Abstract:
The spectral structure of the climatic wind fields at the levels of 850 hPa(
V850) and 200 hPa (
V200) are investigated for a tropical region from 30 °S to 30 °N by the Fourier analysis scheme and the National Centers for Environmental Prediction/National Center fro Atmospheric Reseach (USA) reanalysis data, and the cause is discussed for the stationary wave of the tropical wind field to fill the gap of research in this regard. The basic conclusions are as follows: (1)
V is of low-dimensional and low-order features, with the annual average of the cumulative module square fitting rate up to 90% and 98% for |
m|=0,4 and 0,3 waves for the monthly
V850 and
V200 respectively. (2) The zonal mean component
V0 is the most important, and its annual average of the single-wave fitting rate
ρ0 is up to 52% and 85% for
V850 and
V200 respectively. The 850 hPa
V0 mainly consists of two branches of trade wind belts from the Northern and Southern Hemisphere and is strong in the winter hemisphere and weak in the summer hemisphere, and its axis position corresponds to the centers of hemispheric Hadley circulation. The 200 hPa
V0 is composed of a strong westerly wind belt outside the tropics and weak easterlies within the tropics. The seasonal variations are stronger at 850 hPa than at 200 hPa, and much stronger in the Northern Hemisphere than in the Southern Hemisphere. (3) The maximal components of the stationary wave of the wind field are both
V*|1| for 850 hPa and 200 hPa levels throughout the year, the annual average fitting rate
ρ*|1|, of annual biweekly oscillation, is up to 39% and 55% for 850 hPa and 200 hPa respectively, and its amplitude reaches maximum in the summer and winter and minimum in the autumn and spring of the Northern Hemisphere. The second wave components are 3- and 2-wave in the winter and summer of the Northern Hemisphere and the same for the transitional seasons, with the exception of 4 waves for
V*850 in October to December. (4) The maximal and second wave components of the steady wave OLR
* in January and July are the same as those of
V* during the same period, 1-and 3-wave for January and 1-and 2-wave for July. The vertical configuration of the extreme region of the important OLR
* wave component and the divergence fields of the corresponding
V* wave components is in conformity with the kinetic theory. (5) The vast region of Qinghai-Tibet Plateau and regions to the east and south are the climatic zones where the formation of the precipitation and the release of the latent heat take place most easily over the same latitudes, where the main components
V*|1| and
V*|2| of
V* are both of the convergence at the lower levels and divergenc at the upper levels, and OLR
1* and OLR
2* are both negative.