Observed and simulated Ural blocking and its impact on East Asian winter monsoon

• Main Author: Cheung, Ho Nam (張皓嵐)
• Format: Thesis
• Language: unknown
• Published: City University of Hong Kong 2015
• Subjects: Monsoons > East Asia; Blocking (Meteorology) > Russia (Federation) > Ural Mountains Region; Pacific; laptev; North Atlantic
• Online Access: http://hdl.handle.net/2031/8582; http://lib.cityu.edu.hk/record=b4862787

This study aims to propose that Ural blocking (UB) in boreal winter [the December-January-February (DJF) period] is one of the important dynamic factors contributing to the variability of the East Asian winter monsoon (EAWM) in the current and future climate. The Ural blocking index (UBI) is defined as the area-averaged blocking frequency over 45o–90oE in a winter. In observations, statistical and dynamic relationships between UB and the EAWM are diagnosed by the NCEP–NCAR reanalysis datasets (NCEP data) for the period 1948/49–2013/14. Diagnosis of the height tendency equation suggests that the evolution of a UB event is related to (1) the non-linear interaction of transient eddies between a cyclone over the Euro–Atlantic region and an anticyclone over Europe, and (2) the interaction between the transient eddies over the Urals and the time-mean flow over East Asia. Downstream of UB, the strengthened northerly cold advection reinforces the Siberian high (SH) and potentially triggers a severe cold air outbreak in East Asia. As a result, more frequent occurrence of UB in a winter potentially causes a cooler East Asia and a high frequency of cold extremes, and vice versa. On interannual and interdecadal time scales, the UBI is significantly correlated to the two dominant temperature modes of the EAWM and the dominant mode of cold day frequencies in China. The UBI can be predicted by the sea surface temperature (SST) over the North Pacific in the preceding autumn, suggesting potential predictability of the EAWM. In numerical simulations, biases and the projected uncertainty of UB and the EAWM are analysed by the outputs of general circulation models (GCMs) from the fifth phase of Coupled Models Intercomparison Project (CMIP5). The Siberian high intensity (SHI) is used as a proxy for the EAWM. In a historical run (HIST), the biases of UBI in GCMs are related to the long-term mean circulation bias over the North Atlantic. On seasonal time scales, the GCMs simulating a positive bias of UBI are associated with a stronger westerly momentum fluxes from the North Atlantic to Europe. On synoptic time scales, however, these GCMs tend to be associated with a weaker SH and East Asian trough during the evolution of a UB event. Altogether, there is no apparent linkage between the long-term mean bias of UBI and the EAWM. Future works are needed to explore the teleconnections between UB and the EAWM in the GCMs. The projection focuses on the twenty-first century (2006/07–2099/2100) of two representative concentration pathways (RCPs); namely RCP4.5 and RCP8.5 scenarios. Compared to HIST, the year-to-year frequency variation of UB appears to show a stronger linkage with the large-scale circulation over the Kara and Laptev Seas in the late twenty-first century. Moreover, UB likely exerts a stronger impact on the EAWM on synoptic and seasonal time scales. Because the projection of UBI varies substantially across the GCMs, the uncertainty of UB might present a challenge for accurate prediction of the sub-seasonal and long-term variation of the EAWM. More research should be devoted to the simulations of UB in order to improve preparedness of cold extremes in the EAWM region under a changing climate. CityU Call Number: QC939.M7 C454 2015 xxvii, 188 pages : illustrations (some color) 30 cm Thesis (Ph.D.)--City University of Hong Kong, 2015. Includes bibliographical references (pages 171-183)