomalies over the Eurasian high latitudes and the North Pacific Ocean during this event have strong projection on the negative Arctic Rapid change Pattern (ARP), which has played a decisive dynamic role in systematically and simultaneously causing both rapid Arctic warming and cold Eurasia after the late 1990s (Zhang et al., 2008).
During this event, there was also a Rossby wave train originating from the North Atlantic warm blob region propagat- ing southeastward to East Asia; it was especially well-developed from 4–7 January (Figs. 4b3–b6). The anomalous high cen- ters over the North Atlantic and the Ural Mountains linked the midlatitude circulation to the Arctic positive GHT anom- alies, resulting in intensified ridges or blocking highs and facilitating increased poleward transient eddy heat flux. The anom- alous high center of the wave train reinforced the ridge over western China on 6–7 January, which then strengthened the meri- dional flow described above, enhancing the cold air outbreak and enabling it to reach southeastern China. In addition, com- pared with the first cold event, the pathway of the wave train during this event was located over relatively lower latitudes.
The cold event in North America was more severe and lasted longer than the two East Asia events, as analyzed above. The La Niña event preconditioned an anomalous tropospheric circulation from the central tropical Pacific to the North Pacific and North America. In January and early February 2021, the Niño 4 (5°N–5°S, 150°W–160°E) regional mean SST anomalies reached large negative values exceeding –1.0°C (as low as –1.4°C in mid-January) (https://www.cpc. ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml). As a result, the Pacific/North American (PNA) index became negative from 21 January to 9 February (https://www.cpc.ncep.noaa.gov/products/precip/CWlink/ pna/pna.shtml). Correspondingly, a negative PNA teleconnection pattern (i.e., a Rossby wave train) emanated from the cent- ral tropical Pacific Ocean and propagated to the northeastern North Pacific, the central part of North America, and Eastern Canada (van den Dool et al., 2000; Fig. 4c1). At the same time, the atmospheric circulation anomalies originating from the Arctic further transformed the midlatitude circulation anomalies. A strong low-pressure system, or a tropospheric polar vor- tex (TPV, a recently coined name to be distinguished from the stratospheric polar vortex, SPV), occurred over the Cana- dian Arctic Archipelago, which meridionally stretched and deepened the low center of the PNA pattern over the North Amer- ican continent. The atmospheric circulation was therefore predominantly characterized by a ridge over the eastern North Pacific, a trough ranging from the Canadian Arctic Archipelago down to the Great Plains and the Southern United States, and a southward shifted and intensified jet stream over the southern area of the United States, driving cold air southward.
During the following days, the poleward extended ridges over the North Pacific and Eastern Canada/Baffin Bay favored warm air advection into the Arctic, leading to increased thickness of the Arctic air column according to the QG the- ory (Holton, 2004) and as seen in Figs. 4c2–c4. The negative PNA pattern then gradually weakened and was deformed. Nev- ertheless, the North Pacific ridge was further intensified, extending into the Gulf of Alaska, the Bering Sea, Alaska, and the Chukchi Sea. The TPV over North America deepened and shifted southward. The blocking high strengthened over Baffin Bay and Greenland. As a consequence, cold air was persistently transported southward over the North American continent. Note that during this period an anomalous low center developed and intensified over the western North Pacific, shifting the jet stream southward to around 30°N.
After 11 February, the eastern North Pacific ridge, North American TPV, and Arctic positive GHT anomaly began weak- ening. However, a wave train developed from the western North Pacific low center to North America, maintaining the ridge over the eastern North Pacific and the trough over the Great Plains and the Southern United States for an extended time period (up to 18 February). Due to the relatively southern location of the wave train (particularly the two low centers), the jet stream shifted south of 30°N in the United States, which is unusual, and led to the disastrous and persistent cold weather in Texas and the adjacent states.
So far, we have analyzed the spatial structures and temporal evolutions of the tropospheric circulation anomalies, which have triggered and steered cold polar air outbreaks. However, a number of questions remain open regarding the changes in the tropospheric circulations, including (1) why the wave train was deformed and the North Atlantic low anom- aly center shifted southward in the first East Asia cold event; (2) what additional force drove the intensification of the East Asian trough in the second East Asian event; and (3) why the TPV intensified and moved southward in the North Amer-ican event. We address these questions below through examining stratosphere–troposphere interactions.
4.3. Sudden stratospheric warming and stratospheric polar vortex
The stratospheric atmosphere also experienced tremendously large anomalies in winter 2020/21. In climatology, the SPV emerges and then intensifies in the fall and weakens and dissipates in the spring. During this course, it reaches its strongest state in January with the lowest GHT. However, the 50 hPa GHT dramatically increased from late December 2020 to mid-February 2021 (Fig. 5), coincident with the period of the three extreme cold events shown above. It departed from its climatology by more than one standard deviation and even exceeded two standard deviations in mid-January, indicating an occurrence of a major sudden stratospheric warming (SSW) event with a weakened SPV.
The SSW event and its extended persistence in winter 2020/21 could be ascribed to the increase in the tropospheric pole- ward transient eddy heat transport, which is the source of the wave activity (e.g., Edmon et al., 1980). As shown in Fig. 4, a GHT ridge and a poleward propagating Rossby wave train occurred on 25–26 December 2020, resulting in a heat flux intru- sion from the North Atlantic into the Arctic and, in turn, substantially decreasing sea ice cover in the Barents–Kara seas. The anomalous poleward heat intrusion and the decreased sea ice cover over the warm ocean would also increase atmo- spheric transient eddy heat flux, which is the mechanism inducing an upward propagation of planetary Rossby waves to dis- rupt the SPV. In particular, the long memory of the retreated sea ice and the underlying warm ocean can favorably main- tain surface and lower tropospheric warming and, therefore, increase transient eddy heat flux over a longer time period, sup- porting a persistence of the SSW event. This role of decreased sea ice in disrupting the SPV through planetary Rossby waves was revealed through data analysis and modeling experiments in Kim et al. (2014) and Zhang et al. (2018a). In addi- tion, during the second East Asian event and the North American event, the intermittently occurring blocking highs, or ridges, from the North Atlantic, the eastern North Pacific, and eastern Canada–Baffin Bay would also continually reinforce the wave activity, sustaining the SSW event for a long time (about one and half months).
As a consequence of the SSW event, the weakened SPV demonstrated a deformation in its spatial structure, which can intensify the tropospheric circulation anomalies to cause the extreme cold events. We now discuss these processes in each of the three events. During the first East Asian event, the SPV center was displaced to be over the Eurasian continent with a deep trough located from Scandinavia to Western Europe. It reached its strongest state on 28 December 2020 (Fig. 6a). In cor- respondence to the maximum negative GHT anomaly at 50 hPa, a positive potential vorticity (PV) anomaly, which is defined by the local maximum of PV, would develop. According to PV dynamics (Hoskins, et. al., 1985), the positive PV anomaly spun up cyclonic circulation underneath itself, generated a jet stream under the tropopause, and supported enhance- ment of tropospheric baroclinicity, which finally intensified the tropospheric low pressure system. This downward impact mechanism explains the shift of the anomalous tropospheric low center from the northwestern North Atlantic to the area over the United Kingdom, as shown in Figs. 4a2–a3. This shift facilitated the wave train propagation from the North Atlantic to East Asia and, in turn, amplified meridional circulation to cause a cold air outbreak in East Asia.
The SSW persisted into the second East Asian event. The weakened SPV evolved to be split to two daughter centers (or vortices) situated over Greenland and Northeast Asia, respectively (Fig. 6b). The latter center was stronger than the former one and showed a maximum negative GHT anomaly at 50 hPa. Similar to the PV dynamic processes discussed above, the Northeast Asian center and associated positive PV anomaly would deepen the anomalous tropospheric low cen- ter, or the corresponding East Asian trough, and strengthen the jet stream at the bottom of the trough (Figs. 4b4–b6), amplify- ing the meridional flow and, in turn, driving the cold polar airmass to spread southward.
The role of the SSW in the development of the North American cold event exhibited differences from those of the two East Asian events. In the East Asian events, the dispaced or split SPV center mainly modulated or intensified tropospheric cir- culation anomalies at one particular step/phase. However, the weakened SPV progressively played a role in the tropo- spheric circualtion changes from the begining throughout most of the time during the occurrence and development of the North America event. The SPV was still split into two centers over the western North Pacific and Baffin Bay–Greeland, respectively (Fig. 6c). In early Feburary 2021, the Baffin Bay–Greeland SPV center intensified and meridionally stretched the anomalous tropospheric low center of the PNA pattern following the same downward impact theory mentioned above (Figs. 4c1 and c2). This also strengthened the tropospheric blocking high over the eastern North Pacific and the trough over North America, as shown in Figs. 4c1–c2. Subsequently, both the North America and western North Pacific SPV centers extended considerablly southward, leading to negative GHT anomaly centers at 50 hPa located as far south as 30°N. Under the influence of the two SPV centers, the two underneath tropospehric low GHT centers intensified, and a wave train developed in the far south propagating from the western North Pacific to south of the Great Plains. The jet stream was also located anomalously farther south than its climatology (Figs. 4c3–c6). Therefore, this circuation pattern continously steered snow/ice storms along with cold temperatures toward the southern United States.
