Interannual variation of the wintertime fog-haze days across central and eastern China and its relation with East Asian winter monsoon

The interannual variation of the wintertime fog-haze days across central and eastern China from 1972 to 2014 and its relationship with East Asian winter monsoon (EAWM) are investigated based on the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data and the surface observation data from the weather stations in China. The results show that the wintertime fog-haze days across central and eastern China have close relation with EAWM in the interannual time scale. The stronger (weaker) the EAWM is, the less (more) the wintertime fog-haze days are. In strong (weak) EAWM winters, both near-surface winds and vertical shear of horizontal zonal winds strengthen (weaken). The strengthened (weakened) near-surface winds enhance (reduce) the outward transport of fog and haze and are unfavourable (favourable) for their accumulation over central and eastern China. The enlarged (receded) vertical shear of horizontal zonal winds intensifies (abates) the atmospheric baroclinic instability and vertical diffusion, leading to less (more) fog and haze in near-surface. In addition, a strong (weak) EAWM is also unfavourable (favourable) for the maintenance of the fog and haze in the lower troposphere through the anomalous divergence (convergence) associated with the intense anticyclonic (cyclonic) anomalies in the upper troposphere over southern China.