Inertia-gravity waves observed in the lower stratosphere over Macquarie Island

This study examines the properties of inertia-gravity waves observed in the lower stratosphere over Macquarie Island, how these properties vary with s eason, and the likely source of the waves. The waves are observed in high-r esolution upper-air ozonesonde soundings of wind and temperature released f rom Macquarie Island during the 1994 ASHOE-MAESA program. The properties of the inertia-gravity waves observed in the soundings are q uantified using hodograph and rotary spectral analyses. The analyzed waves have horizontal wavelengths between 100 and 1000 km, vertical wavelengths b etween about 1 and 7 km, intrinsic frequencies between f and 2f; and horizo ntal trace speeds between -50 and 30 m s(-1). There appears to be a seasona l cycle in the inertia-gravity wave activity in the lower stratosphere, the minimum being in the austral winter when the background zonal Row is stron g and westerly and its vertical shear is positive. In contrast, the varianc e of the horizontal perturbation winds does nor show a similar seasonal cyc le. Inertia-gravity waves are detected over Macquarie Island on days with a com mon synoptic pattern. Two features define this synoptic pattern: 1) an uppe r-level jet and associated surface front lying upstream of Macquarie Island , and 2) a 300-hPa height field with Macquarie Island located between the i nflection axis and the downstream ridge. This common synoptic pattern is ob served on 16 of the 21 days on which inertia-gravity waves;were detected. M oreover, 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 seas onal cycle similar to that found for the inertia-gravity wave activity. Ana lyses of the ozonesonde soundings suggest also that the source of the inert ia-gravity waves is in the troposphere. Using GROGRAT, the ray-tracing mode l 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 i nertia-gravity waves are generated in the jet-front system southwest of Mac quarie Island.