Planetary Waves

The middle atmosphere exhibits variation on a time scale of a few days. Short term variations include travelling waves which are found at all seasons (although those occurring in summer have very small amplitudes). However, stratospheric warming which are connected with a very strong intensification of the planetary waves one or two, have the largest effect. They affect the stratosphere and mesosphere over periods varying between a few days and several months. The magnitude of temperature changes which can occur are shown. At some level (e.g., 50 km) changes exceed 70 K over 15 days from 28 December to 18 January. The sudden warming is such a large phenomenon that it strongly affects individual monthly means, giving larger planetary wave amplitudes than for months without large warmings. However, sudden warmings are part of the climatology, and their mean effect needs to be included, but an average over a small number of years for a given month can possibly be inadequate to obtain a reliable mean. Consequently, the means given here must be treated with caution. During the summer season, planetary wave amplitudes are small (a few K) so the year to year variability will cause little absolute error in the amplitude. Year to year variability of the monthly mean is illustrated.