Quasi-biennial Oscillation and Its Analog under the Assumption of Wave Self-Acceleration

Abstract

We have investigated numerically, using a slowly varying model, how the behavior of the quasi-biennial oscillation (QBO) in the equatorial stratosphere and its analog in the laboratory water tank are modified when wave phase velocities are allowed to be variable in time and space. Associated modifications of the wave characteristics were examined as well. The differences between the wave self-acceleration assumption and a constant phase velocity assumption are significant for the equatorial QBO, probably due to strong deformation of the mixed Rossby–gravity waves. The oscillation period of the equatorial QBO is extended approximately 25%, which is too large to be simply neglected in obtaining a correct oscillation period. Easterly wind speed is more accelerated than expected and exceeds the initial value of the mixed Rossby–gravity wave phase velocity. At the same time, westward phase velocity of the mixed Rossby–gravity wave is drastically accelerated at almost twice the initial value. Eastward phase velocity acceleration of the Kelvin wave is restricted to within only 15%. Phase velocity of the mixed Rossby–gravity wave decreases to zero in the westerly regime. Phase velocity acceleration (or deceleration) of the mixed Rossby–gravity wave is predominant in the upper region of the QBO phenomenon and amplitude of the wave must be small there due to strong damping experienced Phase velocities of the internal gravity waves in the case of the QBO analog reproduced in a water tank are accelerated (or decelerated) very little (within 10% of the initial phase velocity) except in the early transient stage. Increase of the oscillation period associated with incorporation of wave self-acceleration is also restricted to within 10%. The reason that the internal gravity waves, together with the Kelvin wave, do not experience significant wave self-acceleration may be principally due to the smaller group velocities.