Date of Award

1-1-2020

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 (xii, 71 pages) : color illustrations, color maps.

Dissertation/Thesis Chair

Paul E Roundy

Committee Members

Kristen L Corbosiero

Keywords

extratropical, intraseasonal, MJO, NAO, phase speed, subseasonal, Madden-Julian oscillation, Tropospheric circulation, Winds, Jet stream

Subject Categories

Atmospheric Sciences | Meteorology

Abstract

Differences in the Madden–Julian oscillation (MJO) phase speed help explain some of the variability in the global circulation patterns associated with the MJO. Select mechanisms that identify these distinctive circulation signals are applied to help explain MJO variability associated with different phase speeds. Wavelet filter analysis is first used to compute regression models of the MJO at different phase speeds, isolating the associated patterns in outgoing longwave radiation (OLR), geopotential height, and upper-level wind. Covariance matrices provide a deeper understanding of how the tropical and extratropical patterns compare between two phase speeds at any point in time. Both Fourier and wavelet-filtered data are then used to calculate the upper- level background wind as a function of regressed advection of the subseasonal wind by the mean wind and the gradient of the subseasonal wind. This reconstructed background wind highlights two primary regions of variability when the MJO is located in the Central Indian Ocean: (1) downwind of the convection over the Maritime Continent, where upper-tropospheric easterlies are strongest when the phase speed is slower, and (2) in the subtropical jet stream, where upper- tropospheric westerlies are stronger when the phase speed is faster. Lastly, Hovmöller diagrams help highlight the unique propagation characteristics of anomalies associated with the reconstructed background wind signals. Increasing the background wind downstream over the Maritime Continent yields a decrease in the phase speed between 60 and 130°E, while increasing the zonal extent of the jet favors an increase in phase speed over the same range of longitudes. Understanding the sources of such global circulation variability originating from the tropics is fundamental to increasing subseasonal forecast skill.

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