Dependence of Sudden Stratospheric Warmings on Internal and External Drivers

Abstract

Plain Language Summary
During some winters the cold polar stratosphere experiences a strong and sudden warming. These sudden stratospheric warmings (SSW) can affect greatly the surface weather in northern Europe and in North America. However, the factors that contribute to the formation of sudden warmings are not entirely known. We study how the two independent solar‐related factors, energetic particles and solar irradiance, and two atmospheric internal factors, the wind in the equatorial stratosphere (QBO) and the weather system of the Pacific (ENSO), affect the occurrence of sudden warmings in the Northern Hemisphere. We confirm the earlier finding that sudden stratospheric warmings are more common in winters with an easterly QBO wind. We find that the QBO effect on SSW occurrence depends on the two solar‐related factors and ENSO. Additionally, we find that the occurrence of sudden stratospheric warmings is affected by energetic particles precipitating to the Earth's atmosphere. Sudden warmings happen more often if the number of energetic particles is small. This effect is especially clear if the QBO wind is easterly. Our study helps to understand in which circumstances sudden stratospheric warmings are more or less likely to form. This information can benefit the forecasting of northern wintertime weather.

Full text

Dependence of Sudden Stratospheric Warmings
on Internal and External Drivers
A. Salminen1, T. Asikainen1, V. Maliniemi2, and K. Mursula1
1ReSoLVE Centre of Excellence, Space Climate Research Unit, University of Oulu, Oulu, Finland, 2Birkeland Centre for
Space Science, Department of Physics and Technology, University of Bergen, Bergen, Norway
Abstract A sudden stratospheric warming (SSW) is a large-scale disturbance of the wintertime
stratosphere, which occurs especially in the Northern Hemisphere. Earlier studies have shown that SSW
occurrence depends on atmospheric internal factors and on solar activity. We examine SSW occurrence in
northern winters 1957/1958–2016/2017, considering several factors that may affect the SSW occurrence:
Quasi-Biennial Oscillation (QBO), El Niño–Southern Oscillation (ENSO), geomagnetic activity, and solar
radiation. We confirm the well-known result that SSWs occur more often in easterly QBO phase than in
westerly phase. We show that this difference depends on how the QBO phase is determined. We find that
the difference in SSW occurrence between easterly and westerly QBO winters strengthens (weakens) if
geomagnetic activity or solar activity is low (high), or if the ENSO is in a cold (warm) phase. In easterly
QBO phase significantly more SSWs occur during low geomagnetic activity than high activity.
Plain Language Summary During some winters the cold polar stratosphere experiences a
strong and sudden warming. These sudden stratospheric warmings (SSW) can affect greatly the surface
weather in northern Europe and in North America. However, the factors that contribute to the formation
of sudden warmings are not entirely known. We study how the two independent solar-related factors,
energetic particles and solar irradiance, and two atmospheric internal factors, the wind in the equatorial
stratosphere (QBO) and the weather system of the Pacific (ENSO), affect the occurrence of sudden
warmings in the Northern Hemisphere. We confirm the earlier finding that sudden stratospheric warmings
are more common in winters with an easterly QBO wind. We find that the QBO effect on SSW occurrence
depends on the two solar-related factors and ENSO. Additionally, we find that the occurrence of sudden
stratospheric warmings is affected by energetic particles precipitating to the Earth's atmosphere. Sudden
warmings happen more often if the number of energetic particles is small. This effect is especially clear
if the QBO wind is easterly. Our study helps to understand in which circumstances sudden stratospheric
warmings are more or less likely to form. This information can benefit the forecasting of northern
wintertime weather.
1. Introduction
A dominant pattern in the wintertime stratosphere is the polar vortex, a strong thermal westerly wind sur-
rounding the cold pole. The polar vortex experiences large variability during winter and between winters.
Planetary waves originating from the tropospheric wind patterns can propagate into the stratosphere if the
stratospheric wind is westerly (Charney & Drazin, 1961), depositing their zonal momentum and thereby
decelerating the polar vortex. Planetary wave activity is higher in the Northern Hemisphere than in the
Southern Hemisphere because of larger mountain ranges and continent-ocean temperature contrasts (van
Loon et al., 1973). Thus, the northern polar vortex is much more variable than its southern counterpart
(Waugh & Randel, 1999). In the extreme cases planetary waves can initiate a temporary reversal and collapse
of the polar vortex. Such events, called sudden stratospheric warmings (SSW), are accompanied by strongly
enhanced downwelling of air within the polar vortex and associated warming of the polar stratosphere
(Dunkerton et al., 1981; Matsuno, 1971). A midwinter SSW is a common event in the Northern Hemisphere,
occurring approximately six times in a decade (Polvani et al., 2017). The polar vortex eventually breaks down
during spring in a so-called final warming.
The occurrence of SSWs depends on the flux of planetary waves into the stratosphere and the state of the
polar vortex (e.g., de la Cámara et al., 2017; Matsuno, 1971; Scott & Polvani, 2004). Their conditions can be
RESEARCH LETTER
10.1029/2019GL086444
Key Points:
• SSWs occur more often in QBO-E
than QBO-W winters with the largest
difference found if the QBO at 30 hPa
of preceding autumn is used
• QBO effect on SSW occurrence
strengthens (weakens) if
geomagnetic or sunspot activity is
low (high) or if the ENSO is in cold
(warm) phase
• The highest SSW occurrence
rate is found in winters with
low geomagnetic activity and
easterly QBO
Correspondence to:
A. Salminen,
antti.salminen@oulu.fi
Citation:
Salminen, A., Asikainen, T., Malin-
iemi, V., & Mursula, K. (2020).
Dependence of sudden stratospheric
warmings on internal and external
drivers. Geophysical Research Letters,
47, e2019GL086444. https://doi.org/
10.1029/2019GL086444
Received 27 NOV 2019
Accepted 27 FEB 2020
Accepted article online 29 FEB 2020
©2020. American Geophysical Union.
All Rights Reserved.
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