An unusually prolonged Pacific-North American pattern promoted the quad-state tornado outbreak on 10-11 December 2021

doi:10.21203/rs.3.rs-3934404/v1

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An unusually prolonged Pacific-North American pattern promoted the quad-state tornado outbreak on 10-11 December 2021

https://doi.org/10.21203/rs.3.rs-3934404/v1

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This study examines the large-scale atmosphere-ocean environments that led to the winter tornado outbreak across the Ohio Valley on 10–11 December 2021, also known as the quad-state tornado outbreak. Here, we show that the quad-state tornado outbreak occurred under an exceptionally strong and prolonged negative Pacific-North American pattern (PNA), which developed around December 1 and persisted for a month. This unusual PNA produced a strong atmospheric ridge along the south and eastern US seaboard, which in turn helped warm up the Gulf of Mexico and produced large-scale environments conducive for tornadogenesis across the Ohio Valley. Further analysis shows that a broad region across the Ohio Valley is particularly vulnerable to extensive winter tornado outbreaks during long-lived negative PNA, whereas a limited region in the central US is exposed to winter tornado activity during short-lived negative PNA. Finally, although the PNA is a mode of internal variability that occurs with or without El Niño - Southern Oscillation, the occurrence of prolonged negative PNA is more frequent during La Niña than during El Niño.

Tornadoes are one of the deadliest and most costly natural disasters in the United States (US). The occurrence of tornadoes increases sharply in boreal spring and then decreases after reaching its peak in April, May, and June. As a result, previous studies have largely focused on warm-season (i.e., boreal spring and summer) US tornado activity^1–12. However, the occurrence of significant tornadoes (i.e., Enhanced Fujita [EF] scales greater than or equal to two) is not only limited to the warm-season but also to the cold-season ^13–17. Although relatively rare, winter tornado outbreaks do occur and when they do, they tend to produce disproportionally large socioeconomic impacts on populated US regions, partly due to a lack of societal awareness ^16,18.

On December 10, 2021, one of the most destructive winter tornado outbreaks developed in northeastern Arkansas. This tornado outbreak, known as the quad-state tornado outbreak, struck portions of the Ohio Valley, including Missouri, Illinois, Tennessee, and Kentucky, from December 10 to 11, 2021. According to the National Oceanic and Atmospheric Administration (NOAA, https://www.weather.gov/pah/December-10th-11th-2021-Tornado ), the quad-state tornado outbreak resulted in 66 confirmed tornadoes, 89 fatalities, 672 injuries, and at least $3.9 billion in property damages. The outbreak occurred during a strong multi-year La Niña (i.e., the cold phase of the El Niño -Southern Oscillation, ENSO) and warm sea surface temperature anomalies (SSTAs) were prevalent throughout the Gulf of Mexico (GoM; Fig. 1a). As well-documented in previous studies, both La Niña and warm GoM SSTAs are known to produce favorable large-scale atmospheric conditions for US tornado activity ^{3, 4, 13, 19, 20}. Specifically, La Niña, through extratropical teleconnections, tends to increase the frequency and intensity of winter storm activity over the US, while warm GoM tends to increase the supply of warm and moist low-level air to the US, fueling the winter storms ^{7, 14, 15, 17, 20–22}.

Coinciding with the cold tropical Pacific and warm GoM, a strong negative Pacific North American (PNA) pattern developed more than 10 days prior to the quad-state tornado outbreak and then persisted throughout the event days and beyond until around January 4, 2022 (Fig. 1d). Associated with this unusually prolonged negative PNA, a strong atmospheric ridge developed along the south and eastern US seaboard, increasing the southwesterly low-level wind and the associated flux of warm and moist low-level air from the GoM into the central and eastern US. (Fig. 1b). The increased low-level wind, humidity and near-surface temperature promoted atmospheric instability, evidenced by a strong negative lifted index (LI) and strong low-level wind shear (LLWS) between 1000 and 850 hPa (Fig. 1c), producing highly conducive atmospheric conditions for tornado outbreaks in and around the Mississippi-Ohio Valley.

As briefly discussed above, it is very likely that the negative PNA, warm GoM and La Niña conditions in the Pacific all played sizable roles in fostering the quad-state tornado outbreak. However, in order to achieve a useful forecast capability for future winter US tornado outbreaks, it is necessary to further investigate large-scale atmospheric processes preceding the quad-state tornado outbreak and other historical winter outbreak events. Since a negative PNA developed more than 10 days prior to the quad-state tornado outbreak and then persisted for an unusually long period throughout the event days, it is important to address if such a long-lived PNA event is a precursor (or necessary condition) for the development of historical winter US tornado outbreaks. Additionally, given that SSTAs in the GoM can be forced by surface heat flux anomalies associated with the PNA, it is also important to address if there is any link between the PNA and SSTAs in the GoM. Finally, since negative PNA events occur more frequently during La Niña conditions in the Pacific ^23–26, it is useful to further explore if long-lived PNA events are preferred than short-lived ones during La Niña. These key questions are addressed in this study using observational datasets and a fully-coupled model simulation.

Impacts of negative PNA and warm GoM on winter US tornado outbreaks

We first carried out a composite analysis to examine the roles of negative PNA and warm GoM in winter US tornado activity. As shown in Fig. 2a, the negative PNA is associated with the development of an anomalous atmospheric ridge around the eastern US. The associated anticyclonic circulation anomalies enhance the low-level southwesterly wind anomalies, increasing the flux of warm and moist low-level air from the GoM to the central and eastern US and LLWS in these regions (Fig. 2b). As a result, near-surface temperature ^27–28 and atmospheric instability (e.g., LI) increase over the central and eastern US (Fig. 2b), leading to an increase in winter tornadogenesis in and around the Ohio Valley (Fig. 2c).

Interestingly, the warm GoM is also linked to a negative PNA-like pattern (Fig. 2d). An atmospheric ridge prevails along the eastern US, similar in location albeit weaker than the negative PNA composite (Fig. 2a), with associated anticyclonic circulation anomalies that enhance the low-level southwesterly wind anomalies. In this case, however, the low-level southwesterly anomalies strengthen predominantly over the southern US; thus, LLWS, the flux of warm and moist low-level air from the GoM, and negative LI are present along the Gulf states (i.e., Texas, Louisiana, Mississippi, and Alabama; Fig. 2e). This suggests that warm GoM is linked to enhanced tornado activity over the southern US and a portion of the southern Ohio Valley (Fig. 2f). The composites of SSTAs and geopotential height at 500 hPa derived from Community Earth System Model Large Ensemble Simulation (CESM-LENS) are largely consistent with the results from observations and reanalysis datasets (Fig. S1).

Relationship between PNA and GoM SSTAs

The negative PNA-like pattern in the warm GoM composite (Fig. 2d) suggests that GoM SSTAs can be forced by the PNA phase. Indeed, it is noted that the GoM is also warm in the negative PNA composite (Fig. 2a). To further examine the relationship between the PNA and GoM SSTAs, we show a histogram of GoM SSTAs during negative, positive, and neutral PNA phases in Fig. 3a. It clearly shows an inverse relationship between the PNA and GoM SSTAs. Specifically, during the negative PNA, the occurrence of warm GoM is more frequent than that during the positive PNA phase. For instance, the probability of extremely warm GoM (> 1.2K) increases by ~ 300% during the negative PNA compared to that during the positive PNA. During the positive PNA, on the other hand, the occurrence of cold GoM is more frequent than that during the negative PNA. This result supports the hypothesis that GoM SSTAs are physically linked to the PNA phases. The histogram derived from CESM-LENS also supports this result from observational and reanalysis datasets (Fig. S2a).

To further examine the physical relationship between the PNA and GoM SSTAs, we carried out a lead-lag correlation analysis between the PNA and GoM SSTAs. As shown in Fig. 3b, the negative PNA leads to warm GoM with a time-lag of approximately six days. A consistent result is found from CESM-LENS (Fig. S2b). The lagged relationship between the PNA and GoM SSTAs is largely due to PNA-induced surface heat flux anomalies over the GoM (Fig. 3b). Further analysis indicates that surface turbulent heat flux (i.e., latent and sensible) anomalies modulated by PNA-induced surface wind speed are the major contributors to the surface heat flux anomalies (Fig. S3).

Impact of long- versus short-lived PNA on winter US tornado outbreaks

The lead-lag correlation analysis (Fig. 3b) suggests that if the negative PNA and associated atmospheric ridge over the eastern US persist for six days or longer, GoM warms up significantly. In contrast, if the negative PNA dissipates within six days, GoM SSTAs are not significantly changed. This indicates that long-lived PNA helps warm up the GoM and thus provides a set of ideal environmental conditions for winter US tornado activity. To test this hypothesis, we carried out a composite analysis for the long-lived negative PNA (i.e., exceeding half a standard deviation and persisting for at least six days; Table S2) versus the short-lived negative PNA (i.e., exceeding half a standard deviation and dissipating in less than six days; Table S2).

As shown in Fig. 4a and 4e, 500 hPa geopotential anomalies exhibit a similar spatial pattern during the short-lived and long-lived negative PNA. However, the associated anomalous ridge over the US eastern seaboard is much more robust during the long-lived PNA. During the short-lived PNA, the atmospheric ridge extends meridionally across the eastern US, producing weak low-level southerly wind anomalies over the central US. This, in turn, increases LLWS, the flux of warm and moist air from the GoM, and decreases LI over the central US, producing favorable conditions for tornadogenesis in this region (Fig. 4b and c). In contrast, during the long-lived PNA, the anomalous ridge is tilted in such a way to produce strong low-level southwesterly anomalies across both the central and eastern US. This increases warm and moist air flux from the GoM and the eastern US coast into the US while increasing LLWS over the eastern US (Fig. 4f). This, in turn, increases atmospheric instability (i.e., LI) over the central and eastern US (Fig. 4g). During both the short- and long-lived PNA, surface heat flux is enhanced over the GoM as well as along the eastern US coast (Fig. 4c and g). However, since the reduction of near-surface wind speed is greater during the long-lived negative PNA than during the short-lived negative PNA (Fig. S4), the surface flux anomalies are much stronger and more extensive during the long-lived negative PNA (Fig. 4c and g). This explains why the long-lived PNA is more frequently accompanied by warm GoM. These large-scale environmental conditions associated with the short- and long-lived PNA are consistent with those derived from CESM-LENS (Fig. S5).

Largely consistent with the large-scale environmental patterns, winter US tornadogenesis is increased over a limited region in the central US (i.e., Arkansas, Missouri, and Illinois) during the short-lived negative PNA (Fig. 4d), while the area of increased winter tornadogenesis is much more extensive across the central and eastern US, including Mississippi, Tennessee, Kentucky, and Indiana, during the long-lived negative PNA (Fig. 4h). Indeed, the area of increased winter tornadogenesis during the long-lived negative PNA is surprisingly consistent with the area affected by the quad-state tornado outbreak (Fig. 4h).

The role of La Niña in the development of long- and short-lived PNA

Thus far, we have shown that a long-lived negative PNA pattern provides a set of atmosphere and ocean conditions ideal for fostering winter US tornado outbreaks similar to the quad-state tornado outbreak on 10–11 December 2021. Previous studies have shown that the PNA is a mode of internal variability and thus external forcing (e.g., ENSO) is not required for its development ^{25–26, 29}. However, during La Niña, suppression of deep convection in the tropical Pacific promotes a negative PNA-like pattern through the atmospheric bridge ^29–32. Consequently, cold SSTAs tend to appear in the tropical Pacific during both the short- and long-lived PNA events (Fig. 4a and 4e).

A related outstanding question is if the 2021-22 La Niña promoted the unusually prolonged negative PNA event that in turn fostered the quad-state tornado outbreak. While it is very difficult to find a definitive answer to this question, we may instead explore statistical relationships between the histogram of negative PNA days and ENSO conditions. As expected, the number of negative PNA days in a winter season (i.e., 92 days from November 1 to January 31) increases from 26.4 days per one El Niño season to 33.1 days per one La Niña season (a 25% increase). However, as shown in Fig. 5, the persistence of negative PNA is greater during La Niña than during El Niño. Specifically, about 39 out of 100 negative PNA events are long-live events (≥ 6 days) during El Niño conditions, whereas about 52 out of 100 negative PNA events are long-live events during La Niña conditions. Interestingly, unusually prolonged negative PNA events (≥ 20 days), as in the case of the quad-state tornado outbreak, occurred only during La Niña conditions. These results, also supported by the histogram derived from CESM-LENS (Fig. S6), suggest that the exceptionally long-lived negative PNA event during December 2, 2021 - January 4, 2022 (a total of 34 days) was promoted by the 2021-22 La Niña.

Concluding remarks

This study shows that the quad-state tornado outbreak on December 10–11, 2021 is closely tied to the long-lived negative PNA and associated warm GoM that produced a set of highly favorable environmental conditions for a winter tornado outbreak (i.e., increased LLWS and flux of warm and moist air from the GoM, and decreased LI) in and around the Ohio Valley. Further analysis indicates that the long-lived negative PNA produces a robust anomalous atmospheric ridge along the eastern US seaboard, producing anomalous low-level southwesterly wind and flux of warm and moist low-level air from the GoM into the central and eastern US. Through the associated surface turbulent heat flux, the long-lived negative PNA warms the GoM and thus further enhances the flux of warm and moist low-level air to the US. These atmospheric environments lead to a large increase in atmospheric instability over a broad region of the central and eastern US, increasing the likelihood of tornadogenesis in the region including the Ohio Valley where the quad-state tornado outbreak predominantly occurred. It is also shown that the occurrence of long-lived negative PNA events is more frequent during La Niña condition than during El Niño condition. This supports the idea that the 2021-22 La Niña promoted the exceptionally long-lived negative PNA event associated with the quad-state tornado outbreaks.

In contrast, the short-lived negative PNA produces a relatively weak and meridionally elongated atmospheric ridge over the eastern US. The atmospheric ridge produces southerly low-level wind anomalies, increasing LLWS and atmospheric instability over the central US. However, due to its short duration, the GoM SSTAs remain relatively neutral. During the short-lived negative PNA, tornadogenesis is increased over the central US. However, due to the relatively weak amplitude of the atmospheric instability (i.e., LI) and the near-neutral GoM SSTAs, winter tornadogenesis during the short-lived PNA is increased in a limited region in Arkansas, Missouri, and Illinois.

There remain several questions that are unexplored in this study. First, it is unclear through what processes the long-lived negative PNA is generated. Previous studies have shown that the negative PNA can be forced by the suppression of tropical Pacific convection (e.g., during La Nina) and associated extratropical Rossby wave propagation ^29–31. The negative PNA can be also triggered by transient eddy vorticity fluxes over the extratropics ^{23, 28} and by extraction of kinetic energy from the zonally varying basic state ^{24, 29, 31, 33–35}. Further analysis is required to better understand the potential link between these drivers and the persistence of the PNA. Another important question is if the frequency of long-lived negative PNA will change in the future. According to several previous studies ^36–38, the amplitude of PNA is projected to increase partly due to the increasing variance of SSTAs in the tropical Pacific in the future, while the frequency of PNA is not expected to change significantly. However, it is still unclear whether the duration of the PNA will change in the future climate. Addressing the above questions could potentially help achieve a useful forecast capability for future winter US tornado outbreaks.

The National Centers for Environmental Prediction reanalysis version 1 (NCEP1)³⁹ is used to derive daily atmospheric variables. Daily SST data is derived from NOAA optimum interpolation SST version 2.1 (OI-SST)^40–41. The Severe Weather Database (SWD) from NOAA’s Storm Prediction Center is used to compute the number of tornadoes. We only counted the tornadoes with the Enhanced Fujita (EF) scale equal to or greater than 1 (EF1-5) to avoid a spurious long-term trend in the tornado dataset^{4, 12, 42}.

The study period covers 40 years from 1982 to 2021, focusing on the November-January (NDJ; n = 3680) period, or cold season from here onward. To minimize any spurious trend, all data are linearly detrended. To obtain the indices for ENSO and GoM SSTAs, we used a spatial average of SSTAs over the NINO3.4 region (170°-120°W, 5°S-5°N) and SSTAs over the GoM region (100°W-80°W, 18°N-31°N), respectively. La Niña years are defined as when NDJ seasonal NINO3.4 SSTAs fall below − 0.5 K. Warm GoM events are defined as when the daily GoM index is greater than half a standard deviation (i.e., > 0.27 K; n = 802; Table S1). The daily PNA index, following the method used at the NOAA Climate Prediction Center, is derived from NCEP1 500 hPa geopotential height anomalies. Negative PNA phases are defined when the daily PNA index falls below half a standard deviation (< -82.18 gpm; n = 1361; Table S1).

To further test the results derived from the observational and reanalysis datasets, we analyzed a 400-model-year in the Community Earth System Model Large Ensemble Simulation (CESM-LENS) with a constant preindustrial CO₂ level ⁴³. All daily indices from CESM-LENS are obtained in the same manner as in the observational and reanalysis datasets. The thresholds for defining warm GoM and negative PNA in the CESM-LENS are 0.18K (n = 12,260), and − 67.28 gpm (n = 12,401), respectively (Table S1).

Data availability

The National Centers for Environmental Prediction - National Center for Atmospheric Research Reanalysis version 1 (NCEP1) data were downloaded from NOAA PSL at https://psl.noaa.gov/data/gridded/data.ncep.reanalysis.html. NOAA optimum interpolation SST version 2.1 (OI-SST) data were downloaded from NOAA PSL at https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.highres.html. US tornado reports were downloaded from NOAA’s Storm Prediction Center (SPC) at https://www.spc.noaa.gov/wcm/#data. CESM-LENS data were downloaded from National Center for Atmospheric Research (NCAR) at https://www.cesm.ucar.edu/community-projects/lens.

Code availability

All statistical analyses were performed using the Grid Analysis and Display System (GrADS), which is publicly available from the Center for Ocean-Land-Atmosphere Studies at http://cola.gmu.edu/grads and NCL, which is publicly available from the NCAR Command Language (NCL) at https://www.ncl.ucar.edu/ The GrADS, NCL, and Fortran codes used to perform the analyses can be accessed upon request to D. K.

Acknowledgments

The authors thank Breanna Zavadoff for helpful comments and suggestions. This work was carried out under the auspices of the Cooperative Institute for Marine and Atmospheric Studies (CIMAS), a cooperative institute of the University of Miami, and NOAA, cooperative agreement NA20OAR4320472, and supported by NOAA’s Oceanic and Atmospheric Research, NOAA’s Climate Program Office, Climate Variability and Predictability Program, and NOAA’s Atlantic Oceanographic and Meteorological Laboratory.

Author contributions

D.K. and S.-K.L. conceived the study, D.K. performed the analysis and wrote the initial draft of the paper. All authors (D.K., S.-K.L., H.L., J.-H. J., and J.-S. H.) significantly contributed to the discussion and interpretation of results, and reviewed the paper.

Competing interests

The authors declare no competing interests.

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published 15 Jun, 2024

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An unusually prolonged Pacific-North American pattern promoted the quad-state tornado outbreak on 10-11 December 2021

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Journal Publication

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Abstract

Figures

Introduction

Results

Impacts of negative PNA and warm GoM on winter US tornado outbreaks

Relationship between PNA and GoM SSTAs

Impact of long- versus short-lived PNA on winter US tornado outbreaks

The role of La Niña in the development of long- and short-lived PNA

Concluding remarks

Data and methods

Declarations

References

Additional Declarations

Supplementary Files

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Journal Publication

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