Since 1966, two types of wave motions have been discovered in the tropical stratosphere. These have been identified with the two gravest modes of a family of equatorial waves. These waves are characterized by downward phase propagation, which renders them important in the vertical transport of energy and zonal momentum. In the tropical lower troposphere there exists a separate class containing wave modes which do not propagate vertically, one of these being the familiar easterly wave.
The role of these two classes of waves in the tropical general circulation is discussed and the possible energy sources for the waves are enumerated.
1 Contribution No. 208, Department of Atmospheric Sciences, University of Washington, Seattle, Wash.