Initiation Processes of the Tropical Intraseasonal Variability Simulated in an Aqua‐Planet Experiment: What is the Intrinsic Mechanism for MJO Onset?

Abstract

To understand the intrinsic onset mechanism of the Madden‐Julian Oscillation (MJO), we simulated a set of initiation processes of MJO‐like disturbances in 10 year aqua‐planet experiments using a global atmospheric model with a 56 km horizontal mesh and an explicit cloud scheme. Under a condition with a zonally nonuniform sea surface temperature (SST) in the tropics, we reproduced MJO‐like disturbances over the western warm pool region. The lagged‐composite analysis of detected MJO‐like disturbances clarifies the time sequence of three‐dimensional dynamic and moisture fields prior to the onset. We found that midtropospheric moistening, a condition that is favorable for deep convection, is particularly obvious in the initiation region 5–9 days before onset. The moistening is caused by two‐dimensional horizontal advection due to cross‐equatorial shallow circulations associated with mixed Rossby‐gravity waves, as well as anomalous poleward flows of a negative Rossby response to suppressed convection. When the midtroposphere is sufficiently moistened, lower tropospheric signals of circumnavigating Kelvin waves trigger active convection. The surface latent heat flux (LHF) feedback contributes to the initial stages of convective organization, while the cloud‐radiation feedback contributes to later stages. Sensitivity experiments suggest that circumnavigating Kelvin waves regulate the period of MJO‐like disturbances because of efficient convective triggering and that the LHF feedback contributes to rapid convective organization. However, the experiments also reveal that both conditions are not necessary for the existence of MJO‐like disturbances. Implications for the relevance of these mechanisms for MJO onset are also discussed.

Observational Evidence of Mixed Rossby‐Gravity Waves as a Driving Force for the MJO Convective Initiation and Propagation

“…Figures d and S3b show that the midtropospheric equatorial IO has positive horizontal advective tendencies mainly by cross‐equatorial flows from off‐equatorial humid regions (e.g., 0–10°N, 60–80°E) associated with MRG shallow circulations or broadening of moistened areas around the IO. This is consistent with a preconditioning process in the aqua‐planet MJO study by Takasuka et al ().…”

“…The proposed mechanism covers a classical view that MJO convection comprises westward convective disturbances (Nakazawa, 1988) and insights from the long-term aqua-planet simulation (Takasuka et al, 2018). The next step is to examine the roles of MRGs in MJO mechanics in observation and numerical studies for more realistic cases.…”

“…The contributions of MRGs to MJO initiation have also been confirmed in the modulation of moisture fields through dry air intrusion (Kerns & Chen, ) and in vertical and horizontal moisture transports (Chen et al, ; Muraleedharan et al, ) in the Dynamics of the MJO (DYNAMO) campaign. Moreover, to understand the intrinsic initiation mechanism of the MJO, Takasuka et al () conducted long‐term aqua‐planet experiments using the Nonhydrostatic Icosahedral Atmospheric Model and suggested the importance of midtropospheric moistening by cross‐equatorial shallow circulations for triggering MJO convection. This result implies that MRGs may have an essential role in MJO initiation.…”

“…in the Dynamics of the MJO (DYNAMO) campaign. Moreover, to understand the intrinsic mechanism of the MJO,Takasuka et al (2018) conducted long-term aqua-planet experiments using the Nonhydrostatic Icosahedral Atmospheric Model and suggested the importance of midtropospheric moistening by cross-equatorial shallow circulations for triggering MJO convection. This result implies that MRGs may have an essential role in MJO initiation.…”

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

“…By examining the third MJO event during the DYNAMO period as observed in December 2011, Kubota et al (2015) found the advective pre-moistening effect of the westward-propagating disturbances with a 2-day period, which is consistent with the findings of Yu et al (2018). More recently, using the in situ observation collected during the Year of Maritime Continent (YMC)-Sumatra 2017, Takasuka et al (2019) found evidently westward-moving positive moisture and meridional wind disturbances of the quasi-3-8-day mixed Rossby-gravity waves preceding the onset of the December primary MJO event, validating their earlier findings using the aqua-planet experiments in Takasuka et al (2018).…”