Impact of middle stratospheric water vapor increase on polar vortices: a case study of the Tonga volcanic eruption

Stratospheric water vapor has been found to significantly affect atmospheric circulation. Previous studies mainly focus on the lower stratospheric water vapor. The impact of middle stratospheric water vapor is not yet well understood. The volcanic eruption in Tonga is observed to inject an unprecedented approximately 146 Mt of water vapor into the stratosphere, particularly into the middle stratosphere. Here we investigate the effects of the middle stratospheric water vapor (SWV) increase similar to the Tonga volcanic eruption on stratospheric circulation and polar vortices using SWV-locking experiment with CESM/CAM5. It is found that a middle SWV increase leads to radiative cooling in the middle stratosphere and warming in the upper troposphere and lower stratosphere (UTLS). The subtropical UTLS warming increases the meridional temperature gradient in the high latitudes, which tends to enhance the circumpolar westerly winds. Furthermore, this warming reduces the meridional temperature gradient in the low latitudes and weakens subtropical jet streams, which results in a weakening of stratospheric Brewer-Dobson circulation and a strengthening of both the Arctic and Antarctic polar vortices.