Abstract
Abstract. The Madden–Julian oscillation (MJO) is the main controller of the weather in the tropics on intraseasonal timescales, and recent research provides evidence 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 indicate 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 was used and enabled us to probe the direction and frequency of the interactions. It was found that equatorial stratospheric ozone and stratospheric zonal winds are connected with the MJO at periods of 1–2 months and 1.5–2.5 years. We explore the role of normal-mode interactions behind the stratosphere–troposphere coupling 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, influence the MJO on interannual timescales. MJO-related modes, such as Kelvin waves and Rossby waves with a symmetric wind structure with respect to the Equator, are shown to have significantly different dynamics during MJO events depending on the phase of the QBO.
Highlights
The Madden–Julian oscillation (MJO) and the quasi-biennial oscillation (QBO) are two of the main elements of atmospheric low-frequency variability in the tropics
We study the interactions between the stratosphere and the tropical troposphere, with particular emphasis on the MJO
The results indicate that the interaction of internal westward gravity waves, which are responsible for the QBO as well as Kelvin and Rossby waves associated with the MJO, partially explains the stratospheric influences on the MJO
Summary
The Madden–Julian oscillation (MJO) and the quasi-biennial oscillation (QBO) are two of the main elements of atmospheric low-frequency variability in the tropics. The QBO manifests in the tropical stratosphere as a reversal of the zonal winds with descending cycles with a mean period of 28 months, with important impacts on the global circulation of the atmosphere (Holton and Tan, 1980) Both are important players for the Earth system’s weather and climate. The same approach was used to study the conditions that led to the 2016 QBO disruption (Raphaldini et al, 2020) In this context a natural question arises: what is the role of these normal modes in the MJO interaction with the stratosphere? We perform the PDC analysis with the time series for the energies associated with each of the Hough modes responsible for the MJO dynamics (as in Žagar et al, 2015) and the stratospheric zonal wind. The results indicate that the interaction of internal westward gravity waves, which are responsible for the QBO as well as Kelvin and Rossby waves associated with the MJO, partially explains the stratospheric influences on the MJO
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