ASSESSING THE MADDEN JULIAN OSCILLATION’S (MJO) DECAY RATE AND FREQUENCY USING THE REAL-TIME MULTIVARIATE MJO (RMM) INDEX
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2022
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The Madden-Julian Oscillation (MJO), a dominant mode of variability on intraseasonal timescales, is a key phenomenon with pronounced global impacts and thus of great importance in the weather and climate continuum. In this study, we proposed the simplest linear stochastically forced damped oscillator model for MJO, by designating a complex MJO state variable with its real and imaginary parts by the widely used RMM index. With this 2-degree freedom conceptual MJO model, we developed a simple approach to assessing MJO’s decay rate and frequency as well as their seasonal and ENSO modulations, whereas these two key intrinsic properties of MJO have great implications for its predictability.
Our approach gives a relatively robust estimation of MJO’s main decay rate, frequency, and their annual cycle modulations. We found that the decay rate is about −0.05 day^-1, and the frequency is about 2\pi/45 day^-1. The decay rate has a significant annual cycle modulation with an amplitude of 27% of its mean decay rate. Its minimum and maximum are in March and September respectively. We only found a modest ENSO modulation on the MJO frequency which is subject to large uncertainty with 40 years of data. The relative weak ENSO impacts on MJO inferred from the observed MJO index may be partly because this index itself may not adequately capture ENSO’s modulations on MJO patterns.
Our approach may be further used to assess MJO’s two basic properties in comprehensive climate models and operational forecast models, which may be useful for quantifying MJO’s simulations and predictability. Further studies for a better understanding of MJO’s seasonal and ENSO modulations, may shield light on MJO’s fundamental dynamics, which remains an active research subject.
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Atmospheric sciences
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43 pages
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