Stratosphere-troposphere coupling and its impacts on Northern winter subseasonal-to-seasonal forecast uncertainty
Skill of mid-latitude weather predictions largely decays within the first two forecast weeks. Nonetheless, remote coupling with slower-evolving Earth system components can provide predictability beyond the local initial condition memory, at the so-called subseasonal-to-seasonal (S2S) timescale, whic...
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| Format: | Thesis |
| Language: | unknown |
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Ludwig-Maximilians-Universität München
2024
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| Online Access: | https://edoc.ub.uni-muenchen.de/34210/1/Spaeth_Jonas.pdf http://nbn-resolving.de/urn:nbn:de:bvb:19-342108 |
| Summary: | Skill of mid-latitude weather predictions largely decays within the first two forecast weeks. Nonetheless, remote coupling with slower-evolving Earth system components can provide predictability beyond the local initial condition memory, at the so-called subseasonal-to-seasonal (S2S) timescale, which spans approximately from two weeks to two months. This thesis delves into a key source of S2S predictability for mid-latitude weather: stratosphere-troposphere coupling, specifically through the strong circumpolar winds in the Arctic stratosphere known as the polar vortex. Studies of stratosphere-troposphere coupling are often constrained by the scarcity of extreme events in observational records, especially for sudden stratospheric warmings (SSWs), which occur approximately once every second winter. Leveraging a large set of state-of-the-art S2S ensemble forecasts, this thesis improves the statistical characterization of SSWs by employing a sample size roughly 150 times larger than what is available from observations. This approach allows to quantify aspects of stratosphere-troposphere coupling that would be unfeasible to derive from pure observations. Here, this set of SSWs is used to compute the fraction of attributable risk, which demands particularly large statistics. First, it is found that approximately every second split-type SSW and every third displacement-type SSW may be attributable to corresponding wave activity events of tropospheric origin, underscoring that SSW formation relies on suitable conditions in both troposphere and stratosphere. Second, the impact of weak and strong stratospheric polar vortex events on the frequency of large-scale circulation extremes in the troposphere – characterized by the Arctic Oscillation (AO) index – is investigated. The results show that SSWs increase the likelihood of negative AO extremes (exceeding 3 standard deviations) by about 60%, which is expected to also translate into regional weather extremes with potentially high socio-economic impacts. In turn, ... |
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