Effects of MJO Vertically Tilted Structure on Its Phase Speed from the Moisture Mode Theory Perspective

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  • 1 Key Laboratory of Meteorological Disaster, Ministry of Education / Joint International Research Laboratory of Climate and Environmental Change / Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanjing, China
  • 2 Department of Atmospheric Sciences, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii
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Abstract

The effect of vertically tilted structure (VTS) of MJO on its phase propagation speed was investigated through the diagnosis of ERA-I reanalysis data during 1979-2012. A total of 84 eastward propagating MJO events were selected. It was found that all MJO events averaged throughout their life cycles exhibited a clear VTS, and the tilting strength was significantly positively correlated to the phase speed.

The physical mechanism through which the VTS influenced the phase speed was investigated. On the one hand, a stronger VTS lead to a stronger vertical overturning circulation and a stronger descent in the front, which caused a greater positive moist static energy (MSE) tendency in situ through enhanced vertical MSE advection. The stronger MSE tendency gradient lead to a faster eastward phase speed. On the other hand, the enhanced overturning circulation in front of MJO convection lead to a stronger easterly/low pressure anomaly at the top of the boundary layer, which induced a stronger boundary layer convergence and stronger ascent in the lower troposphere. This strengthened the boundary layer moisture asymmetry and favored a faster eastward propagation speed.

Corresponding author: Tim Li, International Pacific Research Center and Department of Atmospheric Sciences, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822. E-mail: timli@hawaii.edu

Abstract

The effect of vertically tilted structure (VTS) of MJO on its phase propagation speed was investigated through the diagnosis of ERA-I reanalysis data during 1979-2012. A total of 84 eastward propagating MJO events were selected. It was found that all MJO events averaged throughout their life cycles exhibited a clear VTS, and the tilting strength was significantly positively correlated to the phase speed.

The physical mechanism through which the VTS influenced the phase speed was investigated. On the one hand, a stronger VTS lead to a stronger vertical overturning circulation and a stronger descent in the front, which caused a greater positive moist static energy (MSE) tendency in situ through enhanced vertical MSE advection. The stronger MSE tendency gradient lead to a faster eastward phase speed. On the other hand, the enhanced overturning circulation in front of MJO convection lead to a stronger easterly/low pressure anomaly at the top of the boundary layer, which induced a stronger boundary layer convergence and stronger ascent in the lower troposphere. This strengthened the boundary layer moisture asymmetry and favored a faster eastward propagation speed.

Corresponding author: Tim Li, International Pacific Research Center and Department of Atmospheric Sciences, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822. E-mail: timli@hawaii.edu
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