Three-Dimensional Structure and Evolution of the Vertical Velocity and Divergence Fields in the MJO

Abstract The features in the planetary-scale wind field that shape the MJO-related vertical velocity field are examined using the linear analysis protocol based on the daily global velocity potential field described in a companion paper, augmented by a compositing procedure that yields a more robust and concise description of the prevalent patterns over the Indo-Pacific warm pool sector (60°E–180°). The analysis elucidates the structural elements of the planetary-scale wind field that give rise to the characteristic “swallowtail” shape of the region of enhanced rainfall and the “bottom up” evolution of the vertical velocity profile from one with a shallow peak on the eastern end of the region of enhanced rainfall to one with an elevated peak on the western end. These distinctive features of the vertical velocity field in the MJO reflect the juxtaposition of deep overturning circulation cells in the equatorial plane and much shallower frictionally driven cells in the meridional plane to the east and west of the regions of enhanced rainfall. The zonal overturning circulations determine the pattern of ∂u/∂x and the meridional overturning circulations determine the pattern of ∂υ/∂y in the divergence profiles. These features are at least qualitatively well represented by the Matsuno–Gill solution for the planetary wave response to a stationary equatorial heat source–sink dipole.