A mechanism‐denial study on the Madden‐Julian Oscillation with reduced interference from mean state changes

Abstract

Mechanism‐denial experiments using Superparameterized Community Atmosphere Model are conducted to investigate the importance of extratropical and circumnavigating waves, wind‐evaporation feedback, and radiative‐convective feedback to the Madden‐Julian Oscillation (MJO). A common issue with mechanism‐denial studies is the interference from mean state changes when processes are turned off in the model. Here time‐invariant forcing and nudging on effective timescales longer than the intraseasonal timescale are implemented to maintain the mean state. The MJO activity remains largely unchanged with suppressed extratropical and circumnavigating waves when the mean state is maintained to be close to that of the control run, suggesting that excitation of MJO by extratropical and circumnavigating waves is not necessary for the existence of MJO in this model. It is also shown that the wind‐evaporation feedback slows down eastward propagation of the MJO, and the radiative‐convective feedback amplifies the MJO.

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

“…It is difficult to discuss the role of the extratropics by comparing our study with previous works directly, but ultimately the MJO can be initiated without the aid of dynamical processes from the extratropics. This is consistent with Ma and Kuang (), who used a sensitivity experiment in which perturbations in the extratropics were damped without changing the mean state to show that extratropical disturbances have no influence on the existence of MJO events.…”

“…In addition, the east‐west ratio of the spectral power for 15 S–15 N OLR filtered for zonal wavenumbers 1–4 and periods of 20–100 days is 1.82. These imply that the absence of circumnavigation of Kelvin waves is irrelevant to the existence of MJO‐like disturbances, consistent with Zhao et al (), Maloney and Wolding (), and Ma and Kuang (). However, the frequency at the spectral peak associated with MJO‐like disturbances was lower in the KDAMP experiment (more than 50 days), which suggests that it takes longer for large‐scale deep convective activities to initiate without circumnavigating Kelvin waves.…”

Review of Tropical‐Extratropical Teleconnections on Intraseasonal Time Scales

“…Ray and Li () also performed experiments in which the effects of extratropical disturbances impinging on the tropics were cut off, in this case by the imposition of wall‐like boundary conditions in the subtropics, whose effect is to eliminate the MJO. Ma and Kuang () point out, however, that such mechanism denial experiments lead to a change in the basic state of the atmosphere and that repeating these experiments with care taken to keep the basic state unchanged (with nudging on very long time scales) leads to the opposite conclusion: the MJO can exist quite well without extratropical forcing. Clearly, this is an area in which more study is needed.…”

A Census of Atmospheric Variability From Seconds to Decades

“…It features eastward propagation of large‐scale patterns of rainfall, zonal wind, and humidity at an average speed of 5 m s −1 along the equator from the Indian Ocean to the Pacific Ocean (Zhang, ). Tremendous progress has been made in the study of the MJO during the past four decades (Zhang et al, ) and recently in its understanding (Ma & Kuang, ; Xu & Rutledge, ; Yang & Ingersoll, ), simulation (Ajayamohan et al, ; Ling et al, ; Song & Seo, ), and prediction (Chen et al, ; Shelly et al, ). Because of its timescale and expected eastward propagation, the MJO is considered to be a major source of predictability on subseasonal to seasonal timescales (Vitart, ; Waliser et al, ).…”