| 1. |
- Chiacchio, Marc, et al.
(författare)
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On the links between meteorological variables, aerosols, and tropical cyclone frequency in individual ocean basins
- 2017
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 122:2, s. 802-822
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Tidskriftsartikel (refereegranskat)abstract
- A generalized linear model based on Poisson regression has been used to assess the impact of environmental variables modulating tropical cyclone frequency in six main cyclone development areas: the East Pacific, West Pacific, North Atlantic, North Indian, South Indian, and South Pacific. The analysis covers the period 1980-2009 and focuses on widely used meteorological parameters including wind shear, sea surface temperature, and relative humidity from different reanalyses as well as aerosol optical depth for different compounds simulated by the Goddard Chemistry Aerosol Radiation and Transport model. Circulation indices are also included. Cyclone frequency is obtained from the International Best Track Archive for Climate Stewardship. A strong link is found between cyclone frequency and the relative sea surface temperature, Atlantic Meridional Mode, and wind shear with significant explained log likelihoods in the North Atlantic of 37%, 27%, and 28%, respectively. A significant impact of black carbon and organic aerosols on cyclone frequency is found over the North Indian Ocean, with explained log likelihoods of 27%. A weaker but still significant impact is found for observed dust aerosols in the North Atlantic with an explained log likelihood of 11%. Changes in lower stratospheric temperatures explain 28% of the log likelihood in the North Atlantic. Lower stratospheric temperatures from a subset of Coupled Model Intercomparison Project Phase 5 models properly simulate the warming and subsequent cooling of the lower stratosphere that follows a volcanic eruption but underestimates the cooling by about 0.5 degrees C.
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| 2. |
- 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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| 3. |
- 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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| 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. |
- Ge, ShuCan, et al.
(författare)
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Characteristic analysis of layered PMSEs measured with different elevation angles at VHF based on an experimental case
- 2021
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Ingår i: Earth and Planetary Physics. - : Earth and Planetary Physics. - 2096-3955. ; 5:1, s. 42-51
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Tidskriftsartikel (refereegranskat)abstract
- Polar Mesosphere Summer Echoes (PMSEs) are very strong radar echoes observed at altitudes near the polar summer mesopause. One of the essential properties of these radar echoes is that they can give useful diagnostic information about the physics of the scattering process. In this paper, the related characteristics of PMSEs measured with the European Incoherent SCATter Very High Frequency (EISCAT VHF) 224 MHz radar on 13-15 July 2010 are studied at different elevation angles from 78 degrees to 90 degrees. It is found that the PMSEs peak power and strongest PMSEs average power occur at the same elevation angles. Also interesting is that the strongest PMSEs occur at off-vertical angles when a PMSEs has a layered (multilayer) structure. And reflection may have more significant effects on PMSEs when there are double or multilayer PMSEs. Possible explanations regarding these observations are discussed.
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| 6. |
- 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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| 7. |
- Hannachi, Abdel, et al.
(författare)
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On the Nonlinearity of Winter Northern Hemisphere Atmospheric Variability
- 2019
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 76:1, s. 333-356
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Tidskriftsartikel (refereegranskat)abstract
- Nonlinearity in the Northern Hemisphere’s winter time atmospheric flow is investigated from both an intermediate complexity model of the extratropics and reanalyses. A long simulation is obtained using a three-level quasi-geostrophic model on the sphere. Kernel empirical orthogonal functions (EOFs), which help delineate complex structures, are used along with the local flow tendencies. Two fixed points are obtained, which are associated with strong bimodality in two-dimensional kernel PC space in consistency with conceptual low-order dynamics. The regimes reflect zonal and blocked flows. The analysis is then extended to ERA-40 and JRA-55 reanalyses using daily sea level pressure (SLP) and geopotential heights in the stratosphere (20-hPa) and troposphere (500-hPa). In the stratosphere, trimodality is obtained, representing disturbed, displaced and undisturbed states of the winter polar vortex. In the troposphere the probability density functions (PDFs), for both fields, within the two-dimensional (2D) kernel EOF space are strongly bimodal. The modes correspond broadly to opposite phases of the Arctic Oscillation with signature of negative North Atlantic Oscillation (NAO). Over the North Atlantic/European sector a trimodal PDF is also obtained with two strong and one weak modes. The strong modes are associated, respectively, with the north (or +NAO) and south (or –NAO) positions of the eddy-driven jet strteam. The third weak mode is interpreted as a transition path between the two positions. A climate change signal is also observed in the troposphere of the winter hemisphere, resulting in an increase (decrease) in the frequency of the polar high (low) consistent with an increase of zonal flow frequency.
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| 8. |
- Hannachi, Abdel
(författare)
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Regularised empirical orthogonal functions
- 2016
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 68
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Tidskriftsartikel (refereegranskat)abstract
- Empirical orthogonal functions, extensively used in weather/climate research, suffer serious geometric drawbacks such as orthogonality in space and time and mixing. The present paper presents a different version, the regularised (or smooth) empirical orthogonal function (EOF) method, by including a regularisation constraint, which originates from the field of regression/correlation of continuous variables. The method includes an extra unknown, the smoothing parameter, and solves a generalised eigenvalue problem and can overcome, therefore, some shortcomings of EOFs. For example, the geometrical constraints satisfied by conventional EOFs are relaxed. In addition, the method can help alleviate the mixing drawback. It can also be used in combination with other methods, which are based on downscaling or dimensionality reduction. The method has been applied to sea level pressure and sea surface temperature and yields an optimal value of the smoothing parameter. The method shows, in particular, that the leading sea level pressure pattern, with substantially larger explained variance compared to its EOF counterpart, has a pronounced Arctic Oscillation compared to the mixed North Atlantic Oscillation/Arctic Oscillation pattern of the leading EOF. The analysis of the remaining leading patterns and the application to sea surface temperature field and trend EOFs are also discussed.
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| 10. |
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