  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.
