MJO Seasonality in Its Scale Selection: Perspectives From Space‐Time Spectral Analysis of Moisture Budget

Abstract

The Madden–Julian Oscillation (MJO) displays an evident seasonality in its spatiotemporal scale selection. In boreal winter, the MJO is best behaved at wavenumber 2 and oscillates on a narrow time scale centering on a ∼55‐day period. In contrast, a wavenumber‐1 selection and a broad oscillation frequency centering on a ∼33‐day period are observed in boreal summer. Using the space‐time cross‐spectral analysis between convection and moisture budget, we reveal that column processes determine seasonal variations in the MJO spatial organization, while the MJO is strongly damped by the horizontal moisture advection mainly due to its meridional component (Vadv). A timescale decomposition analysis suggests that the damping effect of Vadv results primarily from high‐frequency synoptic‐scale disturbances, and the Vadv component related to the seasonal‐mean moisture generally supports the MJO growth at large wavenumbers while inhibits the growth of small wavenumbers, which is the most evident in boreal summer. The advection of seasonal‐mean moisture by anomalous zonal winds supports the MJO growth at all wavenumbers in boreal winter, but in boreal summer the growth becomes weak and even turns negative for small wavenumbers. Furthermore, different MJO timescale selections in winter versus summer are rooted in the Vadv, as both the column processes and zonal moisture advection mainly support the propagation of high‐frequency modes. Observational results related to the horizontal seasonal‐mean moisture advection are reasonably validated in a dynamical moisture model. These findings advance our understanding of the MJO seasonality and offer alternative diagnoses for evaluating the MJO simulation fidelity in contemporary climate models.