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Analysis of the variability of the North Atlantic eddy-driven jet stream in CMIP5
Stockholm University, Faculty of Science, Department of Meteorology .ORCID iD: 0000-0003-0698-2677
Stockholm University, Faculty of Science, Department of Meteorology .
Number of Authors: 32018 (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.

Place, publisher, year, edition, pages
2018. Vol. 51, no 1-2, p. 235-247
Keywords [en]
North Atlantic jet, CMIP5, evaluation, Jet variability
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-158384DOI: 10.1007/s00382-017-3917-1ISI: 000435522000014OAI: oai:DiVA.org:su-158384DiVA, id: diva2:1237909
Available from: 2018-08-10 Created: 2018-08-10 Last updated: 2018-11-27Bibliographically approved
In thesis
1. On atmospheric low frequency variability, teleconnections and link to jet variability
Open this publication in new window or tab >>On atmospheric low frequency variability, teleconnections and link to jet variability
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:nbn:se:su:diva-162321 (URN)978-91-7797-518-2 (ISBN)978-91-7797-519-9 (ISBN)
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

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