An analytical framework for understanding tropical Meridional Modes

TL;DR

This paper develops a theoretical framework to analyze the transient growth and propagation of meridional mode-like structures in the tropics, highlighting mechanisms behind the dominance of certain SST modes in tropical variability.

Contribution

It introduces a simplified non-normal system model that separates symmetric and anti-symmetric SST modes, revealing the dynamical processes governing their evolution and the role of WES feedback.

Findings

Transient growth of specific SST modes toward lower order modes.

Symmetric and anti-symmetric modes are governed by similar mechanisms, with Kelvin wave affecting symmetric decay.

WES feedback is positive for all modes except the symmetric mode with Kelvin wave influence.

Abstract

A theoretical framework is developed for understanding the transient growth and propagation characteristics of thermodynamically coupled, meridional mode-like structures in the tropics. The model consists of a Gill-Matsuno type steady atmosphere under the longwave approximation coupled via a wind-evaporation-sea surface temperature (WES) feedback to a "slab" ocean model. When projected onto basis functions for the atmosphere the system simplifies to a non-normal set of equations that describes the evolution of individual sea surface temperature (SST) modes, with clean separation between symmetric and anti-symmetric modes. The following major findings result from analysis of the system: (i) a transient growth process exists whereby specific SST modes propagate toward lower order modes at the expense of the higher-order modes; (ii) the same dynamical mechanisms govern the evolution of symmetric and anti-symmetric SST modes except for the lowest-order wave number, where for symmetric structures the atmospheric Kelvin wave plays a critically different role in enhancing decay; and (iii) the WES feedback is positive for all modes (with a maximum for the most equatorially confined antisymmetric structure) except for the most equatorially confined symmetric mode where the Kelvin wave generates a negative WES feedback. Taken together, these findings explain why equatorially anti-symmetric "dipole"-like structures may dominate thermodynamically coupled ocean / atmosphere variability in the tropics. The role of non-normality as well as the role of realistic mean states in meridional mode variability are discussed.