La Niña-related tropospheric column ozone enhancement over East Asia

Abstract

The El Niño-Southern Oscillation (ENSO), a dominant driver of interannual climate variations around the world, is characterized in the central-to-eastern tropical Pacific by anomalous sea-surface warming during the El Niño phase and cooling during the La Niña phase. Although ENSO strongly affects atmospheric circulation in East Asia, its effects on tropospheric ozone there have not been fully explored. Here we use satellite measurements of tropospheric column ozone and chemistry-climate model simulations to assess the effects of atmospheric circulations driven by ENSO on tropospheric column ozone levels in East Asia. We find that the observed ozone tends to increase in the East Asian troposphere 4 months after the La Niña peak, corresponding to anomalous northerly downward motions and suppressed convection. The circulation changes can be attributed to anomalous cyclonic flows in the western North Pacific, which links the La Niña and East Asian climate by modulating meridional transport of water vapor. These post-La Niña changes are also evident in the chemistry-climate model results, albeit with a slightly longer (5-month) lag. The model also reveals that La Niña-related ozone enhancement in East Asia is largely due to the intensified southward transport of ozone-rich air from higher latitudes. This suggests that ENSO should be considered to estimate the ozone concentration in East Asia, requiring much attention of ENSO properties in a changing climate.