3. Observations of Tropical Climate Dynamics and Convectively Coupled Waves

The first evidence of equatorially trapped waves in observational records appeared in 1966 in the work of Yanai and Maruyama [294], at the same time as the theoretical work of Matsuno [187]. Yanai and Maruyama [294] found signals of wave-like motion with strong cross equatorial wind in US Navy stratospheric wind data (which were apparently used to monitor nuclear activity during the cold war) when they were looking for evidence of eddy momentum transport as a plausible energy source for the quasi-biannual oscillation (QBO) in the equatorial stratosphere [188]. These waves correspond to the mixed Rossby-gravity waves from the Matsuno theory, which are also sometimes called Yanai-Maruyama or simply Yanai waves. Two years later Wallace and Kousky [271] (see also [83]) published their work on the discovery of Kelvin waves in the tropical stratosphere, which unlike those identified earlier by Yanai and Maruyama they are characterized by dominating zonal winds in phase with pressure perturbations. They were also motivated by the search for an energy source for the QBO in the form of wave eddy momentum.