Downward propagation from the stratosphere to the troposphere: A comparison of the two hemispheres

The variability in the middle- and high-latitude stratosphere and troposphere on annual and intraannual timescales is investigated with focus on the differences between the two hemispheres. Results obtained with the National Centers for Environmental Prediction reanalysis data are compared to a general circulation model simulation. Although downward propagation of zonal mean zonal wind anomalies from the stratosphere to the troposphere are found on intraannual timescales in both hemispheres the two hemispheres show several differences. First, the intraannual variability is around 30% weaker in the southern hemisphere (SH) than in the northern hemisphere (NH). Second, in the SH the downward propagation is concentrated to the spring season while it is found during the whole winter half-year in the NH. Third, the coupling between the stratosphere and the troposphere is weaker and faster in the SH compared to the NH. Also, the annual cycle of the zonal mean zonal wind is quite different in the two hemispheres. In the SH the annual cycle is strong, shows a downward propagation with a timescale of two months, and includes higher annual harmonics of considerable strength. In the NH the annual cycle is weaker, shows no downward propagation, and is more sinusoidal. In both hemispheres the annual cycle and the intraannual variability are weaker in the model than in the reanalysis. The downward propagation in the model closely resembles that of the reanalysis both on intraannual timescales and in the annual cycle. In the SH the model underestimates the importance of the higher annual harmonics.