The Behavior of Wave 2 in the Southern Hemisphere Stratosphere during Late Winter and Early Spring

Abstract

A detailed analysis is presented of the behavior of the zonal wavenumber 2 component of the flow (wave 2) for July through October in the Southern Hemisphere stratosphere. Wave 2 in the stratosphere is characterized by a broad meridional structure peaking between 55° and 65°S, and regular eastward propagation, with periods ranging from 5 to 40 days. Maximum geopotential height amplitudes for a year range from approximately 600 to 1000 m. Examination of vertical structure suggests that during episodes of largest growth, wave 2 propagates upward from the upper troposphere. Regular eastward propagation is, however, evident only within the stratosphere. There are also episodes of wave 2 growth that do not appear connected with the troposphere; in general, the wave 2 amplitude is not as large in these cases. There are several years when wave 1 and wave 2 amplitudes are strongly anticorrelated in time during September and October. There are also years with strong positive correlation during August and September. While wave 1 is usually quasi-stationary, a number of instances where wave 1 moves eastward with wave 2 are observed, lasting from 4 to 10 days. Calculations show that the zonal mean state of the Southern Hemisphere stratosphere frequently satisfies conditions for instability. It is suggested that instability of zonally symmetric and asymmetric states, and nonlinear interactions between wave 1 and wave 2 both play a role in determining the behavior of wave 2 in the Southern Hemisphere winter and spring stratosphere.