Cross‐tropopause and interhemispheric transports into the tropical free troposphere over the Indian Ocean

Ozone (O 3 ) and relative humidity (RH) profiles over the Indian Ocean, obtained during the 1999 Indian Ocean Experiment, were analyzed. Upper‐tropospheric O 3 mixing ratios were generally very high (varying from 100 ppbv up to 150 ppbv at the (upper) tropopause). Maxima were observed as laminae (between 14–16 km) and as O 3 ‐ rich layers between the chemopause in O 3 and the tropopause (between 15–18 km), resembling a transition zone between tropospheric and stratospheric O 3 mixing ratios. Very dry, high O 3 air is observed in the midtroposphere (between 5–8 km). The 1999 laminae are similar to those observed in 1998 in a pre‐INDOEX campaign, although they appear older. This complicates the attribution of an origin but there are indications, i.e., by back trajectory analyses, that the air in all these O 3 maxima originates in the subtropical jet stream (STJ). This is a favored region for stratosphere‐troposphere exchange (STE). A new feature is that the transition zone is supplied with air from both the northern and the southern STJ. A pair of anticyclones located over the western Pacific/Australia directs this interhemispheric transport (IHT). IHT also occurs in the midtroposphere where a flow channel along the east coast of Africa connects the upper troposphere of both hemispheres. Although our analysis, involving the ozone profiles, trajectories, and ECMWF model data, supports the idea that all O 3 maxima have sources similar to 1998, the upper‐tropospheric flow is anomalously easterly in 1999. This is probably associated with the La Nina phase of the tropospheric quasi‐biennial oscillation as opposed to the 1998 El Niño. Thus, although climatological conditions are very different, the tropospheric O 3 budget over the Indian Ocean during the winter monsoon is still largely influenced by stratospheric intrusions.