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Northern Hemispheric Cold Spells and their Tropospheric-Stratospheric Link
Stockholm University, Faculty of Science, Department of Meteorology .ORCID iD: 0000-0002-3636-9622
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Cold spells have severe consequences for society. They require early warnings for elaborate mitigation strategies on sub-seasonal to seasonal time-scales. Intense stratospheric westerlies and a polar vortex breakdown (SSW) may enhance extended-range forecast skill for Eurasian and North American cold extremes through a dynamic coupling to the troposphere. Understanding the complex interplay remains a challenging task that requires further investigation.

Since fine-grained observational stratospheric data is limited to the satellite era, climate model simulations, such as atmosphere-only simulations (AMIP) from the Coupled Model Intercomparison Project Phase 6, can be considered. Application of the common empirical orthogonal function method in Paper II, a tool for multimodel comparison and evaluation, unveiled differences in daily winter 2m temperatures (T2m) across four reanalyses while stratospheric geopotential height varies across AMIP models. Results show a link between a weak polar vortex and cold T2m anomalies over Eurasia in reanalysis data.

In addition, quantile regression is a simple but proficient statistical method that neatly enables modeling the response variable’s complete conditional distribution. Thereby, information about extremes, which hide in the distribution’s tails, is extracted. Application to boreal winter ERA5 reanalysis data and teleconnection indices in Paper I reveals significant asymmetries in duration, strength, and direction of the stratosphere-troposphere connection across quantiles. 

Regionally specific, lagged composite analysis of ERA5 data in Paper III verifies the canonical warm stratosphere - cold Eurasia relation. However, persistent Eurasian cold spells may also coincide with a strong polar vortex. We find stratospheric reflection of upward propagating planetary waves toward the North Atlantic to potentially influence mid-tropospheric circulation anomalies that travel towards Eurasia. By interacting with a quasi-stationary anticyclone over the Barents Sea, which promotes a cold Eurasia, these circulation anomalies likely influence the persistence and strength of the cold spell.

Paper IV discusses the relationship between the 2018/2019 winter SSW and the subsequent North American cold spell using the JRA-55 reanalysis. An unusual wave number 3 planetary wave pulse in the stratosphere led to a polar vortex split. Further, wave reflection at the stratospheric Aleutian high likely fostered the circulation configuration, i.e., positive North Pacific and negative North American geopotential height anomalies that facilitated the cold temperatures.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2024. , p. 30
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-224415ISBN: 978-91-8014-629-6 (print)ISBN: 978-91-8014-630-2 (electronic)OAI: oai:DiVA.org:su-224415DiVA, id: diva2:1820341
Public defence
2024-02-16, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2024-01-24 Created: 2023-12-18 Last updated: 2024-01-16Bibliographically approved
List of papers
1. Tropospheric Response to Stratospheric Variability via Lagged Quantile Regression
Open this publication in new window or tab >>Tropospheric Response to Stratospheric Variability via Lagged Quantile Regression
2022 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 49, no 15, article id e2022GL099231Article in journal (Refereed) Published
Abstract [en]

Stratospheric variability may affect tropospheric weather on time scales between a few weeks and a few months, thus providing great potential for sub-seasonal to seasonal weather forecasts of extremes. Unlike standard linear regression, which is based on the conditional mean of the response variable, lagged quantile regression is used here, which enables modeling the response variable's complete conditional distribution. Using the ERA5 reanalysis, we explore the relation of prominent teleconnection indices and gridded tropospheric fields to the stratosphere during boreal winter for various quantiles and lags. Significant differences in duration, strength, and direction of the stratosphere-troposphere connection in outer quantiles compared to the median are identified. In particular, tropospheric extremes and their relation to the polar vortex strength are discussed, and an outlook is given.

Keywords
surface temperature extremes, lagged response, stratosphere-troposphere interaction, quantile regression
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-209481 (URN)10.1029/2022GL099231 (DOI)000838384300001 ()2-s2.0-85135915371 (Scopus ID)
Available from: 2022-09-21 Created: 2022-09-21 Last updated: 2023-12-18Bibliographically approved
2. Common EOFs: a tool for multi-model comparison and evaluation
Open this publication in new window or tab >>Common EOFs: a tool for multi-model comparison and evaluation
2023 (English)In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 60, no 5-6, p. 1689-1703Article in journal (Refereed) Published
Abstract [en]

With the increase in the volume of climate model simulations for past, present and future climate, from various institutions across the globe, there is a need for efficient and robust methods for model comparison and/or evaluation. This manuscript discusses common empirical orthogonal function analysis with a step-wise algorithm, which can be used for the above objective. The method looks for simultaneous diagonalisation of several covariance matrices in a step-wise fashion ensuring thus simultaneous monotonic decrease of the eigenvalues in all groups, and allowing therefore for dimension reduction. The method is applied to a number of tropospheric and stratospheric fields from the main four reanalysis products, and also to several historical climate model simulations from CMIP6, the Coupled Model Intercomparison Project (Phase 6). Monthly means as well as winter daily gridded data are considered over the Northern Hemisphere. The method shows consistency between mass fields as well as mid-tropospheric and stratospheric fields of the reanalyses, but also reveals significant differences in the 2 m surface-air temperature in terms of explained variance. CMIP6 models, on the other hand, show differences reflected in the percentage of explained variance of the leading common EOFs with inter-group variation ranging from 5–10% in the troposphere to about 25% in the stratosphere. Higher order statistics within the leading common modes of variability, in addition to further merits of the method are also discussed. 

Keywords
Common EOF, Model comparison, CMIP6
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-208411 (URN)10.1007/s00382-022-06409-8 (DOI)000825205500002 ()2-s2.0-85134293623 (Scopus ID)
Available from: 2022-08-29 Created: 2022-08-29 Last updated: 2023-12-18Bibliographically approved
3. Exceptionally persistent Eurasian cold events and their stratospheric link
Open this publication in new window or tab >>Exceptionally persistent Eurasian cold events and their stratospheric link
2023 (English)In: Asia-Pacific Journal of Atmospheric Sciences, ISSN 1976-7633, Vol. 59, no 1, p. 95-111Article in journal (Refereed) Published
Abstract [en]

Persistent boreal winter cold spells (PCEs) can heavily strain the economy and significantly impact everyday life. While sudden stratospheric warmings are considered a precursor for Eurasian (EUR) cold events, these temperature extremes may occur during the full range of stratospheric variability. We investigate PCEs relative to the prevailing stratospheric polar vortex regime before their onset, with a particular focus on extremely weak (SSW) and strong (SPV) stratospheric winds by performing (lagged) composite analysis based on ERA5 reanalysis. On average, SPV PCEs that are concentrated over central EUR, are colder, shorter and set in more abruptly compared to SSW PCEs. A quasi-stationary, mid-tropospheric anticyclone over the Arctic Ocean that blocks warm air advection toward EUR is connected to the canonical downward progression of the negative North Atlantic Oscillation for SSW PCEs. In contrast, during SPV PCEs, the anticyclone is part of a Rossby wave having an origin co-located with negative wave activity flux anomalies over and being influenced by stratospheric wave reflection toward the North Atlantic. Its slow east-ward propagation is likely related to Arctic surface warming and unusually weak zonal winds over EUR. 

Keywords
Cold spells, Sudden stratospheric warming, Eurasia, Wave activity fluxes, Blocking, Extreme events
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-215152 (URN)10.1007/s13143-022-00308-y (DOI)000920184500001 ()2-s2.0-85146218833 (Scopus ID)
Available from: 2023-03-02 Created: 2023-03-02 Last updated: 2023-12-18Bibliographically approved
4. The stratospheric polar vortex and surface effects: The case of the North American 2018/19 cold winter
Open this publication in new window or tab >>The stratospheric polar vortex and surface effects: The case of the North American 2018/19 cold winter
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography; Meteorology
Identifiers
urn:nbn:se:su:diva-224416 (URN)
Available from: 2023-12-11 Created: 2023-12-11 Last updated: 2023-12-18

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