Wave–Wave Interactions in the Stratosphere: Observations during Quiet and Active Wintertime Periods

Abstract

Using satellite data from the Nimbus 7 LIMS instrument, a previous study by Smith showed that interactions among planetary waves 1, 2 and 3 in the stratosphere were significant during January 1979. That month was characterized by an exceptionally large wave 1 amplitude in the stratosphere. The present study extends the analysis to the period November 1978–March 1979 to determine the conditions under which wave–wave interactions have a significant effect on variations in wave activity and on wave-mean flow interactions. A quantitative measure of how wave–wave interactions affect the wave activity of zonal waves 1 and 2 is obtained from the potential enstrophy budget. The results demonstrate that the relative importance of wave–wave versus wave-mean flow interactions depends on the magnitude of the eddy mean wind and potential vorticity relative to the zonal means. When the zonal mean wind is weak, a relatively small amplitude wave tends to behave nonlinearly, whereas when the mean wind is strong, only large amplitude waves are significantly nonlinear. In the 1978–79 winter, the zonal mean wind was weaker and wave–wave interactions were more important in middle and late winter than during November–December. Further evidence is presented that the vacillation between waves 1 and 2, which has been observed in the winter stratosphere of both hemispheres, is as strongly influenced by wave–wave interactions in the stratosphere as by variations in the forcing from the troposphere.