| 1. |
- Finke, Kathrin, et al.
(författare)
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Exceptionally persistent Eurasian cold events and their stratospheric link
- 2023
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Ingår i: Asia-Pacific Journal of Atmospheric Sciences. - : Springer Science and Business Media LLC. - 1976-7633 .- 1976-7951. ; 59:1, s. 95-111
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Finke, Kathrin, 1991-
(författare)
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Northern Hemispheric Cold Spells and their Tropospheric-Stratospheric Link
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 3. |
- Finke, Kathrin, et al.
(författare)
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Revisiting remote drivers of the 2014 drought in South-Eastern Brazil
- 2020
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 55, s. 3197-3211
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Tidskriftsartikel (refereegranskat)abstract
- South-Eastern Brazil experienced a devastating drought associated with significant agricultural losses in austral summer 2014. The drought was linked to the development of a quasi-stationary anticyclone in the South Atlantic in early 2014 that affected local precipitation patterns over South-East Brazil. Previous studies have suggested that the unusual blocking was triggered by tropical Pacific sea surface temperature (SST) anomalies and, more recently, by convection over the Indian Ocean related to the Madden-Julian Oscillation. Further investigation of the proposed teleconnections appears crucial for anticipating future economic impacts. In this study, we use numerical experiments with an idealized atmospheric general circulation model forced with the observed 2013/2014 SST anomalies in different ocean basins to understand the dominant mechanism that initiated the 2014 South Atlantic anticyclonic anomaly. We show that a forcing with global 2013/2014 SST anomalies enhances the chance for the occurrence of positive geopotential height anomalies in the South Atlantic. However, further sensitivity experiments with SST forcings in separate ocean basins suggest that neither the Indian Ocean nor tropical Pacific SST anomalies alone have contributed significantly to the anomalous atmospheric circulation that led to the 2014 South-East Brazil drought. The model study rather points to an important role of remote forcing from the South Pacific, local South Atlantic SSTs, and internal atmospheric variability in driving the persistent blocking over the South Atlantic.
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| 4. |
- Finke, Kathrin, et al.
(författare)
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Tropospheric Response to Stratospheric Variability via Lagged Quantile Regression
- 2022
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 49:15
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Hannachi, Abdel, 1961-, et al.
(författare)
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Common EOFs : a tool for multi-model comparison and evaluation
- 2023
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 60:5-6, s. 1689-1703
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
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