Four Theories of the Madden‐Julian Oscillation

Abstract

Studies of the Madden‐Julian Oscillation (MJO) have progressed considerably during the past decades in observations, numerical modeling, and theoretical understanding. Many theoretical attempts have been made to identify the most essential processes responsible for the existence of the MJO. Criteria are proposed to separate a hypothesis from a theory (based on the first principles with quantitative and testable assumptions, able to predict quantitatively the fundamental scales and eastward propagation of the MJO). Four MJO theories are selected to be summarized and compared in this article: the skeleton theory, moisture‐mode theory, gravity‐wave theory, and trio‐interaction theory of the MJO. These four MJO theories are distinct from each other in their key assumptions, parameterized processes, and, particularly, selection mechanisms for the zonal spatial scale, time scale, and eastward propagation of the MJO. The comparison of the four theories and more recent development in MJO dynamical approaches lead to a realization that theoretical thinking of the MJO is diverse and understanding of MJO dynamics needs to be further advanced.

Can geostrophic adjustment of baroclinic disturbances in the tropical atmosphere explain MJO events?

“…We should emphasize that, in spite of the abundant literature, there is no consensus on the dynamical explanation of the MJO, e.g. (Fuchs and Raymond 2017;Majda and Stechmann 2009;Matthews 2000;Sobel and Maloney 2013;Wedi and Smolarkiewicz 2010;Yang and Ingersoll 2011;Yano and Tribbia 2017) and the recent review (Zhang et al 2020) † . However, an essential ingredient, the vertical structure, which is well-pronounced in the MJO events, cf.…”

Large-scale controls of propagation of the Madden-Julian Oscillation

“…Meanwhile, a lack of consensus exists on the fundamental physics of the MJO 6,10 . One traditional body of thought considers the MJO to be a couplet of the equatorial Kelvin and Rossby waves.…”

Interactions of large scale dynamics in multi-model MJO simulations

“…The large scale flow is more divergent and zonally oriented with a Kelvin wave-like horizontal wind structure east of MJO convection. This wind distribution is similar to the Gill (1980) model shown in Figure 2.3, which has become a main starting point for explaining the large scale dynamics associated with the MJO (Zhang, 2005;Zhang et al, 2020).…”

“…Much of the MJO community's recent modeling efforts have been on improving the representation of cumulus convection over the Maritime Continent in order to improve mean state dry biases (Kim et al, 2017;Ahn et al, 2020). However, there has been less focus on investigating the role of the large scale circulations coupled to MJO convection and which play an instrumental role in the MJO's slow eastward propagation (Wang et al, 2018a;Zhang et al, 2020).…”

“…While numerous modeling studies have focused on the impact of MSE and moisture on the MJO, fewer studies research the large scale circulation aspects as emphasized recently by a review article about the current state of MJO theory (Zhang et al, 2020). Despite the need to improve the understanding of MJO dynamics, it has become apparent that there is a spatial asymmetry between the circulations to the west and east of the MJO's convection and the asymmetry is a necessary component to simulate an eastward propagating MJO Wang and Lee, 2017).…”