Theoretical Research And Observational Facts On The Influence Of The Three-dimensional Structure Of MJO On Its Eastward Transmission

This paper investigates the influence of the three-dimensional structure of MJO on its eastward propagation and the physical mechanisms from the perspective of theoretical model and observation.Based on the original theoretical framework,this paper establishes a linear"moist-dynamic"theoretical model by combining the first and second free atmospheric baroclinic modes and the boundary layer equations.This model involves the consideration of CFC feedback,the"moisture mode"theory and the role of the stratiform clouds.Based on 33-year observational data,this paper analyzes the existence of the vertical tilt structure of the MJO,its relationship with the eastward propagation speed and the physical mechanism.Besides,the applicability of two types of"moisture mode"theories to explain the eastward propagation of MJO also has been studied.The main conclusions are as follows:(1)The"moist-dynamic"theoretical model can simulate the slow eastward propagating MJO with the selection of zonal planetary scale.For wavenumber 1,the speed is around5.2ms^-1,which is close to the observation.Horizontal MSE advection,vertical MSE advection and boundary layer moisture change contribute 4ms^-1,3ms^-1 and 1ms^-1 to the propagation of MJO,respectively,while the surface latent heat flux and radiative flux contribute-1ms^-1 and-2ms^-1.Their net effects make MJO move eastward slowly at a speed of about 5ms^-1.(2)All eastward-propagating MJO events are accompanied by a vertical tilt structure,and the tilt degree varies from 9°to 23°.The stronger the tilt,the faster the MJO eastward propagating.The correlation coefficient between the tilting degree and MJO propagation speed is 0.50,exceeding a 0.01 significant level.On the one hand,the more tilted the vertical structure of the MJO,the stronger the sinking motion to the east of MJO,resulting in stronger poleward flow response at the lower troposphere.The mean large moisture along the equator would be advected by the stronger poleward flow,causing stronger meridional moisture advection.On the other hand,the stronger the sinking motion,the stronger the easterly Kelvin wave response at the top of the boundary layer,resulting in stronger boundary layer convergence and upward motion.The mean large moisture at the surface would be advected by the stronger ascending motion.Both aspects will lead to stronger moisture in the boundary layer,leading to a faster eastward propagation.Besides,the greater the tilting of the MJO,the more negative vertical MSE advection to the west,leading to a more negative MSE tendency.A stronger sinking motion to the east will cause positive vertical MSE advection,leading to more positive MSE tendency to the east.The greater MSE tendency gradient will lead to MJO faster propagation.(3)The background mean state modulates the vertical tilt structure of MJO.To the west of the MJO,the warmer sea surface temperature,more positive moisture,and stronger convective instability in the upper troposphere are beneficial to the formation of a more tilted MJO.Meanwhile,the warmer SST and positive moisture in the eastern Pacific,similar to the El Ni(?)o pattern,are also favorable to the formation of more tilted MJO.(4)When MJO convective center is located in the Indian Ocean and Maritime Continent(west of 140°E),both schools of“moisture mode”theory can explain MJO eastward propagation.When it propagates to the east of 140°E,the first type of“moisture mode”theory is still applicable,while only 23%of MJO meets the second type of“moisture mode”theory.