Ozonesonde observations in the Indonesian maritime continent: a case study on ozone rich layer in the equatorial upper troposphere

Ozonesonde observation campaigns were conducted over the Indonesian maritime continent in September–October 1998 and in August–September 1999. Three stations were used for each campaign, Watukosek (7.5°S,112.6°E), Kototabang (0.20°S,100.3°E), and Pontianak (0.03°N,109.3°E) for the 1998 campaign, and Watukosek, Kototabang, and Darwin (12.25°S,130.55°E) for the 1999 campaign. Both periods were basically characterized as the La Niña period, and the tropospheric ozone concentrations showed normal values. Temporal variation and horizontal distribution of an ozone layered structure with a 1–1.5-km thickness were obtained just below the tropopause at the two equatorial stations during the 1998 campaign. Meteorological data analyses including the reverse domain filling technique suggested that the most plausible explanation for the layer is the quasi-horizontal, thin intrusion from the northern midlatitude lower stratosphere associated with a breaking Rossby wave and large-scale flow pattern.