Pattern of Wintertime Southern Rainfall and Northern Pollution over Eastern China: The Role of the Strong Eastern Pacific El Nino

Air quality over the North China Plain (NCP) and rainfall over southern China exhibit robust and correlated interannual variations during winter from 1979 to 2018. A correlation coefficient of 0.7 between the PM2.5 concentration over the NCP and rainfall in southern China indicates that poor air quality and rainfall are linked (referred to as the south rainfall-north pollution pattern), which is likely due to the strong eastern Pacific (EP) El Nino. In the mature phase of the strong EP El Nino, the Northeast Asian anomalous anticyclone (NAAA) is strengthened by two Rossby wave trains modulated by warmer sea surface temperatures. One wave train is directly related to the strong EP El Nino and originates in the tropical eastern Pacific, propagating eastward to East Asia along the middle and high latitudes, which weakens the Ural ridge and strengthens the NAAA. The other wave train is derived from the northern Indian Ocean and propagates into East Asia along the great circle route, which strengthens an anomalous cyclone in southern China and the NAAA. This results in an anomalous horizontal trough of 850 hPa being captured in eastern China at similar to 30 degrees N. The southwesterly flow along the southern margin of the trough transports abundant moisture to southern China, which leads to heavy rainfall in southern China combined with an anomalous ascending motion related to the anomalous cyclone. The southeasterly flow to the north of the trough weakens the East Asia winter monsoon due to a weakened Ural ridge, which then leads to a high PM2.5 concentrations over the NCP with the help of an anomalous descending motion related to the NAAA.