Rapid transitions in the zonal wind (U) around the tropical tropopause within several days are investigated by using the global analysis data and their relation to the amplified Kelvin waves are considered. At 100 hPa, cases with a rapid increase in U (TypeU+ events) are concentrated in the eastern hemisphere, where such cases far outnumber cases that show a rapid decrease in U (TypeU- events). The difference in the number of the two types of events is greatest during the period November-March in the region 90°E-180°E. When only considering cases with a large zonal extent and eastward propagation, the dominance of TypeU+ events in the eastern hemisphere is much more distinct. The asymmetry between the two types is only weakly recorded at lower levels. An amplified and nonlinearly distorted Kelvin wave possibly accounts for predominance of these TypeU+ events at 100hPa. The dominance of TypeU+ events is detected in the basic easterly flow within the upper troposphere, which possibly facilitates the distortion by enhancing the upward propagation of energy and enabling the high intrinsic speed of the wave. In the distorted Kelvin wave, the latitudinal extent of the westerly signal is much smaller than that of the easterly signal.
2007 by the Meteorological Society of Japan