Madden–Julian Oscillation analog and intraseasonal variability in a multicloud model above the equator

Abstract

The Madden-Julian Oscillation (MJO) is the dominant component of tropical intraseasonal variability, and a theory explaining its structure and successful numerical simulation remains a major challenge. A successful model for the MJO should have a propagation speed of 4 -7 m/s predicted by theory; a wavenumber-2 or -3 structure for the planetary-scale, low-frequency envelope with distinct active and inactive phases of deep convection; an intermittent turbulent chaotic multiscale structure within the planetary envelope involving embedded westward-and eastward-propagating deep convection events; and qualitative features of the low-frequency envelope from the observational record regarding, e.g., its zonal flow structure and heating. Here, such an MJO analog is produced by using the recent multicloud model of Khouider and Majda in an appropriate intraseasonal parameter regime for flows above the equator so that rotation is ignored. Key features of the multicloud model are (i) systematic low-level moisture convergence with retained conservation of vertically integrated moist static energy, and (ii) the use of three cumulus cloud types (congestus, stratiform, and deep convective) together with their differing vertical heating structures. Besides all of the above structure in the MJO analog waves, there are accurate predictions of the phase speed from linear theory and transitions from weak, regular MJO analog waves to strong, multiscale MJO analog waves as climatological parameters vary. With all of this structure in a simplified context, these models should be useful for MJO predictability studies in a fashion akin to the Lorenz 96 model for the midlatitude atmosphere.coherent planetary intraseasonal variability ͉ multiscale structure ͉ intermittency in convection ͉ nonlinear analog model T he dominant component of intraseasonal variability in theTropics is the 40-to 50-day tropical intraseasonal oscillation, often called the Madden-Julian Oscillation (MJO) after its discoverers (1). In the troposphere, the MJO is an equatorial planetary-scale wave envelope of complex multiscale convective processes that begins as a standing wave in the Indian Ocean and propagates across the western Pacific at a speed of Ϸ5 m⅐s Ϫ1 (2-5). The planetary-scale circulation anomalies associated with the MJO significantly affect monsoon development and intraseasonal predictability in midlatitudes and the development of the El Niño southern oscillation in the Pacific Ocean, which is one of the most important components of seasonal prediction (6, 7). Present-day computer general circulation models typically poorly represent the MJO (8, 9). One conjectured reason for the poor performance of general circulation models is the inadequate treatment across multiple spatial scales of the interaction of the hierarchy of organized structures that generate the MJO as their envelope.There have been a large number of theories attempting to explain the MJO through a specific linearized mechanism, such as evaporation-wind feedback (10, 11), boundary layer...

The skeleton of tropical intraseasonal oscillations

“…Equatorial high-pressure anomalies are colocated with the westerly wind burst as in Kelvin waves; and they are flanked by off-equatorial low-pressure anomalies and cyclonic Rossby gyres, in broad agreement with the observational record (27)(28)(29). Rectification of the vertical structure and some of the phase relationships is likely due to effects of higher vertical modes (13,14,19,20).…”

“…The important role of synoptic scale wave activity in driving the MJO is documented in a growing body of evidence in the form of observations (28,37,38), simulations (13)(14)(15)(16)(17)23), and theory (18)(19)(20)(21)(22). This synoptic scale wave activity is a complex menagerie of convectively coupled equatorial waves, such as 2-day waves, convectively coupled Kelvin waves, etc.…”

“…It is well-known that this low-level moisture content plays a key role in regulating mesoscale convection, and there is a growing body of evidence that shows it also plays a key role in regulating convection on the scales of synoptic scale convectively coupled waves and the MJO (13,14,31,32,(34)(35)(36). A fundamental part of the model presented below is the effect of low-level moisture on the envelope of synoptic scale wave activity.…”

“…Nevertheless, they are all interesting theories that contribute to our understanding of certain aspects of the MJO. Other insight has been gained through the study of MJO-like waves in multicloud model simulations (13,14) and in superparameterization computer simulations (15-18), which appear to capture many of the observed features of the MJO by accounting for smallerscale convective structures within the MJO envelope. The role of convective momentum transport from synoptic scale waves in producing key features of the MJO's planetary scale envelope has also been elucidated by multiscale asymptotic models (19)(20)(21)(22)(23).…”

A Review of the Nonlinear Dynamics of Intraseasonal Oscillations

“…As a particular example, the multicloud moisture models of Khouider and Majda (2006a, b, c, 2007, 2008a and Majda et al (2007) can be processed through the asymptotic procedure stipulated in the derivation of IPESD. Ideally, this approach would yield a closed MJO model where synoptic scale fluctuations drive upscale fluxes of temperature and momentum while large scale flows modify the local synoptic scale environment where the fluctuations are generated.…”