The Dependence of Tropical Modes of Variability on Zonal Asymmetry

Tropical modes of variability, including the Madden-Julian Oscillation (MJO) and the El Nino-Southern Oscillation (ENSO), are challenging to represent in climate models. Previous studies suggest their fundamental dependence on zonal asymmetry, but such dependence is rarely addressed with fully coupled ocean dynamics. This study fills the gap by using fully coupled, idealized Community Earth System Model (CESM) and comparing two nominally ocean-covered configurations with and without a meridional boundary. For the MJO-like intraseasonal mode, its separation from equatorial Kelvin waves and the eastward propagation of its convective and dynamic signals depend on the zonal gradient of the mean state. For the ENSO-like interannual mode, in the absence of the ocean's meridional boundary, a circum-equatorial dominant mode emerges with distinct ocean dynamics. The interpretation of the dependence of these modes on zonal asymmetry is relevant to their representation in realistic climate models.

Automated summary

This study investigates the dependence of tropical modes of variability, such as MJO and ENSO, on zonal asymmetry using an idealized CESM model. The findings suggest that the separation and propagation characteristics of these modes are influenced by the zonal gradient of the mean state. The study highlights the importance of considering zonal asymmetry in realistic climate models for accurate representation of these modes.