The relationship between the subtropical Western Pacific SST and haze over North-Central North China Plain

Haze is a serious issue in China today, presenting a substantial pollution problem to the public. To enhance understanding of haze mechanisms in recent years, this study examined the relationship between the subtropical western Pacific sea surface temperature (SST) and the haze pollution over North-Central North China Plain. The empirical orthogonal function result showed an opposite trend of boreal winter (December-January-February) haze days (WHD) between the south and north of the Huaihe River. The negative sea surface temperature anomaly (SSTA) over the subtropical western Pacific could intensify the WHD over North-Central North China Plain. As an effective external forcing, the preceding boreal autumn (Sepember-October-November) negative SSTA could weaken the East Asian winter monsoon (EAWM) circulation, leading to conditions such as a weaker Siberian High, reduced wind speed, higher humidity and an abnormal southerly wind. These conditions provide a favourable environment for haze with static stability and potential for hygroscopic growth. A series of SST sensitivity experiments were carried out using version 3.1 of the Community Atmosphere Model. When negative SSTA with different strengths were added into the model, all the key factors exhibited favourable features for haze, such as smaller EAWM index, stronger inversion potential, weaker vertical velocity, lower boundary layer height and higher relative humidity.