An Analytical Framework for Understanding Tropical Meridional Modes

A theoretical framework is developed for understanding the transient growth and propagation characteristics of thermally 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 equatorially 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 meridional 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 thermally 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. Finally, the relevance of this framework in explaining observed thermally coupled variability, such as the Pacific Meridional Mode, is addressed.