Downward propagating Equatorial Annual Oscillation and QBO generated multi-year oscillations in stratospheric NCEP reanalysis data

• Review of stratospheric zonal wind and temperature NCEP reanalysis data.
• Observed wave driven and downward propagating 12-month Equatorial Annual Oscillation.
• Observed multi-year oscillations with periods between 3 and 10 years.
• Wave mechanism for multi-year oscillations generated by QBO interactions with seasonal variations.
• Numerical model results for 5-year and 9-year quasi-decadal variations in support of observations.

In this brief review we discuss and summarize the results of an analysis of zonal wind and temperature variations from the National Center for Environmental Prediction (NCEP) reanalysis, which provide observational evidence for the 12-month Equatorial Annual Oscillation (EAO), in addition to the classical equatorial oscillations, the 6-month Semi-annual Oscillation (SAO) and Quasi-biannual Oscillation (QBO). The EAO is observed slowly propagating down with a velocity of about 3 km/month in agreement with the results from a numerical model, and characteristic of wave mean flow interactions at low latitudes that generate the QBO at 1.3 km/month. For data samples that cover as much as 40 years, the NCEP zonal winds reveal a rich spectrum of oscillations with periods between 3 and 10 years. Such multi-year oscillations can be generated by the QBO interacting with the seasonal variations through wave filtering. As shown with model simulations, the 30-month QBO can interact with the 12-month Annual Oscillation (AO) to generate a 5-year oscillation, which is prominent in the NCEP data. And the 27-month QBO can generate a 9-year quasi-decadal oscillation, which is also observed along with the related modulation signatures close to 3 years.