Inertia–Gravity Waves Observed in the Lower Stratosphere over Macquarie Island

Abstract

This study examines the properties of inertia?gravity waves observed in the lower stratosphere over Macquarie Island, how these properties vary with season, and the likely source of the waves. The waves are observed in high-resolution upper-air ozonesonde soundings of wind and temperature released from Macquarie Island during the 1994 ASHOE?MAESA program. The properties of the inertia?gravity waves observed in the soundings are quantified using hodograph and rotary spectral analyses. The analyzed waves have horizontal wavelengths between 100 and 1000 km, vertical wavelengths between about 1 and 7 km, intrinsic frequencies between f and 2f, and horizontal trace speeds between ?50 and 30 m s?1. There appears to be a seasonal cycle in the inertia?gravity wave activity in the lower stratosphere, the minimum being in the austral winter when the background zonal flow is strong and westerly and its vertical shear is positive. In contrast, the variance of the horizontal perturbation winds does not show a similar seasonal cycle. Inertia?gravity waves are detected over Macquarie Island on days with a common synoptic pattern. Two features define this synoptic pattern: 1) an upper-level jet and associated surface front lying upstream of Macquarie Island, and 2) a 300-hPa height field with Macquarie Island located between the inflection axis and the downstream ridge. This common synoptic pattern is observed on 16 of the 21 days on which inertia?gravity waves were detected. Moreover, the pattern is not observed on 15 of the 21 days in which inertia?gravity waves are not identified. This common synoptic pattern shows a seasonal cycle similar to that found for the inertia?gravity wave activity. Analyses of the ozonesonde soundings suggest also that the source of the inertia?gravity waves is in the troposphere. Using GROGRAT, the ray-tracing model developed by Marks and Eckermann, a cone of rays is released 21 km above Macquarie Island and traced backward in time. These rays suggest that the inertia?gravity waves are generated in the jet?front system southwest of Macquarie Island.