Tibetan middle tropospheric ozone minimum in June discovered from GOME observations

Global Ozone Monitoring Experiment (GOME) observations from 1997– 2000 have revealed the Tibetan Middle Tropospheric Ozone Minimum (TMTOM), a lowozone layer that occurs in the middle troposphere over the Tibetan Plateau (TP) in June. Ozone profiles were derived from GOME observations and validated by ozonesonde measurements at two stations (Lhasa and Xining) over the TP. The mean bias is 5– 10% within the troposphere. The ozone profiles reveal the TMTOM phenomenon occurs in the middle troposphere (8 – 13 km) over the middle and eastern TP in June. Dynamical field analyses showed that the TMTOM accompanies the onset of the Asian summer monsoon. The low-ozone air from the Bay of Bengal is transported into the middle troposphere over the TP by southwest currents while the lower troposphere over the TP is still occupied by ozone-rich air blocked by the transport barrier of the Himalayas. The TMTOM was most prominent in June 1998 likely linked to the occurrence of the intense El Niño of 1997 – 1998 because the Tibetan anticyclone is of large area and has strong intensity during El Niño years. The occurrence of the TMTOM can serve as an indicator of the phase of evolution of the Asian summer monsoon.