Toward an Understanding of the Madden-Julian Oscillation: With a Mesoscale-Convection-Resolving Model of 0.2 Degree Grid

Abstract

This paper describes results from numerical experiments which have been performed as the author's first step toward a better understanding of the Madden-Julian oscillation (MJO). This study uses the author's mesoscale-convection-resolving model that was developed in the 1980s to improve parametrization schemes of moist convection. Results from numerical experiments by changing the SST anomaly in the warm pool area indicate that the period of the MJO does not monotonously change with increasing SST anomaly. Between the two extreme cases (no anomaly and strong anomaly), there is a regime in which the period varies in a wide range from 20 to 60 days. In the case of no warm pool, eastward-propagating Kelvin waves are dominant, whereas in the case of a strong warm pool, it produces a quasi-stationary convective system (with pronounced time variation). In a certain regime between the two extreme cases, convective activities with two different properties are strongly interacted, and the period of oscillations becomes complicated. The properties and behaviors of large-scale convective system (LCS), synoptic-scale convective system (SCS), mesoscale convective system (MCS), and mesoscale convection (MC), which constitute the hierarchical structure of the MJO, are also examined. It is also shown that cloud clusters, which constitute the SCS (such as super cloud cluster SCC), consist of a few MCS, and a new MCS forms to the west of the existing MCS. The northwesterly and southwesterly low-level flows contribute to this feature. In view of recent emphasis of the importance of the relative humidity above the boundary layer, it is shown that the model can simulate convective processes that moisten the atmosphere, and the importance of latent instability (positive CAPE), which is a necessary condition for the wave-CISK, is emphasized.

Why do Super Clusters and Madden Julian Oscillation Exist over the Equatorial Region?

“…This asymmetric feature may come from the easterly zonal winds. The similar results were obtained by Oouchi and Yamasaki (2001) and Yamasaki (2011), in which intense convection takes place in the large gradient area of the SST.…”

Toward a Better Understanding of the Moist Atmosphere: Cooperative Interaction between Moist Convection and the Larger-Scale

“…As is well known, wave-CISK studies were extensively carried out by a great number of researchers after the middle of the 1980s, to explain, in particular, the Madden-Julian oscillation discovered by Julian (1971, 1972). A review of these studies was given by Yamasaki (2011), and is also given in Section 5. In this subsection, a brief summary on wave-CISK is described, primarily based on the author's studies.…”

“…The hydrostatic versions of the MCRMs (Yamasaki, , 2001) have been applied to not only TCs and tropical disturbance but also to other phenomena such as Kelvin wave-CISK, the Madden-Julian oscillation (Yamasaki, 2011), cloud clusters, and a meso-α-scale low associated with Baiu/Meiyu fronts (Yamasaki, 2002(Yamasaki, , 2005a(Yamasaki, , 2007b(Yamasaki, , 2008.…”

“…A number of studies have been made since the middle 1980s (e.g., Hayashi and Sumi, 1986). The author's study with the MCRM was reported as Yamasaki (2011). The 40-50 day period of the Madden-Julian oscillation, and the super cloud cluster (SCC) embedded in it were simulated by the MCRM under the condition that a warm pool of the western Pacific exists.…”

“…As for studies on the planetary-scale phenomena and the global warming effects (on tropical cyclones), the author considers that a coarse-grid model such as the MCRM will still be important in the coming years. The author's recent study on the Madden-Julian oscillation (Yamasaki, 2011) was made with the use of the MCRM. The behavior of the super cloud clusters (SCC) calculated in the numerical experiments was approved by Nakazawa (2016), who studied the behavior of cloud clusters embedded in SCCs associated with the Madden-Julian oscillation (Nakazawa, 1988).…”