Nonlinear optimal moisture perturbations as excitation of primary MJO events in a hybrid coupled climate model

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.Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Distinct MJOs Under the Two Types of La Niña

“…Also different from the observations, the vertical pattern of Column is largely opposite to that of Vadv. These model biases are also found in many other GCMs (e.g., Li et al., 2022; Wei et al., 2020).…”

Diurnal Cycle of Precipitation Over the Maritime Continent Under Modulation of MJO: Perspectives From Cloud‐Permitting Scale Simulations

“…As a dominant mode of the TISO, the Madden‐Julian Oscillation (MJO) (Madden & Julian, 1971, 1972) initiated from the western Indian Ocean (Matthews, 2008; Wei et al, 2019, 2020) can modulate the behavior of the DCP (e.g., Fujita et al, 2011; Hassim et al, 2016; Ichikawa & Yasunari, 2006, 2008; Kanamori et al, 2013; Lu et al, 2019; Vincent & Lane, 2016, 2018). In general, the amplitude of the island DCP reaches its maximum during a local MJO suppressed period and reaches its minimum in an MJO active period, while the opposite is true for the oceanic DCP (e.g., Birch et al, 2016; Oh et al, 2012; Peatman et al, 2014; Rauniyar & Walsh, 2011; Sakaeda et al, 2017; Vincent & Lane, 2017).…”

Westward‐Propagating Disturbances Shape Diverse MJO Propagation

“…The fundamental characteristics and dynamics of MJO have been well‐documented (Jiang et al., 2020; Zhang et al., 2020) since its first discovery (Madden & Julian, 1971, 1972). After being initiated from the Indian Ocean (IO) (Matthews, 2008; Wei et al., 2019, 2020, 2023; Zhao et al., 2013), MJO deep convection expands and migrates eastward slowly (∼5 m/s) and may weaken when encountering the Maritime Continent (MC) barrier effect (Ling et al., 2019; Wei et al., 2022; Zhang & Ling, 2017); nevertheless, it can usually reintensify over the Western Pacific (WP) (Waliser et al., 2009; Wei & Pu, 2021) and finally damp over the Central‐Eastern Pacific (Hendon & Salby, 1994; Wang & Chen, 2017; Wei et al., 2018).…”

“…Besides eastward‐propagating intraseasonal oscillation (ISOe), westward‐propagating intraseasonal oscillation (ISOw) is also active in the tropics, such as equatorial Rossby waves (Gonzalez & Jiang, 2019; Kiladis et al., 2009) or westward‐propagating “moisture mode” (Fuchs‐Stone et al., 2019; Mayta et al., 2022). Previous studies have noticed that ISOw might shape the boreal summer intraseasonal oscillation (Wang & Rui, 1990; Wang & Xie, 1997; Wang et al., 2021), anchor or initiate MJO deep convection by interplaying with high topography (Feng & Li, 2016; Roundy & Frank, 2004; Takasuka & Satoh, 2021; Wei et al., 2019, 2020, 2023), and block eastward propagation of MJO moist convection through dry intrusion (DeMott et al., 2018; Feng et al., 2015; Fu et al., 2018; Huang & Pegion, 2022). In this study, we unexpectedly found that ISOw, which cooperates with ISOe, over the WP, can offer a plausible explanation of diverse MJO propagation initiated from the IO (WCL19).…”