Evolution of the vertically tilted structure of MJO during its eastward propagation

Abstract

The existence and evolution of MJO vertically tilted structure (VTS) across its eastward propagation have been validated through the diagnosis of observational data during 1979–2022 boreal winter. A total of 53 eastward-propagating MJO events, comprising 215 pentads, were selected based on cluster diagnosis. By comparing the range of ascending motion between the upper and lower layers in the rear of MJO convective centers, it has been demonstrated that the VTS exists only on the intraseasonal time scale and is not presented in the high-frequency or low-frequency fields. 70 % of MJO pentads are occupied with VTS. The proportion and intensity of VTS vary as the MJO propagates eastward from 60°E to 180°, both exhibiting a bimodal distribution. In most basins, MJO with VTS is a prominent feature, except where MJO convection is just forming (60°-70°E) or about to dissipate (170°E to 180°), in which the proportion of VTS is lower than that of no-VTS. The intensity of VTS follows a similar evolutionary pattern, being strongest in the Western Pacific and weakest in the western Indian Ocean and central Pacific. There is positive (negative) relationship between phase speed and intensity of VTS (proportion of no-VTS), the correlation coefficient of which is 0.59 (-0.66), all exceeding the 99 % significant level. The evolution of VTS would be regulated by the low-frequency background. The precipitation has a prominently positive (negative) impact on the intensity of VTS (no-VTS proportions). The vertical wind shear and upper-layer zonal velocity have a significantly negative (positive) effect on the intensity of VTS (no-VTS proportions).