Stratospheric ozone and QBO interaction with the tropical troposphere on intraseasonal and interanual time-scales: a wave interaction perspective

Abstract. The Madden Julian Oscillation (MJO) is the main controller of the weather in the tropics on intraseasonal time-scales and recent research provide evidences that the Quasi-Biennial Oscillation (QBO) influences the MJO interannual variability. However the physical mechanisms behind this interaction are not completely understood. Recent studies on the normal mode structure of the MJO indicates the contribution of global scale Kelvin and Rossby waves. In this study we test whether these MJO-related normal modes are affected by the QBO and stratospheric ozone. The Partial Directed Coherence method enabled us to probe the direction and frequency of the interactions. It was found that the tropical stratosphere influences the MJO, with ozone and stratospheric wind influencing the MJO at periods of 1–2 months and 1.5–2.5 years, and on the decadal time-scale, the latter probably related to solar cycle forcing. We explore the physical mechanisms behind the stratosphere-troposphere interactions by performing a linear regression between the MJO/QBO indices and the amplitudes of the normal modes of the atmosphere obtained by projections on a normal mode basis using ERA-Interim reanalysis data. The MJO is dominated by symmetric Rossby modes but is also influenced by Kelvin and asymmetric Rossby modes. The QBO is mostly explained by westward propagating inertio gravity waves and asymmetric Rossby waves. We explore the previous results by identifying interactions between those modes and between the modes and the ozone concentration. In particular, westward inertio-gravity waves, associated with the QBO, influences the MJO on interannual time-scales. These modes have their largest intensities above the Himalayas in the stratosphere, suggesting that this region plays a key role in the QBO-MJO interaction.