Earth Planets Space, Vol. 61 (No. 4), pp. 525-533, 2009
Toshitaka Tsuda1, M. Venkat Ratnam1,2, Simon P. Alexander1, Toshiaki Kozu3, and Yukari Takayabu4
1Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Japan
2National Atmospheric Research Laboratory (NARL), Gadanki, India
3Faculty of Science and Engineering, Shimane University, Japan
4Center for Climate System Research (CCSR), University of Tokyo, Japan
(Received January 30, 2008; Revised April 25, 2008; Accepted May 15, 2008; Online published May 14, 2009)
Using stratospheric temperature profiles derived from GPS radio occultation (RO) measurements made by the German CHAMP satellite from June 2001 to May 2006, we studied the climatological behavior of atmospheric wave activity in the tropics. The wave potential energy, Ep, is calculated from temperature fluctuations with vertical scales shorter than 7 km in a longitude and latitude cell of 20° × 10° at 19-26 km. Ep is then averaged every 3 months (June-July-August (JJA), September-October-November (SON), December-January-February (DJF), March-April-May (MAM)), and the averages are compared with the cloud top temperature from outgoing long-wave radiation (OLR) and the convective rain rate from the TRMM precipitation radar (PR). Ep at 19-26 km in the western Pacific to Indian Ocean is found to show a clear seasonal variation, with a large Ep during DJF and MAM and a considerably enhanced Ep in SON; it becomes minimum during JJA near the equator, when the center of the enhanced Ep region appears over north India and the Indochina peninsula. Localized enhancement of Ep seems to be mainly due to atmospheric gravity waves. In addition, the longitudinally elongated portion of Ep is partially affected by Kelvin wave-like disturbances with short horizontal scales. In DJF and MAM, the convective clouds are located over the western Pacific and around Indonesia, at which time the Kelvin wave-like disturbances are effectively generated. The spatial and seasonal variations of Ep are closely related to the distribution of clouds, implying that convective wave generation is very important in the tropics. However, wave-mean flow interactions due to the wind shear of the QBO become important in the lower stratosphere, which considerably modifies our analysis of the Ep distribution at 19-26 km. Therefore, both wave generation and propagation characteristics must be taken into account in describing the climatological behavior of atmospheric wave activity in the equatorial stratosphere.
Key words: Gravity wave, Kelvin wave, equatorial atmosphere, GPS occultation.