Tropical Variability of Idealized Coupled Aqua and Ridge Planets using the Community Earth System Model

Tropical modes of variability in Earth's climate system, such as the Madden Julian Oscillation (MJO) and the El Niño Southern Oscillation (ENSO), are prominent features of major scientific and societal interest. They also prove to be challenging to represent in contemporary, state-of-the-art climate models. In this study, we examine the tropical modes of variability with relevance to MJO and ENSO in two configurations of fully coupled Community Earth System Model (CESM) with idealized ocean geometry. The Aqua configuration is ocean-covered except for two minimal polar land caps, and the Ridge configuration has an additional grid-cell-wide pole-to-pole strip continent. In their mean climate states, Aqua is characterized by a global cold belt of equatorial upwelling, while Ridge exhibits Pacific-like zonal asymmetry with a western warm pool and eastern cold tongue. On the subseasonal time scale, Ridge shows a number of Earth-like tropical wave modes, including a MJO-like mode that distinguishes itself from equatorial Kelvin waves. These features are quite different on Aqua, which lacks a MJO-like mode distinguishable from Kelvin waves. On the interannual time scale, Ridge shows an ENSO-like mode of variability, with variance concentrated in its eastern cold tongue. In comparison, Aqua's interannual variability is also concentrated in its equatorial belt of upwelling, but at higher frequency than Ridge. We discuss the interpretation of these tropical modes of variability with respect to the role of Ridge's zonal asymmetry, and their relevance to the understanding of MJO and ENSO as represented in realistic climate models. Furthermore, we explore the implications of these tropical modes for scale interaction, such as the impact on tropical cyclones.