Formation of the Summertime Ozone Valley over the Tibetan Plateau: The Asian Summer Monsoon and Air Column Variations

The summertime ozone valley over the Tibetan Plateau is formed by two influences, the Asian summer monsoon (ASM) and air column variations. Total ozone over the Tibetan Plateau in summer was \sim 33 Dobson units (DU) lower than zonal mean values over the ocean at the same latitudes during the study period 2005--2009. Satellite observations of ozone profiles show that ozone concentrations over the ASM region have lower values in the upper troposphere and lower stratosphere (UTLS) than over the non-ASM region. This is caused by frequent convective transport of low-ozone air from the lower troposphere to the UTLS region combined with trapping by the South Asian High. This offset contributes to a \sim20-DU deficit in the ozone column over the ASM region. In addition, along the same latitude, total ozone changes identically with variations of the terrain height, showing a high correlation with terrain heights over the ASM region, which includes both the Tibetan and Iranian plateaus. This is confirmed by the fact that the Tibetan and Iranian plateaus have very similar vertical distributions of ozone in the UTLS, but they have different terrain heights and different total-column ozone levels. These two factors (lower UTLS ozone and higher terrain height) imply 40 DU in the lower-ozone column, but the Tibetan Plateau ozone column is only ～33 DU lower than that over the non-ASM region. This fact suggests that the lower troposphere has higher ozone concentrations over the ASM region than elsewhere at the same latitude, contributing ～7 DU of total ozone, which is consistent with ozonesonde and satellite observations.