Master thesis defense by Luwei Shen

Title: The Connection between Stratospheric Sudden Warming and Tropospheric Blocking Events

Abstract: Characteristics of stratospheric sudden warming(SSW) events and their relationships with tropospheric blocking events during 1979-2019 are studied using the ERA-Interim reanalysis dataset. Two kinds of methods are applied to identify and classify the sudden warming events. After the integration of the results, a total of 28 events were detected with 19 of them belonging to the displacement type and others belonging to the split type. Displacement type events were further divided into three sub-types West European(WE), North America(NA) and Atlantic Basin(AB), events belong to the split type were further divided into two subtypes Eurasian-North America(E-NA) and Atlantic-East Asian(A-EA) according to different distributions of the polar vortex. Composite analyses in the stratosphere are performed to analyze systems disturbing the polar vortex. Results indicate that polar vortex of displacement SSWs are either influenced by one high-pressure system or two adjacent high-pressure systems while the movement of the polar vortex in split events is controlled by either one or two high-pressure systems that are not adjacent in the stratosphere. Activities of polar vortex in WE could be ascribed by activities of the high-pressure system in North American; For NA, the polar vortex is disturbed by two adjacent high-pressure systems in the Asian continent; The movement of the polar vortex in AB is influenced by high-pressure systems in Aleutian Island and Central Asia; The high pressure system in the Aleutian Island and the positive anomalies in the Atlantic basin were critical for the evolution of the polar vortex in E-NA. High-pressure systems in North America and Eurasian continent play an important role in A-EA.These SSWs are preceded by different blocking systems. For the events belonging to the displacement type, there are mainly two blocking systems distributed in the Euro-Atlantic region and Pacific-North American region separately. Various patterns of the evolution for the polar vortex in the stratosphere are related to geopotential height patterns with different intensities and/or different building orders of blocking systems in these two regions. There are blockings in the Eurasian continent and the west coast of North America preceding E-NA, and blockings in the Ural Mountain regions and the North American continent preceding A-EA. The effect of blocking on SSWs is achieved by controlling the upward transmission of EP flux. Developing of the blocking corresponds to gradually increasing upward EP-flux transmissions, the poleward EP-flux transportation will lead to the deceleration of the zonal mean wind and breakdown of the stratospheric polar vortex.

Supervisors:
Eigil Kaas (NBI)
External supervisors:  Bo Christiansen and Shuting Yang (DMI)

Censor:
Peter Aakjær