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Stratosphere‐Troposphere Coupling of Extreme Stratospheric Wave Activity in CMIP6 Models

Journal of Geophysical Research: Atmospheres

August 2023
128(16)

DOI:10.1029/2023JD038811

Authors:

Xiuyuan Ding

Princeton University

Gang Chen

University of California, Los Angeles

Weiming Ma

University of California, Los Angeles

Extreme stratospheric wave activity has been suggested to be connected to surface temperature anomalies, but some key processes are not well understood. Using observations, we show that the stratospheric events featuring weaker‐than‐normal wave activity are associated with increased North American (NA) cold extreme risks before and near the event onset, accompanied by less frequent atmospheric river (AR) events on the west coast of the United States. Strong stratospheric wave events, on the other hand, exhibit a tropospheric weather regime transition. They are preceded by NA warm anomalies and increased AR frequency over the west coast, followed by increased risks of NA cold extremes and north‐shifted ARs over the Atlantic. Moreover, these links between the stratosphere and troposphere are attributed to the vertical structure of wave coupling. Weak wave events show a wave structure of westward tilt with increasing altitudes, while strong wave events feature a shift from westward tilt to eastward tilt during their life cycle. This wave phase shift indicates vertical wave coupling and likely regional planetary wave reflection. Further examinations of CMIP6 models show that models with a degraded representation of stratospheric wave structure exhibit biases in the troposphere during strong wave events. Specifically, models with a stratospheric ridge weaker than the reanalysis exhibit a weaker tropospheric signal. Our findings suggest that the vertical coupling of extreme stratospheric wave activity should be evaluated in the model representation of stratosphere‐troposphere coupling.

Composites of geopotential height at 10 hPa for weak (top) and strong (bottom) stratospheric wave events in the ERA5 reanalysis. (a and e) day −5, (b and f) day 0, (c and g) day 5, and (d and h) day 10. Shading denotes anomalies, and contours denote absolute values (contour intervals: 500 m) with 29,000 m contour emboldened. The time evolution is smoothed by a 5‐day running average. Stippling represents the regions where the anomalies are significant at the 95% confidence level based on the Student's t‐test. The green box (60°–90°N, 60°W–180°) in (f) indicates the region where stratospheric ridge anomalies are calculated.

…

Composites of geopotential height at 500 hPa for weak (top) and strong (bottom) stratospheric wave events in ERA5 reanalysis. As in Figure 1, but for 500 hPa geopotential height. Shading denotes anomalies, and contours denote absolute values (contour intervals: 100 m). The green box (40°–70°N, 30°–130°W) in (h) indicates the region where tropospheric trough anomalies are calculated.

…

Composites of surface anomalies and SAT extremes for weak (top) and strong (bottom) stratospheric wave events in ERA5. (a and d) SAT anomalies in shading and SLP anomalies in contours (contour intervals: 1 hPa) during days ‐15–0 and during days 5–20. (b and e) the risk ratio of cold extremes during days ‐15–0 and during days 5–20. (c and f) the risk ratio of warm extremes during days ‐15–0 and during days 5–20. The risk ratio of cold (warm) extremes is defined as the probability of cold (warm) extreme days (SAT anomalies exceed 1.5 SD) during the period of interest divided by the probability of cold (warm) extreme days in all the winter days. Stippling represents the regions significant at the 95% confidence level based on the Student's t‐test. The green box (40°–70°N, 70°–130°W) in (d) indicates the region where NA SAT anomalies are calculated.

…

Composites of AR and precipitation anomalies for weak (top) and strong (bottom) stratospheric wave events in ERA5. (a and d) same as Figures 3a and 3d. (b and e) AR frequency anomalies during days ‐15–0 and during days 5–20. (c and f) precipitation anomalies during days ‐15–0 and during days 5–20. Contours in (b and e) denote regions where the climatological AR frequency exceeds 10%. Stippling represents the regions significant at the 95% confidence level based on the Student's t‐test.

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Vertical wave coupling during weak (top) and strong (bottom) wave events in ERA5. (a–d) composites of the zonally asymmetric component of anomalous geopotential height (shading) and the vertical and zonal components of anomalous Plumb wave activity flux (vector) averaged over 50°–70°N as a function of longitude and pressure on day −5, 0, 5, and 10 for weak wave events. (e–h) as in (a–d), but for strong wave events. Black lines are zero contours of the wave‐1 component of anomalous geopotential height, indicating the phase tilt of wave‐1. The time evolution is smoothed by a 5‐day running average. To account for the smaller air density with decreasing pressure, the magnitude of the Plumb flux is scaled by (1,000/p)1/2, and geopotential height is scaled by (p/1,000)1/2, where p is pressure. The vertical component of the Plumb flux is also scaled by a factor of 200. See Figure S5 in Supporting Information S1 for the total (zonally asymmetric + zonal mean) field of anomalous geopotential height and absolute (anomalous + climatological) Plumb flux.

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1. Introduction

It is widely recognized that stratospheric variability is connected to tropospheric weather conditions during boreal

winter (e.g., Baldwin etal., 2021; Domeisen etal., 2020b; Sigmond etal., 2013). In the winter stratosphere, a

powerful cyclonic system termed the polar vortex develops over the polar region as the result of large solar

radiation gradients. The strength of a stratospheric polar vortex is also modulated by wave drag due to planetary

waves propagating from the troposphere to the stratosphere (e.g., Charney & Drazin,1961; Matsuno,1970). Both

weak and strong polar vortex events have been shown to influence the troposphere and surface on sub-seasonal to

seasonal timescales (e.g., Baldwin & Dunkerton,2001; Limpasuvan etal.,2004,2005).

Several mechanisms are proposed for the stratospheric impacts on the surface, involving different aspects of the

planetary wave-zonal flow interaction. Planetary waves propagate upwards into the stratosphere, where waves

are absorbed and under certain conditions result in an extremely weak polar vortex, known as sudden strato-

spheric warming (SSW; Charney & Drazin,1961; Garfinkel etal.,2010; Polvani & Waugh,2004). Following

Abstract Extreme stratospheric wave activity has been suggested to be connected to surface temperature

anomalies, but some key processes are not well understood. Using observations, we show that the stratospheric

events featuring weaker-than-normal wave activity are associated with increased North American (NA) cold

extreme risks before and near the event onset, accompanied by less frequent atmospheric river (AR) events on

the west coast of the United States. Strong stratospheric wave events, on the other hand, exhibit a tropospheric

weather regime transition. They are preceded by NA warm anomalies and increased AR frequency over the

west coast, followed by increased risks of NA cold extremes and north-shifted ARs over the Atlantic. Moreover,

these links between the stratosphere and troposphere are attributed to the vertical structure of wave coupling.

Weak wave events show a wave structure of westward tilt with increasing altitudes, while strong wave events

feature a shift from westward tilt to eastward tilt during their life cycle. This wave phase shift indicates vertical

wave coupling and likely regional planetary wave reflection. Further examinations of CMIP6 models show that

models with a degraded representation of stratospheric wave structure exhibit biases in the troposphere during

strong wave events. Specifically, models with a stratospheric ridge weaker than the reanalysis exhibit a weaker

tropospheric signal. Our findings suggest that the vertical coupling of extreme stratospheric wave activity

should be evaluated in the model representation of stratosphere-troposphere coupling.

Plain Language Summary It is challenging to predict cold spells beyond 2weeks due to chaotic

weather systems. Taking into account stratospheric variability may push predictions beyond this limit. Previous

studies have suggested that a stratospheric circulation pattern is connected to surface temperature. Strong and

weak stratospheric wave events are defined based on the phase of this pattern. In observations, we find more

frequent cold spells over North America and less precipitation over the west coast before and around weak

wave events. On the other hand, strong wave events correspond to significant weather changes. Warmer weather

over North America and more precipitation over the west coast occur before strong wave events, while more

frequent cold spells over North America take place around 10days later. We reveal that the weather changes

during strong wave events correspond to a transition in the vertical structure of planetary waves. State-of-the-

art climate models capture the overall linkage between stratospheric wave events and surface conditions.

We further illustrate that the models incapable of simulating a realistic stratospheric wave pattern tend to

show larger biases in the troposphere during strong wave events. Our findings urge the need to improve the

stratospheric wave representation in climate models.

DING ETAL.

Stratosphere-Troposphere Coupling of Extreme Stratospheric

Wave Activity in CMIP6 Models

Xiuyuan Ding1 , Gang Chen1 , and Weiming Ma2

1Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA, 2Atmospheric Sciences

and Global Change Division, Pacific Northwest National Laboratory, Richland, WA, USA

Key Points:

• Increased risks of cold spells over

North America follow strong

stratospheric wave events or precede

weak stratospheric wave events

• The vertical wave coupling during

strong wave events features a shift

of vertical wave phase line from

westward tilt to eastward tilt

• Intermodel spread in stratospheric

wave structure correlates with

tropospheric circulation anomalies

during strong wave events

Supporting Information:

Supporting Information may be found in

the online version of this article.

Correspondence to:

X. Ding and G. Chen,

dingxy@ucla.edu;

gchenpu@ucla.edu

Citation:

Ding, X., Chen, G., & Ma, W. (2023).

Stratosphere-troposphere coupling

of extreme stratospheric wave

activity in CMIP6 models. Journal of

Geophysical Research: Atmospheres,

128, e2023JD038811. https://doi.

org/10.1029/2023JD038811

Received 2 MAR 2023

Accepted 4 AUG 2023

10.1029/2023JD038811

RESEARCH ARTICLE

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The possible influence of changes in the Arctic stratospheric polar vortex on the extratropical troposphere especially in the mid-to-high latitudes of the Northern Hemisphere is still not well understood. Using the ERA5 reanalysis and based on the k-mean clustering algorithm, the northern winter stratospheric polar vortex is categorized into several regimes, which mainly reflect the difference in the intensity and central position of the vortex. As a consequence, the stratospheric polar vortex can be clustered into six groups, including the homogeneously-intensified (HI), homogeneously-weakened (HW), North America-intensified (NAI), North America-weakened (NAW), Eurasia-intensified (EUI), and Eurasia-weakened (EUW) shapes. Statistics of each polar vortex clustering confirms that the yearly frequency of the HI state shows a decreasing trend in past decades, while the HW increases as inferred from the long-term trend. Further, the evolutions of the tropospheric circulation and climate anomalies are explored following each clustering. It is revealed that both the strength and central position of the stratospheric polar vortex significantly modulate the behavior of tropospheric circulation and near surface climate. The relationship between the stratospheric polar vortex regimes and the tropospheric teleconnections are examined. The conventional stratosphere–troposphere coupling via the downward propagation of the North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) signal is confirmed. Other tropospheric teleconnections are also associated with the stratospheric regimes. The Pacific-North American pattern (PNA) is well correlated with the shift of the stratospheric polar vortex, and the Eurasian pattern (EU) is sensitive to the HI and NAW patterns. The patterns of rainfall and temperature anomalies following the six stratospheric regimes are different.

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Natural variations in the strength of the northern stratospheric polar vortex, so-called polar vortex events, help to improve subseasonal-to-seasonal (S2S) predictions of winter climate. Past research on polar vortex events has been largely focused on sudden stratospheric warming events (SSWs), a class of relatively strong weakenings of the polar vortex. Commonly, SSWs are defined when the polar vortex reverses its climatological wintertime westerly wind direction. In this study, however, we use an alternative definition, based on the weighted time-integrated upward wave activity flux at the lower stratosphere. We use a long control simulation with a stratosphere-resolving model and the ERA5 reanalysis to compare various aspects of the wave activity definition with common SSWs over the Arctic. About half of the wave events are identical to common SSWs. However, there exist several advantages for defining stratospheric weak extremes based on wave events rather than using the common SSW definition: the wave activity flux definition captures with one criterion a variety of different event types, detects strong SSWs and strong final warming events, avoids weak SSWs that have little surface impact, and potentially lengthens the prediction horizon of the surface response. We therefore conclude that the wave driving represents a useful early indicator for stratospheric polar vortex events, which exploits the stratospheric potential for creating predictable surface signals better than common SSWs.

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Interannual variability of the winter AR activities over the Northern hemisphere is investigated. The leading modes of AR variability over the North Pacific and North Atlantic are first identified and characterized. Over the Pacific, the first mode is characterized by a dipole structure with enhanced AR frequency along the AR peak region at about 30° N and reduced AR frequency further north. The second mode exhibits a tri-pole structure with a narrow band of positive AR anomalies at about 30° N and sandwiched by negative anomalies. Over the Atlantic, the first mode exhibits an equatorward shift of the ARs with positive anomalies and negative anomalies located on the equatorward and poleward side of the AR peak region at about 40° N , respectively. The second mode is associated with the strengthening and eastward extension of the AR peak region which is sandwiched by negative anomalies. A large ensemble of atmospheric global climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6), which shows high skills in simulating these modes, is then used to quantify the roles of sea surface temperature (SST) forcing versus internal atmospheric variability in driving the formation of these modes. Results show that SST forcing explains about half of the variance for the Pacific leading modes, while that number drops to about a quarter for the Atlantic leading modes, suggesting higher predictability for the Pacific AR variability. Additional ensemble driven only by observed tropical SST is further utilized to demonstrate the more important role that tropical SST plays in controlling the Pacific AR variability while both tropical and extratropical SST exert comparable influences on the Atlantic AR variability.

Stratosphere‐Troposphere Coupling of Extreme Stratospheric Wave Activity in CMIP6 Models

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