Impacts Of The Siberian High On Wintertime Air Quality Over North China

As a key member of East Asian winter monsoon system,the Siberian High influences the weather and climate over East Asia and even the whole Northern Hemisphere.This thesis analyzes the features of large-scale circulation patterns during the haze month January 2013 and establishes the relations between the longitudinal position of the Siberian High and wintertime air quality over Northern China on the interannual time scale.Using results from the Earth System Models in the Coupled Model Intercomparision Project?CMIP5?archive,the future variations of the position of the Siberian High are projected under different warming scenarios and the resulting changes in future wintertime air quality over North China are investigated.The main results and conclusions are as follows:?1?The Siberian High was significantly weaker and extended extremely eastward in January 2013.Composite analysis of sea level pressure?SLP?in high-AOD and low-AOD winters over North China over the period from 2001 to 2013 does not reveal any significant decrease of SLP from low-AOD to high-AOD winters over Mongolia where the climatological center of the Siberian High is located.Instead,significant positive changes of SLP are located from east of Mongolia to Japan and negative changes are located over west of Mongolia.?2?The spatial pattern of SLP was used to develop a Siberian High position index?SHPI?that depicts the longitudinal variations of the Siberian High.This SHPI explains58%?r=0.76?of the interannual variability in detrended wintertime AOD over North China during 2001-2013,and 67%?r=-0.82?of the interannual variability in wintertime AOD over South China.?3?The wind speed is lower in the troposphere from 850 hPa to 500 hPa and the surface relative humidity is higher over North China in high-SHPI winters.Both the wind and relative humidity changes are responsible for enhanced AOD over North China during the high-SHPI winters.?4?Using the longer-term MODIS AOD from 2001 to 2016 to verify the interannual variations of SHPI and North China wintertime AOD,we find after the linearly increasing trend of AOD from 2001 to 2013 and the linearly decreasing trend after 2014 are removed,SHPI explains 37%of the interannual variability in detrended North China winter AOD.Meanwhile,83%of the high SHPI years correspond to high AOD in North China.?5?The simulation results of GEOS-Chem indicate SHPI explains 30%of the simulated interannual variability in AOD over North China due to meteorology from2001 to 2016 and 38%of that in PM_2.5.Meanwhile,60%of the high SHPI years have high AOD in North China.?6?The ensemble-mean results of the five selected coupled models reveal that the frequency of high SHPI winters would reduce under RCP2.6 and RCP4.5 but increase by 1.7 times under RCP8.5 during 2007-2099 compared to that during 1956-2005.This indicates the polluted conditions in winter over North China due to the change of the Siberian High are dependent on climate warming scenarios and a reduction of greenhouse emissions may lead to improved winter air quality over North China in the future.