Synoptic-scale features associated with winter monsoon surges over South China
Student thesis: Master's Thesis
The features associated with the two kinds of winter monsoon surges over South China are studied: the easterly surge (ES) and the northerly surge (NS). Surface and upper-level meteorological parameters over the region (90"-13O0E, 15"-5O0N) and (70"-16O0E, 0"-6O0N) respectively for the surges that occurred in the three winters (October-March) from 1988 to 1991 are analyzed. For the northerly surge, the surface features found are 1) an abrupt temperature drop and wind direction turning from easterly to northerly, which can be related to the passage of a cold front; 2) an increase in the dewpoint depression; and 3) a large north-south pressure gradient. On the other hand, the easterly surge is found to be associated with strong easterly winds up to 40 km h-', little temperature or pressure change, and a southward motion of a high pressure centre from Dahingganling to the Yellow Sea together with a sharp pressure ridge along the east China coast. The outbreak of an NS is associated with a breakdown of the Siberia-Mongolia high. As observed from the features at the upper-levels, the evolution of the Siberia-Mongolia high in an NS is related to an eastward passage of a shortwave trough and polar jet according to the quasigeostrophic (QG) theory. Intensification of the Siberia-Mongolia high is found to be mainly caused by the adiabatic rising associated with the secondary circulation at the polar jet exit region. When the trough and polar jet have passed, the Siberia-Mongolia high breaks down in response to the sinking motion upstream of the trough. The low-level features of the NS are a large-scale meridional flow accompanied with strong cold advection and a southward push of height contours. A cold-air mass is observed to surge southward around the eastern edge of the Tibetan Plateau. In addition, there is the suggestion of a gravity wave associated with the NS since there is an abrupt and transient gradient in the high-passed 850-hPa height. The response at the low-latitude region to the NS is the enhancement of the local Hadley cell as can be observed from the strengthening of the upper easterlies (westerlies) before (after) the NS and this in turn intensifies the subtropical jet. Other features observed are that minimum zonal index is attained and there is strong north-south thermal contrast at the outbreak of the NS. For the easterly surge, passage of upper-ridge is observed and the zonal index is increasing in an ES whereas the subtropical jet is weakening. The high pressure centre responsible for the ES (the Dahingganling high) is found to be a split out from the Siberia- Mongolia high and the splitting related to the ridge passage. Significant transformation is happens along the eastward migration of the Dahingganling high and temperature inversion is only observed when the high is not far apart from its parent high (i.e., the Siberia- Mongolia high). Unlike the NS, an ES is found not to be associated with strong north-south thermal contrast. Significant differences are observed when comparing the features of the ES and NS. On the whole, the results suggest a clear distinction between the ES and NS on the synoptic scale.
- Synoptic meteorology, Monsoons, China, Southeast