Multiscale Structure of the Slow Kelvin Waves That Make up the MJO

Examining MJO events during CINDY/DYNAMO, we have shown that slow and zonally-narrow Kelvin waves are major building blocks of the MJO based on spatio-temporal wavelet transform (STWT) analysis of satellite-borne infrared radiation data. These Kelvin waves have a range of phase speed of 3-9 m s^-1, which is in line with the MJO’s phase speed, while they are distinct from the typical Kelvin waves (~16 m s^-1) that are shown up in climatological spectra. This finding is further corroborated by statistical analysis of data over 30 years. Composite analysis reveals the difference between the typical and slow Kelvin waves. Although both types of waves have similar convective features in which maximum convective activity is observed on and around the equator, the degree to which they are trapped in the equator is different. The e-folding scale in the meridional direction of the slow Kelvin waves is a smaller than that of the typical Kelvin waves, which is qualitatively consistent with the fact that the slow Kelvin waves have smaller equivalent depth and thus have a slow phase speed. We also analyze the dynamical and thermodynamical structures of the slow Kelvin waves. These features are expected to be a key to understanding some fundamental features of the MJO such as its phase speed and its diversity.