The SO is a seesaw in surface pressure between the tropical Indian Ocean/eastern Pacific and the other parts of the tropical Pacific, which influences tropospheric temperature and other climate parameters on a global scale. A typical period range of the connected oscillation is 3 - 4 years.

The SO also influences the stratosphere, as has been shown by van Loon and Labitzke (1987). During so-called "warm events" of the SO in winter the Aleutian high is intensified and the stratospheric polar vortex is disturbed. The reason for that is that during warm events the warm Pacific sea surface then leads to stronger convection in the tropics, thus stronger radiation from the cloud tops in the lower stratosphere and a colder tropical stratosphere then (e.g. Labitzke, 1999).

As the SO has some influence upon the stratosphere, it can be assumed that it also influences the upper layers of the middle atmosphere, namely the mesosphere/lower thermosphere (MLT) region around 90 - 100 km altitude, which is dynamically connected with the stratosphere e.g. through planetary and gravity wave propagation. In Figure 1 the year-to-year variations of the Collm zonal prevailing wind at 95 km are plotted vs. the year-to-year variations of the Southern Oscillation Index (SOI). The positive correlation is in correspondence with the stratospheric bahaviour, i.e. a weaker stratospheric polar vortex - and thus weaker westerly winds - during warm events (i.e. lower SOI).

Detailed analysis of Collm winds in relation to the Southern Oscillation is found in a Word-File here.

References:

Labitzke, K., 1999: Die Stratosphäre. Springer, Berlin, 85p.

Van Loon, H., and K. Labitzke, 1987: The Southern Oscillation. Part V: The anomalies in the lower stratosphere of the northern hemisphere in winter and a comparison with the quasi-biennial oscillation. Mon Wea Rev. 115, 357 -369.
 

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3.11.2000, last modification: 23.2.2001