Contribution of Northeastern Asian stratospheric warming to subseasonal prediction of the early winter haze pollution in Sichuan Basin, China

Abstract

Predicting the interannual (IA) variability of the early winter haze pollution (HP) in Sichuan Basin (SCB) and analyzing the contribution of the complex terrain are of great significance and challenge as well. For one thing, this study finds that the second mode of early winter IA variations of the HP in Central Eastern China (CEC) is dominated by the pollutant pattern of SCB, characterized by an extreme value center located over SCB. For another, the mode accounts for 15.1% of the total variance and well represents the IA variability of the areal mean HP in SCB. Furthermore, the anomalous circulations conducive to the severe HP in SCB consist of the strengthened (weakened) north (south) branch of Tibetan Plateau (TP) bypassing westerlies at 700 hPa, enhanced descents over SCB, and a strong inversion layer at 750 hPa, suppressing the precipitation and the development of the planetary boundary layer (PBL) in SCB. As a result, dispersion and wet deposition conditions are poor, favorable for the occurrence of the HP in SCB.Observed evidences show that the regional warming in the upper stratosphere over Northeastern Asia in November can be a possible subseasonal precursory signal for the SCB mode. Above all, the warming at upper stratosphere will propagate downward and arrive at the lower stratosphere in 3–4 weeks. Followed by the southward and downward propagation through the longitudinal tilted isothermal surface, the warming arrives at the tropospheric mid-low latitudes in 2–3 weeks, leading to anomalous warming and descents over TP and SCB. Consequently, SCB sees the aforementioned three-dimensional anomalous circulations. For this reason, a physical-empirical subseasonal prediction model for the IA variability of the early winter HP in SCB is established in terms of the regional stratospheric temperature in November. Apparently, the hindcast shows a promising prediction skill.