Abstract
Although the moisture feedback has been well known to be essential in the Madden–Julian Oscillation (MJO) dynamics, whether its pre-moistening effect plays a key role in exciting the onset of primary MJO events, as has been confirmed in the successive initiation, remains elusive. In this study, using a hybrid coupled climate model that has a good fidelity in simulating the intraseasonal variability, we develop a new framework of methodology to investigate the nonlinear excitation of primary MJO event, of which the key achievement is the successful implementation of the conditional nonlinear optimal perturbation (CNOP). In an application of this new framework, the CNOP-type moisture perturbations are calculated for the pre-chosen non-MJO reference states and generally favor a moistening in the equatorial region while drying in the poleward. Comparisons of the model simulation with observation give credibility to the existence of moisture signals several weeks before some primary MJO events. A suite of numerical experiments confirms that the CNOPs of moisture can contribute to the excitation and propagation of strong primary MJO events while random perturbations cannot. The moisture budget analysis further reveals the central importance of the horizontal moisture advection, especially the nonlinearly upscaled moisture transports associated with the high-frequency disturbances on the quasi-3–4-day and 6–8-day synoptic time scales, in supporting the nonlinear excitation of the primary MJO events. The subgrid-scale processes of evaporation, condensation and eddy transport of moisture are found to be critical for the pre-moistening effect in the boundary layer as well. This study directly supports the vital importance of the moisture perturbations, which are characterized by a particular pattern concentrated at low levels, to the nonlinear growth and propagation of the primary MJO events.
Highlights
The most prominent variability in the tropics at the intraseasonal time scale (20–90 days) has been widely recognized as the Madden–Julian Oscillation (MJO, Madden and Julian 1971, 1972)
We have no idea about how the well-organized, large-scale deep convection-circulation coupling system is excited and whether there exist any significant precursor signals preceding the MJO initiation (Zhang and Yoneyama 2017). This is more so, when it comes to the primary type of MJO events (M08), i.e., those occur as a single event or a first one of a chain of successive events in the real world
Previous studies have shown the critical role of moisture feedback in the MJO dynamics (Raymond and Fuchs 2009; Majda and Stechmann 2009; Sobel and Maloney 2013; Adames and Kim 2016; Liu and Wang 2017), and the composite life cycle of successive MJO events even displays evident pre-moistening signals during the initiation stage (M08)
Summary
The most prominent variability in the tropics at the intraseasonal time scale (20–90 days) has been widely recognized as the Madden–Julian Oscillation (MJO, Madden and Julian 1971, 1972). Utilizing the spectral density and wavelet analysis, Chen et al (2015b) focused on the suppressed phase of the MJOs captured during the field campaign of the Dynamics of the MJO (DYNAMO) (Gottschalck et al 2013; Yoneyama et al 2013) and revealed a deep layer of vapor resurgence during the phases 5–8 of MJO as defined by the real-time multivariate MJO (RMM) index (Wheeler and Hendon 2004) They found that the nonlinear moisture transport associated with the high-frequency oscillating patterns, including the diurnal, quasi-2-, quasi-3–4-, quasi-6–8-, and quasi-16-day perturbations plays an essential role in the MJO initiation. The last section summarizes our main findings and discusses some of their broader implications
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