Equatorial waves in the lower stratosphere. II: Annual and interannual variability

Abstract

AbstractThe annual and interannual variability of idealized, linear, equatorial waves in the lower stratosphere is investigated using the temperature and velocity fields from the ECMWF 15‐year re‐analysis dataset. Peak Kelvin wave activity occurs during solstice seasons at 100 hPa, during December–February at 70 hPa and in the easterly to westerly quasi‐biennial oscillation (QBO) phase transition at 50 hPa. Peak Rossby‐gravity wave activity occurs during equinox seasons at 100 hPa, during June–August/September–November at 70 hPa and in the westerly to easterly QBO phase transition at 50 hPa. Although neglect of wind shear means that the results for inertio‐gravity waves are likely to be less accurate, they are still qualitatively reasonable and an annual cycle is observed in these waves at 100 hPa and 70 hPa. Inertio‐gravity waves with n=1 are correlated with the QBO at 50 hPa, but the eastward inertio‐gravity n=0 wave is not, due to its very fast vertical group velocity in all background winds. The relative importance of different wave types in driving the QBO at 50 hPa is also discussed. The strongest acceleration appears to be provided by the Kelvin wave while the acceleration provided by the Rossby‐gravity wave is negligible. Of the higher‐frequency waves, the westward inertio‐gravity n=1 wave appears able to contribute more to the acceleration of the 50 hPa mean zonal wind than the eastward inertio‐gravity n=1 wave. Copyright © 2006 Royal Meteorological Society