Sources of gravity waves in the lower stratosphere above South Pole

Abstract

Five‐year (2001–2005) high‐resolution radiosonde data were processed to obtain the gravity wave (GW) variabilities in the lower stratosphere over South Pole (SP). Our results show that GW activities in the lowermost section (10–15 km) are strongest in May and September and weakest in the austral summer, whereas in the altitude range of 15–25 km, strongest/weakest GW kinetic energy is observed around September/January. We also explored the relationships of GWs to the synoptic‐scale variations in the troposphere and the ageostrophic motions in the upper troposphere over the Antarctic, which are expected to be significant mechanisms for GW generation. A ray‐tracing model (GROGRAT) was used to explore the relationship between GW propagation and the background field. In the altitude of 15–25 km, the annual cycle of GW activity resembles that of adjustment process. Below 15 km, the annual cycle of GW activity has two peaks in May and September. Our analysis suggests that these two peaks are due to the variation of topographic GW generation and filtering of background atmosphere. Due to critical‐level filtering in the lowermost section, topographic GWs cannot propagate upward, which makes the shape of GW annual cycle at higher altitudes closer to the annual cycle of adjustment processes. The analysis suggests that the minimum of GW activity at SP during the austral summer may be due to the combination of weaker wave generation from adjustment processes associated with synoptic‐scale systems, flow over topography, and unfavorable background field for GW propagation.