An observational and theoretical study of the structure and propagation of the Madden-Julian Oscillation

Thesis (Ph.D.)--University of Washington, 2016-06

This study is compososed of two parts. In the first part of this dissertation, the large-scale circulation features that determine the structure and evolution of MJO- related moisture and precipitation fields are examined using a linear analysis protocol based on daily 850- minus 150-hPa global velocity potential data. The analysis is aug- mented by a compositing procedure that emphasizes the structural features over the Indo-Pacific warm pool sector (60◦E−180◦) that give rise to the eastward propagation of the enhanced moisture and precipitation. It is found that boundary layer (BL) convergence in the low level easterlies to the east of the region of maximum ascent produces a deep but narrow plume of equa- torial ascent that moistens the mid-troposphere, while weakly diffluent flow above the BL spreads moisture away from the equator. Vertical advection of moisture from this plume of ascent accounts for the eastward propagation of the positive mois- ture anomalies across the Maritime Continent into the western Pacific. When the convection is first developing over the Indian Ocean, horizontal moisture advection contributes to both the eastward propagation and the amplification of the positive moisture anomalies along the equator to the east of the region of enhanced convection. The results of the first part of the dissertation are used to develop a linear wave theory for the MJO, shown in part two. The theory is largely based on a framework previously developed by Sobel and Maloney. In this treatment, column moisture is the only prognostic variable and the horizontal wind is diagnosed as the forced Kelvin and Rossby wave responses to an equatorial heat source/sink. In contrast to the original framework, the meridional and vertical structure of the basic equations are treated explicitly, and values of several key model parameters are adjusted, based on observations. A dispersion relation is derived that adequately describes the MJO’s signal in the wavenumber-frequency spectrum and defines the MJO as a dispersive equatorial moist wave with a westward group velocity. On the basis of linear regression analysis of satellite and reanalysis data, it is estimated that the MJO’s group velocity is ∼40% as large as its phase speed. This dispersion is the result of the anomalous winds in the wave modulating the mean distribution of moisture such that the moisture anomaly propagates eastward while wave activity propagates westward. The moist wave grows through feedbacks involving moisture, clouds and radia- tion, and is damped by the advection of moisture associated with the Rossby wave. Additionally, a zonal wavenumber dependence is found in cloud-radiation feedbacks which causes growth to be strongest at planetary scales. Our results suggest that this wavenumber dependence arises from the non-local nature of cloud-radiation feed- backs; that is, anomalous convection spreads upper-level clouds and reduces radiative cooling over an extensive area surrounding the anomalous precipitation.