Extreme particulate matter (PM) air pollution of January 2013 in China wasfound to be associated with an anomalous eastward extension of the SiberianHigh (SH). We developed a Siberian High position index (SHPI), which depictsthe mean longitudinal position of the SH, as a new indicator of thelarge-scale circulation pattern that controls wintertime air quality inChina. This SHPI explains 58 % (correlation coefficient of 0.76) of theinterannual variability of wintertime aerosol optical depth (AOD) retrievedby MODIS over North China (NC) during 2001–2013. By contrast, theintensity-based conventional Siberian High index (SHI) shows essentially noskill in predicting this AOD variability. On the monthly scale, somehigh-AOD months for NC are accompanied with extremely high SHPIs; notably,extreme PM pollution of January 2013 can be explained by the SHPI valueexceeding 2.6 times the standard deviation of the 2001–2013 January mean.When the SH extends eastward, thus higher SHPI, prevailing northwesterlywinds over NC are suppressed not only in the lower troposphere but also inthe middle troposphere, leading to reduced southward transport of pollutionfrom NC to South China (SC). The SHPI hence exhibits a significantlynegative correlation of ?0.82 with MODIS AOD over SC during 2001–2013,although the robustness of this correlation depends on that ofsatellite-derived AOD. The suppressed northwesterly winds during high-SHPIwinters also lead to increased relative humidity (RH) over NC. Both the windand RH changes are responsible for enhanced PM pollution over NC during thehigh-SHPI winters.
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