Relationships Between the Circulation and Eastward Propagation of the Madden-Julian Oscillation

Abstract

The Madden-Julian Oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics and represents a major connection between global weather and climate. Successful prediction of the phenomenon has proved to be a great challenge for both operational and climate models. In particular, the propagation of the MJO around the Maritime Continent remains a lingering question, in part due to differing explanations of the fundamental dynamics necessary for propagation across the Maritime Continent.

The circulation response to convection of the MJO has been suggested to have an impact on its propagation by a number of previous studies. This circulation contains both flanking Rossby waves to the rear and a Kelvin wave leading the convective center. In this study, we use a two-dimensional tracking mechanism to follow individual MJO events from a 40-year database, employ a technique to scale the MJO by its zonal wavelength, and use statistical methods to assess the role of the circulation in impacting propagation downstream.

Results suggest that both the geopotential height and wind anomalies east of the convective center are important to the eastward propagation of the MJO, which also changes depending on the region that the MJO is located over. Continuous eastward propagation is favored by having a Kelvin wave circulation, indicated by an easterly zonal wind anomaly and negative geopotential height anomaly east of MJO convection. Kelvin wave circulation east of MJO convection enhances moistening to support continuous eastward propagation of the MJO, mainly through meridional moisture advection. This is in contrast to several previous studies emphasizing the role of the Kelvin circulation in impacting boundary layer moisture convergence and vertical moisture advection. In addition to the known significance of having Kelvin wave easterly wind anomalies, the results of this study highlight that the existence of negative geopotential height is important to supporting moistening and MJO propagation, especially over the Indian Ocean. This thesis provides more insight into the relationships between MJO circulation and propagation by highlighting the sensitivity of MJO propagation to its circulation structure and its interaction with moisture.