Seasonal variation of upper-level mobile trough development upstream of the Pacific storm track

Abstract

The purpose of this thesis is to investigate seasonal and interannual statistics of troughs associated with the Pacific storm track and quantify the influence of deformation on trough development upstream of the western Pacific. The goal is to provide at least a partial explanation of midwinter suppression of the Pacific storm track. From 19 years of analysis, a strong stretching deformation zone (DZ) is defined over East Asia. We examine the number, average intensity and intensity change of upper-level mobile troughs as they enter and exit the DZ as well as over the western Pacific, using Lefevre and Nielsen-Gammon's trough tracking data (1995). The relationship between deformation and trough intensification upstream of the Pacific storm track is discussed on a seasonal and interannual basis. Midwinter reduction of the Pacific storm track is accompanied by midwinter reduction of trough intensity. Intensification of winter and spring troughs is suppressed compared to fall troughs in DZ. Over the first domain of the Pacific, all troughs continue developing and fall and winter troughs tend to decay in the second domain. However, spring troughs continue intensifying over the second domain. Enhanced development of troughs seems to contribute to the double peak over DZ in fall and over the Pacific in spring, respectively. Although winter troughs intensify over the Pacific despite short residence time and strong shearing deformation, they do not become as intense as fall and spring troughs due to the suppression of trough intensification over DZ. Midwinter suppression of the Pacific storm track is found to be reproduced by seasonal variation of trough intensification in this study. Seasonal analysis shows a strong correlation between deformation extending from Kazakhstan to DZ and weaker mobile trough development over DZ, suggesting that excessive stretching deformation upstream of the Pacific storm track might be responsible for midwinter suppression of the Pacific storm track. Confirming the result of seasonal analysis, an interannual comparison shows that enhanced stretching deformation is present over the Kazakhstan region in weak trough intensification years compared to strong trough intensification years.