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On atmospheric low frequency variability, teleconnections and link to jet variability
Stockholm University, Faculty of Science, Department of Meteorology . (Climate Dynamics)ORCID iD: 0000-0003-0698-2677
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The atmosphere is a complex system with an infinite number of independent variables. The best approximations of the atmosphere are made using numerical models. The use of such models provides an invaluable tool for studying the atmospheric system. In the atmosphere, narrow bands of strong winds at upper levels, called jet streams, impact the underlying large-scale weather conditions. In this Ph.D. thesis, I have studied jet stream variability from reanalyses and climate models. The regional climate model RCA4 simulations over South Asia reveal a good agreement between model results and reanalysis for jet stream representation. Lateral boundary data sources are believed to contribute to discrepancies over the mountainous regions.

Currently, the weather forecasts have an upper limit of around 10 days. The atmospheric variability between 10 to 40 days is known as low frequency variability (LFV). This Ph.D. thesis also examined the LFV from a non-linear perspective, which indicated the existence of multiple recurring atmospheric conditions. The North Atlantic eddy-driven jet, which explains a major part of the winter variability over the North Atlantic region, has three preferred latitudinal positions situated south, closest to, and north of its climatological mean position. These positions represent, respectively, Greenland blocking, a low-pressure system over the North Atlantic, and a high-pressure system over the North Atlantic. An improved representation of this jet is reported from CMIP5 GCMs. However, the existence of three preferred latitudinal positions remains a challenge for these models.

The statistical properties of recurring atmospheric conditions can potentially enhance current weather and climate predictions. Techniques from dynamical system theory, like unstable periodic orbits, can be employed to reconstruct such statistical properties. This has been demonstrated, for the first time, in a three-level baroclinic model, of intermediate complexity, for the Northern Hemisphere winter.

In the Northern Hemisphere winter, there are times when the stratosphere gets warmer due to upward propagation of heat fluxes from the troposphere. This type of situation triggers a major sudden stratospheric warming, resulting in the equatorward shift of the jet streams and yielding much colder than usual surface conditions over the extratropics. I have studied thirty such events from the Japanese reanalysis data in relation to the three preferred latitudinal positions of the North Atlantic eddy-driven jet. The probability of strong upward propagation from the troposphere is significantly higher for the central position of the North Atlantic eddy-driven jet. These findings can potentially improve the troposphere-stratosphere predictions.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2019. , p. 34
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-162321ISBN: 978-91-7797-518-2 (print)ISBN: 978-91-7797-519-9 (electronic)OAI: oai:DiVA.org:su-162321DiVA, id: diva2:1266172
Public defence
2019-01-10, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Manuscript.

Available from: 2018-12-18 Created: 2018-11-27 Last updated: 2018-12-12Bibliographically approved
List of papers
1. Mean climate and representation of jet streams in the CORDEX South Asia simulations by the regional climate model RCA4
Open this publication in new window or tab >>Mean climate and representation of jet streams in the CORDEX South Asia simulations by the regional climate model RCA4
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2017 (English)In: Journal of Theoretical and Applied Climatology, ISSN 0177-798X, E-ISSN 1434-4483, Vol. 129, no 1-2, p. 1-19Article in journal (Refereed) Published
Abstract [en]

A number of simulations with the fourth release of the Rossby Center Regional Climate Model (RCA4) conducted within the COordinated Regional climate Downscaling EXperiment (CORDEX) framework for South Asia at 50 km horizontal resolution are evaluated for mean winter (December-March) and summer (June-September) climate during 1980-2005. The two driving data sets ERA-Interim reanalysis and the general circulation model EC-Earth have been analyzed besides the RCA4 simulations to address the added value. RCA4 successfully captures the mean climate in both the seasons. The biases in RCA4 appear to come from the driving data sets which are amplified after downscaling. The jet streams influencing the seasonal precipitation variability in both seasons are also analyzed. The spatial and quantitative analysis over CORDEX South Asia generally revealed the ability of RCA4 to capture the mean seasonal climate as well as the position and strength of the jet streams despite weak/strong jet representation in the driving data. The EC-Earth downscaled with RCA4 exhibited cold biases over the domain and a weak Somali jet over the Arabian Sea. Moreover, the moisture transport from the Arabian Sea during summer is pronounced in RCA4 simulations resulting in enhanced monsoon rainfall over northwestern parts of India. Both the Somali jet and the tropical easterly jet become stronger during strong summer monsoon years. However, there is robust impact of wet years in summer over the Somali jet. Wet-minus-dry composites in winter indicate strengthening (weakening) of the subtropical jet in RCA4 run by ERA-Interim (EC-Earth). The driving data have clear reflections on the RCA4 simulations.

National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-145361 (URN)10.1007/s00704-016-1755-4 (DOI)000403666400001 ()
Available from: 2017-07-26 Created: 2017-07-26 Last updated: 2018-11-27Bibliographically approved
2. Analysis of the variability of the North Atlantic eddy-driven jet stream in CMIP5
Open this publication in new window or tab >>Analysis of the variability of the North Atlantic eddy-driven jet stream in CMIP5
2018 (English)In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 51, no 1-2, p. 235-247Article in journal (Refereed) Published
Abstract [en]

The North Atlantic eddy-driven jet is a dominant feature of extratropical climate and its variability is associated with the large-scale changes in the surface climate of midlatitudes. Variability of this jet is analysed in a set of General Circulation Models (GCMs) from the Coupled Model Inter-comparison Project phase-5 (CMIP5) over the North Atlantic region. The CMIP5 simulations for the 20th century climate (Historical) are compared with the ERA40 reanalysis data. The jet latitude index, wind speed and jet persistence are analysed in order to evaluate 11 CMIP5 GCMs and to compare them with those from CMIP3 integrations. The phase of mean seasonal cycle of jet latitude and wind speed from historical runs of CMIP5 GCMs are comparable to ERA40. The wind speed mean seasonal cycle by CMIP5 GCMs is overestimated in winter months. A positive (negative) jet latitude anomaly in historical simulations relative to ERA40 is observed in summer (winter). The ensemble mean of jet latitude biases in historical simulations of CMIP3 and CMIP5 with respect to ERA40 are and respectively. Thus indicating improvements in CMIP5 in comparison to the CMIP3 GCMs. The comparison of historical and future simulations of CMIP5 under RCP4.5 and RCP8.5 for the period 2076-2099, shows positive anomalies in the jet latitude implying a poleward shifted jet. The results from the analysed models offer no specific improvements in simulating the trimodality of the eddy-driven jet.

Keywords
North Atlantic jet, CMIP5, evaluation, Jet variability
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-158384 (URN)10.1007/s00382-017-3917-1 (DOI)000435522000014 ()
Available from: 2018-08-10 Created: 2018-08-10 Last updated: 2018-11-27Bibliographically approved
3. Troposphere-stratosphere dynamical coupling in regard to the North Atlantic eddy-driven jet variability
Open this publication in new window or tab >>Troposphere-stratosphere dynamical coupling in regard to the North Atlantic eddy-driven jet variability
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2018 (English)In: Journal of the Meteorological Society of Japan, ISSN 0026-1165Article in journal (Refereed) Submitted
Abstract [en]

The interaction between the troposphere and the stratosphere has attracted the attention of climate scientists for several decades not least for the benefit it has on understanding dynamical processes and predictability. This interaction has been revived recently in regard to downward disturbance propagation effects. The current study investigates such interactions over the North Atlantic region in relation to the eddy-driven jet stream. The winter variability of the North Atlantic sector is mainly associated to variations in the latitudinal positions of the North Atlantic eddy-driven jet stream. The Japanese Reanalysis data has been used to analyse the jet latitude statistics. The results reveal robust trimodality of the North Atlantic jet reflecting the latitudinal i.e. northern, central and southern positions in agreement with similar reanalysis products. Thirty major sudden stratospheric warming events were analysed in relation to the three modes or regimes of the eddy-driven jet. The frequency of occurrence of the eddy-driven jet to be in a specific latitudinal position is strongly effected by the amplitude of the upward wave propagation. The stratospheric polar vortex experiences significant changes via upward wave propagation associated to38the jet positions. It is found that when the jet is close to its central mode the wave propagation of zonal wave number 2 from the troposphere to the stratosphere is significantly high. Eliassen-Palm fluxes from all waves and zonal wave number 1 depict deceleration of the stratospheric polar vortex for the eddy-driven jet with latitudinal position close to the northern mode. Plumb wave activity variations originate mainly in the Atlantic sector for the North Atlantic eddy-driven jet states. These significant associations between preferred latitudinal positions of the North Atlantic eddy-driven jet and the stratospheric dynamics may lead to improved predictability.

Keywords
North Atlantic eddy-driven jet, sudden stratospheric warming, JRA-55, jet latitude index, wave activity, troposphere-stratosphere coupling
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-162316 (URN)
Available from: 2018-11-24 Created: 2018-11-24 Last updated: 2018-11-27Bibliographically approved
4. Dynamical behavior of T21QG model and Unstable Periodic Orbits (UPOs)
Open this publication in new window or tab >>Dynamical behavior of T21QG model and Unstable Periodic Orbits (UPOs)
(English)Manuscript (preprint) (Other academic)
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-162320 (URN)
Available from: 2018-11-24 Created: 2018-11-24 Last updated: 2018-11-27Bibliographically approved

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