基于局地多尺度能量涡度分析法(MS-EVA)的北半球夏季西太平洋MJO动能分析

An energetic study of the boreal summer western Pacific MJO with the localized multi-scale energy and vorticity analysis(MS-EVA)

采用局地多尺度能量涡度分析法(MS-EVA)和基于MS-EVA的局地正则传输与不稳定性理论对北半球夏季MJO的动能变化进行了诊断分析。结果表明:1)引起对流层上层和下层MJO动能变化的主要影响因素是有效位能转换和气压梯度力做功,其中有效位能转换在对流中心以北有明显的正的大值带,是MJO的主要动能源;气压梯度力做功则主要是将从有效位能转换而来的动能在空间重新分布。2)引起对流层中下层MJO动能变化的主要因素是动能跨尺度传输作用,其中大尺度向MJO尺度的跨尺度传输在对流中心附近表现为明显的正值,因此是该高度上MJO的动能源,并受5~15°N区域上空正压不稳定制约。3)MJO与天气尺度系统间的动能传输则主要表现为MJO的动能汇,其与MJO环流场分布以及MJO对天气尺度波动动量通量的平流输送有关。

Using the newly developed localized multiscale energy and vorticity analysis（MS-EVA） and the MS-EVA- based theory of canonical transfer and hydrodynamic instability, this study conducts a diagnosis on the variability of the boreal summer MJO kinetic energy（ MJO KE）.MS-EVA is based on a new functional analysis tool, namely multiscale window transform, which splits a signal orthogonally into different parts, each part characteristic of a specific scale range or scale window.In the present study,the atmospheric fields over the Western Pacific are re- constructed on a large-scale window,an MJO scale window, and a synoptic scale window, and the interactions a- mong these windows are investigated.From the results it is found that, in the upper and lower layers of the tropo- sphere,the MJO KE is governed by buoyancy conversion and pressure work.The positive center of the buoyancy conversion is located in the northern part of the convection,which is considered as the source of the MJO kinetic energy.The pressure work redistributes the energy over the region ,the negative center of which is located near the convection,and the positive center is located around the convection.A deeper study reveals that, in the middle- lower layers,the MJO kinetic energy mainly originates from the canonical transfer across the scales.The canonical transfer between the mean flow and MJO scale windows, which are located in the convection dues to the barotropic instability over the zonal band 5-15°N, and the canonical transfer between the MJO and synoptic scale windows,located in the convection, performs a pattern of the energy transfer from the MJO scale to synoptic scale, and depends mainly on the MJO-scale velocity gradient and transport of the synoptic wave momentum flux. Specifically,as the MJO convection propagates northwestward across the Western Pacific, the MJO KE un- dergoes variabilities, with the maximum located to the north of the convection center in both the mid-lower（ -700 hPa） and upper （- 200 hPa） troposphere. Our MS-EVA diagnosis reveals that the composite KE integrated through all levels increases with time to the north of the convection center.In the composite KE budget,the conversion from the available potential energy （APE） within the same scale and the work done by the pressure gradient dominate.This result is consistent with the Gill model result,in which heating contributes to the MJO development.In addition, geopotential fluxes from extra-tropics should be taken into account.The balance between the energy conversion from APE and pressure work for the composite MJO event is similar to that synoptic-scale dis- turbances.A large amount of KE dissipates near the tropopause due to the cumulus friction.The other important term in the KE balance is the cross-scale KE canonical transfer among the three scale windows.The KE transfer between the MJO and the synoptic scale windows is a sink of the MJO KE, while that between the MJO and large-scale windows is a major source.That is to say,barotropic instability is the main mechanism which extracts energy from the mean flow for MJO to grow;in particular,it has been shown that during the active phase over the Western Pacific,MJO extracts much more KE from the background.