Modeling the effects of Southeast Asian monsoon outflow on subtropical anticyclones and midlatitude ozone over the Southern Indian Ocean

Abstract

This observational and modeling study explores how the Southeast Asian summer monsoon outflow into the Southern Indian Ocean influences the life cycle of local anticyclones and leads to changes in the distribution of column ozone in the Southern Hemisphere (SH). A case study of the evolution of synoptic fields during August 1998 was performed to characterize the circulation leading to the average column ozone distribution known as the “ozone croissant,” with a characteristic ozone maximum south of Australia and a minimum near South America. During this month, five phases of SH circulation were found to lead to distinctive ozone patterns, explaining the monthly location and extent of the SH column ozone maximum. An isentropic trajectory model was used to show the cross‐equatorial flow from the Tibetan High into the SH at the near‐tropopause level of 360 K. Outflow pulses are shown to be responsible for the amplification of observed anticyclones over the SH Indian Ocean and intensification of troughs south of Australia. This couplet establishes an ozone transport pathway from tropical lower stratospheric regions around the edges of anticyclones into the ozone maximum in the amplified troughs. An idealized modeling experiment using the University of Wisconsin‐Madison Nonhydrostatic Modeling System was performed to model strong outflow pulse from tropical convection. Together with model trajectory computations, the modeling study showed a strong anticyclonic response over the Indian Ocean and increased ozone transport into the amplified troughs in a perturbed wave pattern.