Date of Award

1-1-2023

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 (iii, 87 pages) : illustrations (some color)

Dissertation/Thesis Chair

Paul Roundy

Committee Members

Aiguo Dai

Keywords

Madden-Julian oscillation

Subject Categories

Atmospheric Sciences

Abstract

El Niño-Southern Oscillation (ENSO) is an interannual climate mode that is dominant in tropical Pacific and affects regional and global weather and climate. Some may categorize ENSO into two types – eastern Pacific ENSO (EP ENSO) and central Pacific ENSO (CP ENSO). Madden-Julian Oscillation (MJO) is the most dominant intraseasonal mode in the tropics that impacts regional and global weather. Previous studies have established relationships between ENSO and MJO. An empirical orthogonal function (EOF) analysis was performed to retrieve the principal components (PCs), instead of filtering atmospheric data, to approximately trace the MJO to analyze its relationship with ENSO. An ENSO index and ENSO index trend are used to determine the ENSO phases, which are categorized into 4 phases. The analysis is performed by season and ENSO phase. Even though the correlation between two whole-time PCs is about zero, it is not for a sub-period of the whole time-series. The correlations between a pair of the first four dominant modes of the MJO change with the season even though the coefficients are not large but statistically significant. Typically, the magnitudes of the correlations of a PC pair of the first four dominant modes of MJO during a particular season and ENSO phase are about 0.2 (0.4 is an absolute maximum value). The correlation between two PCs implies how their EOFs change together. When combining two EOFs during a positive correlation period, the combined amplitude of the MJO signals over a particular region could be enhanced if the same sign signals occur over the similar areas on both EOFs. However, the combined amplitude of the MJO signals could be reduced if the same sign signals are combined during a negative correlation period. Therefore, the changes in correlation between PCs imply the change in the MJO signals. Seasonality can alter the correlation from negative to positive throughout a year while ENSO phase had lesser impact on the correlation. These correlations influence how scatter plots between two PCs (biplots) of the MJO signal look like by elongating the plots along that correlation direction. Sometimes, the elongation would help increase the magnitude of the MJO over particular regions along the diagonals on a biplot. In addition, the mean PCs position on a biplot would tell the likelihood of strong MJO convection locations. The biplots of PC1 and any other PCs show a separation in the mean PCs between EP ENSO phase 1 (“El Niño like”) and phase 3 (“La Niña like”). However, during summer (JJA), the separation of the mean PCs is in different directions of the direction of the elongation on the biplots. It could suggest that one mechanism related to seasonality influences the correlation between 2 PCs of the first four dominant modes of the MJO signal, and another mechanism related to ENSO phase influences the shift in the mean PCs position on biplots. When linear regressing each of the 4 PCs of the MJO during a particular ENSO phase and season, the relationships are dynamic. For example, during EP ENSO phase 3, SST anomalies in the eastern Pacific are associated with only PC2 of the MJO during JJA, but they are associated with only PC1 of the MJO during SON. Moreover, SST anomalies in the central and eastern Pacific are projected differently to each of the first four dominant modes of the MJO during different seasons. Those spatial patterns based on each PC of the MJO during each ENSO phase and seasons could help explain some phenomena like the eastward extent of the MJO propagation during El Niño. Therefore, this could imply the importance of a particular MJO mode during a particular season and ENSO phase over a particular region.

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