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, obta ined during the 1999 Indian Ocean Experiment, were analyzed. Upper-troposph eric O-3 mixing ratios were generally very high (varying from 100 ppbv up t o 150 ppbv at the (upper) tropopause). Maxima were observed as laminae (bet ween 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 tropos pheric and stratospheric O-3 mixing ratios. Very dry, high O-3 air is obser ved 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 olde r. 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 or iginates in the subtropical jet stream (STJ). This is a favored region for stratosphere-troposphere exchange (STE). A new feature is that the transiti on zone is supplied with air from both the northern and the southern STJ. A pair of anticyclones located over the western Pacific/Australia directs th is interhemispheric transport (IHT). IHT also occurs in the midtroposphere where a flow channel along the east coast of Africa connects the upper trop osphere of both hemispheres. Although our analysis, involving the ozone pro files, trajectories, and ECMWF model data, supports the idea that all O-3 m axima have sources similar to 1998, the upper-tropospheric flow is anomalou sly easterly in 1999. This is probably associated with the La Nina phase of the tropospheric quasi-biennial oscillation as opposed to the 1998 El Nino . Thus, although climatological conditions are very different, the troposph eric O-3 budget over the Indian Ocean during the winter monsoon is still la rgely influenced by stratospheric intrusions.