Convectively Coupled Equatorial Waves. Part II: Propagation Characteristics

Abstract

Following the description of the horizontal and vertical structures of convectively coupled equatorial waves presented in Part I, here their propagation characteristics are investigated. Linear lagged regressions are used to produce their composite evolution, and the Radon transform technique is used to calculate their phase speeds. It is shown that coherent wave structures with convective coupling generally exist for about 1?2 weeks. Typical zonal wavenumbers are 6?8, wavelengths are 42°?64° of longitude, and typical periods are 4?8 days. The eastward phase speed of convectively coupled Kelvin waves is between 10 and 17 m s?1. The westward phase speed of the coupled mixed Rossby?gravity wave is between 10 and 15 m s?1, and the westward phase speed of the coupled n = 1 Rossby wave is between 7 and 9 m s?1. It is found that convection can produce stronger vertical coupling of phase speeds, and Doppler shifting by the ambient flow can modify phase speeds. There is further evidence that some waves tend to act as forcing agents for convection whereas others tend to be forced by convection. Eastward propagation of some n = 0 and 1 modes in the upper troposphere is also examined.