Characteristics of atmospheric gravity wave activity in the polar regions revealed by GPS radio occultation data with CHAMP

Using GPS radio occultation data during 2001-2005, we studied the climatological behavior of atmospheric gravity waves in the polar stratosphere. We calculated temperature fluctuations with vertical wavelengths shorter than 7 km and then determined the wave potential energy, E_p, every month in a longitude-latitude cell of 20° × 10° between 12 km and 33 km. In the Arctic region (50-90°N), E_p shows an annual variation with maximum in winter, consistent with the zonal mean horizontal wind, V, and the Eliassen-Palm (E-P) flux, F_z. The large F_z values indicate higher planetary wave activity, resulting in distortion of the polar vortex. The unbalanced flow can then excite gravity waves through geostrophic adjustment. In the Antarctic region (50-90°S), E_p gradually increases during winter and reaches its maximum in spring before decreasing rapidly. The time derivative of V coincides with the E_p peak and the horizontal distribution of E_p has a similar structure to V, suggesting that the E_p enhancement is closely related to the decay of the polar vortex. During major warming events over the Arctic, the divergence of E-P flux, ΔF, was enhanced, coinciding with large E_p. In the Antarctic, ΔF strongly correlates with E_p in spring. Gravity waves seem to be effectively generated through planetary wave transience and/or breaking. Orographic generation of gravity waves seems to be important in limited areas only, such as Scandinavia and the Antarctic Peninsula, showing that it is less important than the polar night jet in determining the climatological behavior of gravity waves.