Date of Award

8-1-2022

Language

English

Document Type

Master's Thesis

Degree Name

Master of Science (MS)

College/School/Department

Department of Atmospheric and Environmental Sciences

Content Description

1 online resource (v, 82 pages) : illustrations (some color), color maps.

Dissertation/Thesis Chair

Andrea Lang

Committee Members

Ryan Torn

Keywords

Cold Air Outbreak, Polar Vortex, Stratosphere, Sudden Stratospheric Warming, Stratospheric circulation, Earth temperature, Polar vortex

Subject Categories

Atmospheric Sciences | Meteorology

Abstract

Variability in high-latitude stratospheric flow, including major sudden stratospheric warming (SSW) or strong polar vortex events, can modulate tropospheric circulation and have meteorological implications on surface temperature and weather. The largest tropospheric impacts from Northern Hemisphere stratospheric variability – and the regions of focus in the literature – are observed in the North Atlantic, Europe and Asia. Impacts on North America occur but have received less research attention. The goal of this thesis research is to identify and quantify the relationship between high-latitude stratospheric variability and North American wintertime temperature.Using the ERA-Interim and ERA-5 reanalysis for the period of 1999/2000–2018/19, this thesis first identifies stratospheric variability from the commonly used zonal-mean perspective, but also considers variability in the geometry of the stratospheric polar vortex. Then using the zonal-mean and geometry-based diagnostics of stratospheric variability, the statistical relationships in the climatological patterns and responses of tropospheric temperature to stratospheric variability in the continental U.S. (CONUS) are analyzed. Stratospheric variability is considered using several metrics, including the commonly implemented metric of 10-hPa zonal-mean wind at 60˚N and the non-zonal geometric properties of the stratospheric polar vortex. The geometric properties of the stratospheric vortex – or elliptical diagnostics – are calculated using a best-fit ellipse applied to specific contours of the 50 and 10-hPa geopotential height. These elliptical diagnostics include metrics for the stratospheric polar vortex length, width, area, ratio (length/width), and orientation angle with respect to 0˚W longitude. Each of the stratospheric metrics is correlated to winter tropospheric temperature in the CONUS using a Kendall-Tau (K-T) correlation test, and linear regression theilslopes underscore the trends in tropospheric temperature dependent on stratospheric ellipse diagnostics. The period of interest includes SSW and strong vortex events, but also incorporates stratospheric vortex variability that is not categorized into these event types. This uncategorized stratospheric variability can be associated with temperature impacts in the U.S. (e.g., highly elliptical vortex conditions, wave reflection events, and minor stratospheric warming events). The elliptical diagnostics capture the continuous range in stratospheric variability and resolves events that are not described by zonal-mean conditions. Several representative examples of stratospheric variability (e.g., two SSW events, a strong vortex event, and a highly elongated vortex event) serve as case studies in explaining the patterns of K-T correlations and significant linear relationships between ellipse diagnostics and tropospheric temperature in the winter. The results of this research are discussed in the context of advancing predictability for temperature sensitive regions of the United States, such as the northern Great Plains or Ohio River Valley.

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