Polar Regions

A polar climatology of stratospheric gravity wave activity was assembled by Hei et al. (2008) using CHAMP data between 2001 and 2005. In the Arctic region, Ep showed an annual variation with a maximum in winter, which is consistent with stronger zonal mean horizontal wind speeds and larger Eliassen-Palm flux Fz. Larger values of Fz indicate higher planetary wave activity which can distort the polar night jet. Gravity waves can then be emitted as the jet re-aligns through geostrophic adjustment.

In contrast, the Antarctic Ep and horizontal wind speed are maximum in spring, concurrent with the break-up time of the polar night jet (Hei et al. 2008). These Antarctic lower stratospheric results are summarized in Fig. 4 for the period 20012005. Planetary wave activity generates gravity waves via geostrophic adjustment related to planetary wave transience and/or breaking thus increasing the Ep during winter and spring. The spring peak Ep corresponds to the time of the largest rate of change of the mean wind speed. Hei et al. (2008) also showed a correlation between gravity wave energy and the Eliassen-Palm flux divergence. Baum-gaertner and McDonald (2007) explained the smaller summertime Antarctic Ep to be a result of critical level wave filtering. During other seasons, strong eastward winds are observed throughout the troposphere and stratosphere which increase in

Fig. 4 (Top panel) Zonal mean gravity wave potential energy from 12-33 km and 50° S to 80° S. (Second panel) Mean horizontal wind speed, 250-10 hPa. (Thirdpanel) Time derivative of mean horizontal wind speed. (Bottom panel) Planetary wave s = 1,2 amplitude

GWs Energy [J/kg]

GWs Energy [J/kg]

Mean Wind [m/s]
Time derivative of Mean Wind [(m/s)/month] 250- lOhPa 50S-70S QE-360E

Fig. 4 (Top panel) Zonal mean gravity wave potential energy from 12-33 km and 50° S to 80° S. (Second panel) Mean horizontal wind speed, 250-10 hPa. (Thirdpanel) Time derivative of mean horizontal wind speed. (Bottom panel) Planetary wave s = 1,2 amplitude

2001 2002 2003 2004 SODS

strength with height. This results in less wave filtering and can also Doppler shift gravity waves to larger vertical wavelengths with larger amplitudes. The decrease in filtering at the vortex edge leads to wave enhancement.

The topography of the Antarctic Peninsula and the Trans-Antarctic Mountains are a strong local source of gravity wave activity (Baumgaertner and McDonald 2007; Hei et al. 2008) but topography is less important than the stratospheric polar night jet in determining the overall gravity wave climatology (Hei et al. 2008).

4 Future Directions

The benefits of an increasingly dense array of GPS-RO profiles obtained from different instruments over the last ten years has enabled the study of many more atmospheric waves and coupling processes, especially in regions where ground-based profiles of temperature are not obtainable. The GPS-RO results will eventually allow more accurate wave parameterizations in models.

Initial investigations of the spatial and temporal variability of stratospheric gravity waves using COSMIC reveal the potential to increase substantially our knowledge of the atmospheric system and stratosphere-troposphere coupling. The results presented above also show the ability of COSMIC to monitor changes in potential energy on the order of a week over relatively small grids of 20° x 5°.

Future possibilities for using COSMIC include a detailed analysis of regional scale convective wave generation, propagation, and mean-flow interaction; model comparisons to determine the waves responsible for the observed potential energy; and incorporation with ground based instruments to expand understanding of the coupled stratosphere-troposphere system. The Antarctic region is an active area of research using COSMIC.

Acknowledgements The COSMIC research was undertaken while one of the authors (SPA) was in receipt of a Japan Society for the Promotion of Sciences (JSPS) post-doctoral fellowship. This study was supported in part by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) through a Grant-in-Aid for Scientific Research (19403009) and the Kyoto University Active Geosphere Investigation (KAGI) for the 21st century COE program. COSMIC data were obtained from the COSMIC Data Analysis and Archive Center (CDAAC). NCEP zonal wind and uninterpolated OLR data used in the study were provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at http://www.cdc.noaa.gov/. The helpful comments of two anonymous reviewers are gratefully acknowledged.

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