An Analytical Model for Understanding Tropical Meridional Mode Behavior

Meridional modes are important sources of subtropical and tropical ocean / atmosphere variability in the climate system. Meridional mode behavior has been identified in observations, fully coupled models, and in simple coupled models. In this presentation, we describe meridional mode behavior in the context of a simple theoretical model that consists of a Gill (1980)-type equilibrium atmosphere coupled to a simple “slab” ocean model. By projecting the model into a parabolic cylinder function basis the model provides a simple framework within which meridional mode behavior can be explained in terms of atmospheric modes.

A critical finding is that meridional modes involve a cascade toward lower wavenumbers. This downward cascade explains a variety of meridional mode characteristics, including the equatorward and westward propagation of meridional mode structures. Additionally, due to conservation of equatorial symmetry the downward cascade also highlights a fundamental difference between equatorially symmetric and anti-symmetric structures, and demonstrates why equatorially anti-symmetric structures dominate growth under a variety of mean states. The simple model framework provides guidance on limits for modal and transient instability as well. This presentation will introduce the simple model and will apply the model to explain a wide range of characteristics of meridional mode behavior.