Influences of El Nino Southern Oscillation events on haze frequency in eastern China during boreal winters

Statistical results reveal the close relationship between the hazy days in eastern China and El Nino Southern Oscillation (ENSO) events. A significant negative correlation coefficient (CC) centre is located in the western Pacific, while a positive CC centre is located in the eastern equatorial Pacific. Case analyses also confirm that an El Nino (La Nina) event is more likely to bring more (less) hazy days during winter. The key circulation pattern for less haze includes stronger Siberian high and Aleutian low pressure systems, stronger northerly wind over eastern Asia, and also a stronger high-level subtropical westerly jet (i.e. stronger East Asian winter monsoon, EAWM). While more haze often occurs under weaker EAWM pattern. The influencing mechanism of the ENSO event could be explained first, by how it alters the pressure and thermal differences between the Asian continent and the western Pacific Ocean, second, by the change of the circulation pattern of EAWM, and finally by the effect of the northerlies at higher latitudes. Compared to the relationship between ENSO and EAWM, the ENSO-haze relationship is more stable. Results also indicate an ENSO event has a leading influence on the haze frequency and can be used as a predictor in the seasonal forecasts of hazy days.