The impact of El Niño–Southern Oscillation on the total column ozone over the Tibetan Plateau
Authors: Li Yang , Feng Wuhu , Zhou Xin , Li Yajuan , Chipperfield Martyn P. 
Published: July 2023

Abstract. The Tibetan Plateau (TP, approximately 27.5–37.5° N, 75.5–105.5° E) is the highest and largest plateau on Earth with a mean elevation of over 4 km. This special geography causes strong surface solar ultraviolet radiation (UV), with potential risks to human and ecosystem health, and which is controlled by the local total column ozone (TCO). The El Niño–Southern Oscillation (ENSO), the dominant mode of interannual variability on Earth, is characterized by warming of central and eastern equatorial Pacific Ocean sea surface temperature anomalies (SSTA) for the El Niño phase and cooling for the La Niña phase. Although some studies have suggested that there exists a positive correlation between ENSO and the TP TCO, the mechanism underlying this effect of ENSO is not fully understood. Here we use the Copernicus Climate Change Service (C3S) merged satellite dataset and the TOMCAT 3–dimensional (3D) offline chemical transport model forced by ERA5 meteorological reanalyses from the European Centre for Medium–Range Weather Forecasts (ECMWF) over the period 1979–2021 to investigate the influence of ENSO on the TCO over the TP. The correlation coefficient of the time series of monthly TCO anomalies over the TP between the TOMCAT output and C3S dataset is ~0.92 with statistical significance above 99 %. In particular, the correlation coefficients (December–May) are above 0.95, indicating that the TCO variability in TOMCAT is very consistent with the merged satellite observations. Based on the lead correlation between the ENSO index (Niño 3.4 index from the National Oceanic and Atmospheric Administration) and TP TCO time series, we find that ENSO has a significant impact on the TCO, especially from the December of its mature phase to the May of its decaying phase (December–May). That is, the El Niño (La Niña) events induce positive (negative) TCO anomalies over the TP. Through studying the ozone profile from the Stratospheric Water and OzOne Satellite Homogenized (SWOOSH) dataset and TOMCAT results over their overlapping period (1984–2021), we attribute the positive (negative) TCO anomalies mainly to the increased (decreased) ozone at the 200–70 hPa levels caused by the downward (upward) shift of ozone profile associated with the lower (higher) tropopause height during the El Niño (La Niña) events. Our results suggest that the El Niño events lead to the descending upper–level geopotential height and hence cause the decreasing air column thickness, which in turn induces the cooling tropospheric temperature over the TP. This cooling associated with El Niño events causes a decrease of the tropopause height, which contributes to the downward shift of the ozone profile and hence leads to the increase in TCO. The La Niña events affect TP TCO during December–May in a manner resembling the El Niño events, except with anomalies of opposite sign. This work provides a systematic understanding of the influence of ENSO on ozone over the TP. Since climate models project an increase in the frequency of strong El Niño and La Niña events under greenhouse–gas–forced warming, we can expect more ozone variation associated with ENSO, with important implications for 21st–century ozone recovery, surface solar UV, and ecosystems over the TP.