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

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 similar to 55-day period. In contrast, a wavenumber-1 selection and a broad oscillation frequency centering on a similar to 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. The Madden-Julian Oscillation (MJO) is an intraseasonal (20-100-day) wave phenomenon prevailing in the tropics. Herein, we reveal that in boreal winter, the MJO prefers to organize at wavenumber 2 and oscillate regularly with a roughly 55-day period. In boreal summer, the MJO is typically a wavenumber-1 wave but can oscillate with broad frequencies centering on a shorter time scale (similar to 33-day period). We explain these features through the convection-humidity cross-spectrum analysis. The results suggest that the upward transport process of basic-state humidity largely shapes the seasonality in the spatial-scale selection, while the seasonal variations of the oscillation frequency are mainly caused by the meridional-wind advection of basic-state humidity. Moreover, we find that the meridional advection can support the MJO growth of small wavelength modes, while the zonal advection can destabilize all wavenumbers in boreal winter but damps small wavenumbers in boreal summer. We also partly reproduce these observed features in a mathematical model and validate the crucial importance of horizontal moisture advection in shaping the MJO seasonality. Our results help understand MJO simulations in state-of-the-art climate models. Especially, we propose that the moisture-convection coupling processes can be used to attribute the possible modeling biases, if any. The MJO shows an evident seasonality in the spatiotemporal scale selection Convection-coherent moisture processes well explain the MJO seasonality of space-time spectral features A dynamical moisture model partly validates the observed MJO scale seasonality