Improved Global Surface Temperature Simulation using Stratospheric Ozone Forcing with More Accurate Variability

Increasing surface ozone and enhanced secondary organic carbon formation at a city junction site: An epitome of the Yangtze River Delta, China (2014-2017)

This study aims to understand the characteristics of surface ozone (O₃), search for factors affecting the variations in its concentration, and estimate its impacts on the secondary organic carbon (SOC) levels and atmospheric oxidation capacities in the Yangtze River Delta (YRD). Four years of continuous observations (2014-2017) of the surface O₃, organic carbon, elemental carbon, nitrogen oxides, PM_2.5 and meteorological factors along with three years of measurements (2015-2017) of the concentrations of 56 volatile organic compounds were conducted at a rural site. Our measurements showed that the total number of O₃ pollution days more than doubled over the four-year period, from 28 days in 2014 to 76 days in 2017. The annual mean of the maximum daily 8-h average O₃ concentration during the months with the strongest solar radiation (July-September) showed a 6.8% growth rate, from 124.5 (2014) to 149.8 μg m^-3 (2017). Regional transport was shown to be the dominant contributor to the high level of O₃ based on a process analysis of the O₃ variation using the Weather Research and Forecasting-Community Multiscale Air Quality model for this site. The simulation results indicated that the city junction site served well as an epitome of the regional background of the YRD. We also found that the level of SOC, which is a major component of PM_2.5 that results from atmospheric oxidizing processes, gradually increased with the increase in the surface O₃ level, even though the overall PM_2.5 concentration significantly decreased each year. There was an increasingly strong correlation between SOC and O_x (O₃ + nitrogen dioxide) during both the daytime and night-time from 2014 to 2017 when the highest annual O₃ concentration was observed. These findings imply that the atmospheric oxidation capacity increased and likely contributed to the SOC production in the YRD during 2014-2017.

Influence of Wintertime Polar Vortex Variation on the Climate over the North Pacific during Late Winter and Spring

The effects of wintertime stratospheric polar vortex variation on the climate over the North Pacific Ocean during late winter and spring are analyzed using the National Centers for Environmental Predictions, version 2 (NCEP2) reanalysis dataset. The analysis revealed that, during weak polar vortex (WPV) events, there are noticeably lower geopotential height anomalies over the Bering Sea and greater height anomalies over the central part of the North Pacific Ocean than during strong polar vortex (SPV) events. The formation of the dipolar structure of the geopotential height anomalies is due to a weakened polar jet and a strengthened mid-latitude jet in the troposphere via geostrophic equilibrium. The mechanisms responsible for the changes in the tropospheric jet over the North Pacific Ocean are summarized as follows: when the stratospheric polar westerly is decelerated, the high-latitude eastward waves slow down, and the enhanced equatorward propagation of the eddy momentum flux throughout the troposphere at 60° N. Consequently, the eddy-driven jet over the North Pacific Ocean also shows a southward displacement, leading to a weaker polar jet but a stronger mid-latitude westerly compared with those during the SPV events. Furthermore, anomalous anti-cyclonic flows associated with the higher pressure over the North Pacific Ocean during WPV events induce a warming sea surface temperature (SST) over the western and central parts of the North Pacific Ocean and a cooling SST over the Bering Sea and along the west coast of North America. This SST pattern can last until May, which favors the persistence of the anti-cyclonic flows over the North Pacific Ocean during WPV events. A well-resolved stratosphere and coupled atmosphere-ocean model (CMCC-CMS) can basically reproduce the impacts of stratospheric polar vortex variations on the North Pacific climate as seen in NCEP2 data, although the simulated dipole of geopotential height anomalies is shifted more southward.