East Asian trough and its modulations on East Asian winter climate

• Main Author: Leung, Yu Ting (梁宇霆)
• Format: Thesis
• Language: unknown
• Published: City University of Hong Kong 2015
• Subjects: Monsoons > East Asia; Lows (Meteorology) > East Asia; Meteorology > East Asia; East Asia > Climate; Arctic; Midwinter; aleutian low; ural mountains
• Online Access: http://hdl.handle.net/2031/8639; http://lib.cityu.edu.hk/record=b4862876

This study is carried out to explore the coupling relation between the East Asian winter monsoon and the East Asian trough (EAT) at seasonal, intraseasonal, and synoptic temporal scales. The study period of interannual variation of the winter EAT is 52 years, from 1960/1961 to 2011/2012. Three properties of the EAT are considered, including the strength and the meridional and zonal displacements, which can explain most of the 500 hPa height variance over the region of the EAT. Regression analysis shows that Southeast Asian temperature exhibits a close relation with the meridional displacement of the trough. On the other hand, Northeast Asian temperature is modulated by both the strength and zonal displacement of the EAT. The relations between the EAT and other climate systems such as the Siberian high, Aleutian low, East Asian jet, Arctic Oscillation, and atmospheric blocking over the Ural Mountains are examined. In this study, different analysis tools are utilized for investigating the variations of the EAT. A notable difference in the potential energy of stationary eddies is observed over the EAT between its strong and weak phases. During strong (weak) EAT years, the conversion from available potential energy of the zonal mean flow to potential energy of stationary eddies increases (decreases) over the midlatitudes. The positive (negative) available energy anomaly over the midlatitudes, along with the steepened (flattened) meridional height gradient, is responsible for anomalous vertical motion over the midlatitudes and polar region during strong and weak EAT years. Both stationary and transient eddies are responsible for inducing anomalous vertical motion over the midlatitudes and polar region. So anomalous stationary and transient eddies precede the establishment of a strong or weak EAT in midwinter. This study also includes investigation of intraseasonal variation in the strength of the EAT. A strong winter EAT and upstream blocking exert a modulating force on the development of a persistent strong EAT, and vice versa for a persistent weak EAT. Anomalous horizontal absolute vorticity advection is found to be the main contributor to the onset and decay of a persistent strong and weak EAT among dynamic and thermodynamic processes. To study the force of mobile troughs on cold air outbreaks, two mobile trough pathways are identified with a tracking algorithm for 30 winters from 1980/1981 to 2009/2010. One path passes north of Lake Baikal, while the other passes through the southern part of the lake. Despite these two mobile trough pathways being adjacent to each other, they exert an opposite force on the strength of the intraseasonal EAT. In addition to the strength of the EAT, the influences on East Asian temperature associated with the two mobile trough pathways demonstrate opposite polarity. The energetic and momentum force due to the passage of the two mobile troughs is also studied. CityU Call Number: QC939.M7 L48 2015 xxvi, 198 pages : illustrations (some color) 30 cm Thesis (Ph.D.)--City University of Hong Kong, 2015. Includes bibliographical references (pages 169-188)