The impact of Climate Change on the Western Pacific Subtropical High and the related ozone pollution in Shanghai, China

Severe ozone (O-3) episodes occur frequently in Shanghai during late-summers. We define geopotential height averaged over the key area region (122.5 degrees E-135 degrees E, 27.5 degrees N -35 degrees N) at 500 hPa as a WPSH_SHO3 index which has high positive correlation with surface O-3 concentration in Shanghai. In addition, the index has a significant long-term increasing trend during the recent 60 years. Analysis shows the meteorological conditions under the strong WPSH_SHO3 climate background (compared to the weak background) have several important anomalies: (1) A strong WPSH center occurs over the key area region. (2)The cloud cover is less, resulting in high solar radiation and low humidity, enhancing the photochemical reactions of O-3. (3) The near-surface southwesterly winds are more frequent, enhancing the transport of upwind pollutants and O-3 precursors from polluted regions to Shanghai and producing higher O-3 chemical productions. This study suggests that the global climate change could lead to a stronger WPSH in the key region, enhancing ozone pollution in Shanghai. A global chemical/transport model (MOZART-4) is applied to show that the O-3 concentrations can be 30 ppbv higher under a strong WPSH_SHO3 condition than a weak condition, indicating the important effect of the global climate change on local air pollution in Shanghai.