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Title: Anomalous east Asian winter monsoon in relation to symbolic Eurasian blocking patterns
Other Titles: Ou Ya zu sai xing shi yu dong Ya dong ji feng zhi yi chang
Authors: Cheung, Ho Nam ( 張皓嵐)
Department: School of Energy and Environment
Degree: Master of Philosophy
Issue Date: 2011
Publisher: City University of Hong Kong
Subjects: Monsoons -- East Asia.
Blocking (Meteorology) -- Russia (Federation) -- Siberia.
Notes: CityU Call Number: QC939.M7 C45 2011
xx, 193 leaves : ill. (some col.) 30 cm.
Thesis (M.Phil.)--City University of Hong Kong, 2011.
Includes bibliographical references (leaves 169-183)
Type: thesis
Abstract: This study attempts to examine the anomalous state of the East Asian Winter Monsoon (EAWM) in relation to spatial and temporal features of atmospheric blocking over Eurasia at the upstream of the climatological Siberian high region. Atmospheric blocking is identified by geopotential height gradients (zonal indices) over the extratropics. Surface air temperature is used to assess the impact of blocking on the EAWM region. Generally, there are two spatially independent blocking patterns of an omega shape with the ridge centered over the Ural-Siberia region and the European continent respectively. The impact is significant if the downstream cyclonic vortex of the blocking system forms a dynamic contact with the Siberian high, which is more confined to the former case. On the whole, the impact can be viewed from the perspective of a single event and a whole season. Establishment of a blocking high over Ural-Siberia involves interaction between the Siberian high and an upstream cyclone. Three temporal features of the blocking high are investigated: duration, intensity and extension. Firstly, the tropospheric warm-core structure is maintained by which the kinetic energy of the thermal ridge is converted to the potential energy of the blocking ridge. The warm ridge extending poleward induces cold advection to promote sustained development of the Siberian high. Secondly, the amplification of the blocking ridge is dependent on the amount of incoming anticyclonic vorticity advection. The advection can be determined by the zonal pressure gradient between the upstream cyclone and the Siberian high, which tightens when the cyclone is located right to the west of the Siberian high. However, this dynamic factor is not deterministic for the thermodynamic evolution of the Siberian high. Thirdly, the extension may be related to the size of pre-existing cold anomalies over western Siberia. Intense cold air masses tend to tighten the pressure gradient and to amplify the upper-tropospheric trough aloft the upstream cyclone. The thermodynamic feedback from Siberia perhaps supports the blocking high to stay for longer time. Therefore, a long-lasting cold period may take place as a consequence of a long-lasting blocking event. The blocking-EAWM relationship is close (weak) when outstanding blocking frequency is over Ural-Siberia (Europe). These upstream blocking activities may be regarded as a response to a combined signal of the Arctic Oscillation (AO) and El Niño/Southern Oscillation (ENSO). Weakened (strengthened) meridional flow in the positive (negative) phase of the AO is unfavorable (favorable) for blocking high formations. As the AO shows a close relationship with the North Atlantic Oscillation (NAO), the teleconnection between the AO and the Eurasian blocking activity may exist in the form of an eastward propagating wave-train signal generated over the North Atlantic Ocean. Be that as it may, the transmission of a signal across East Asia may be disturbed by the external effect of the ENSO, which probably suppresses (enhances) the sinking motion near Siberia in its positive (negative) phase. In short, the blocking-EAWM linkage is stronger (weaker) when the AO and ENSO are in phase (out of phase). If both the AO and ENSO attain their positive (negative) phase, the blocking frequency is distinctly low (high) over Ural-Siberia and uniform warming (cooling) would be observed in East Asia. Rather, if they are out of phase, the blocking signal would not be clear over Ural-Siberia and the monsoonal flow in northern (southern) East Asia would be stronger in negative AO (negative ENSO).
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