1.15 Some Physical Interactions Between Troposphere and Stratosphere
The mean zonal temperature structure shows that the stratosphere is thermally stable and the troposphere is thermally unstable. Wallace (1973) reviewed the general circulation of the tropical lower stratosphere and discussed two waves and three oscillations. They are the Yanai-Maruyama and Kelvin waves, and Quasi-biennial, Annual, and Semiannual Oscillations. Since 1973 many other oscillations have been discussed, such as the 20-40 day and 30-60 day oscillations (Hastenrath, 1994).
1.15.1 Yanai-Maruyama Wave
The Yanai-Maruyama wave is an upward-propagating and westward-propagating wave with a period of 4 to 6 days, zonal wave number 4 to 5, and vertical wavelength of the order of 5 km. The wave has been observed in the western Pacific (Yanai and Maruyama, 1966; Yanai et. al., 1968).
1.15.2 Kelvin Wave
The Kelvin wave is an upward-propagating and eastward-propagating wave with period of 10 to 15 days, zonal wave number 1 to 2, and vertical wavelength of the order of 10 km. The wave has been observed in the western Pacific (Wallace and Kousky, 1968).
1.15.3 Ouasi-biennial Oscillation
- The Ouasi-biennial Oscillation (QBO) in tropical stratospheric winds is defined as a reversal of the zonal wind directions over a fixed location from easterlies to westerlies. This reversal occurs with mean periods around 26-27 months, and a range of 23 to 30 months (McCreary, 1959; Ebdon, 1960; Reed, et al., 1961; Hastenrath, 1994). The maximum zonal wind speed is found within the layer between 20 to 34 km ( Fig. 2.24). The other observable characteristics of QBO are:
- (a). Downward displacement of zero wind speed line (Fig. 2.24) is close to 1 km per month. The westerly shear zone where westerly above easterly displaces faster than 1 km per month, and the easterly shear zone where easterly above westerly displaces slower than 1 km per month.
- (b). The extreme zonal wind speeds of QBO are located above the equator. The wind speed pattern of QBO is symmetric about the equator. The meridional span of QBO is about 15 degrees north and south latitudes.
- (c). The QBO is almost zonally symmetric, i.e. the QBO has little zonal variation.
- (d). At about the 25-km level, the maximum speed of the westerly wind regime is located above the equator and decreases poleward. The isotach pattern of easterly wind regime has more spread than that of the westerly wind regime.
- While the QBO observations mount and the notions on QBO formation increase, QBO simulations still remain as difficult tasks. Lack of understandings and hypotheses regarding factors such as vertically propagating waves, stratospheric temperature oscillation and ozone heating rate that may control QBO formation contribute to the difficulty.
- 1.15.4 Annual Oscillation
- The Annual Oscillation (AO) is defined as the tendency of the lower stratospheric winds to become easterlies in the summer hemisphere and westerlies in the winter hemisphere. The AO is primarily an extratropical phenomenon and does not interact strongly with tropical circulation systems.
- 1.15.5 Semiannual Oscillation
- The Semiannual Oscillation (SAO) of lower stratospheric zonal wind speed reaches its extreme in the extratropics and is an indicator of midwinter breakdowns in the polar night jet (Matsuno, 1970).
- The subject of possible relations between phases of waves or oscillations and yearly tropical cyclone frequency in the Atlantic and the north Western Pacific oceans have been receiving attentions by professional meteorologists, but no clear conclusions have yet been reached. The understandings of wave disturbances and tropical cyclone numbers, and their causes and effects are far from complete (Gray, 1984a,b; Burroughs, 1992).
- ***** The end of Section 1 *****
- Section 1.14
Section 2.1
Chapter 2