21 Jul 2022
21 Jul 2022
Status: this preprint is currently under review for the journal WCD.

Using large ensembles to quantify the impact of sudden stratospheric warmings on the North Atlantic Oscillation

Philip E. Bett1, Adam A. Scaife1,2, Steven C. Hardiman1, Hazel E. Thornton1, Xiaocen Shen3,4, Lin Wang3,4, and Bo Pang5 Philip E. Bett et al.
  • 1Met Office, FitzRoy Road, Exeter EX1 3PB, United Kingdom
  • 2College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, United Kingdom
  • 3Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
  • 4College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
  • 5State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029, China

Abstract. Sudden stratospheric warming events (SSWs) are often followed by significant weather and climate impacts at the surface. By affecting the North Atlantic Oscillation (NAO), SSWs can lead to periods of extreme cold in parts of Europe and North America. Previous studies have used observations and free-running climate models to try to identify features of the atmosphere prior to an SSW that can determine the subsequent impact at the surface. However, the limited observational record makes it difficult to accurately quantify these relationships. Here, we instead use a large ensemble of seasonal hindcasts. We first test whether the hindcasts reproduce the observed characteristics of SSWs and their surface signature. We find that the simulations are statistically indistinguishable from the observations, in terms of the overall risk of an SSW per winter (56 %), the frequency of SSWs with negative NAO responses (65 %), the magnitude of the NAO responses, and the frequency of wavenumber-2 dominated SSWs (26 %). We also assess the relationships between prior conditions and the NAO response following an SSW. We find that there is little information in the precursor state to guide differences in the subsequent NAO behaviour between one SSW and another, reflecting the substantial natural variability between SSW events. The strongest relationships with the NAO response are from pre-SSW sea level pressure anomalies over the polar cap, and from zonal wind anomalies in the lower stratosphere, both exhibiting correlations of around 0.3. The pre-SSW NAO has little bearing on its post-SSW state. The strength of the pre-SSW zonal wind anomalies at 10 hPa is also not significantly correlated with the NAO response. Finally, we find that there is no significant difference in the likelihood of a post-SSW negative NAO response between wave-1 and wave-2 dominated SSWs, although the latter result in a stronger negative NAO anomaly on average.

Philip E. Bett et al.

Status: open (until 01 Sep 2022)

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Philip E. Bett et al.

Philip E. Bett et al.


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Short summary
Sudden stratospheric warming events can severely affect the subsequent weather at the surface. We use a large ensemble of climate model hindcasts to investigate features of the climate that make strong impacts more likely, through negative NAO conditions. This allows a more robust assessment than using observations alone. Air pressure over the Arctic prior to an SSW, and the zonal mean zonal wind in the lower stratosphere, have the strongest relationship with the subsequent NAO response.