Upward and downward atmospheric Kelvin waves over the Indian Ocean

Stratospheric Kelvin waves are often understood as plane gravity waves, yet tropospheric Kelvin waves have been interpreted as a superposition between the baroclinic modes. Fourier filtering is used to decompose the ECMWF-Interim reanalysis dynamical fields into upward and downward propagating components. Then wavelet regression is used to isolate the propagating Kelvin waves over the Indian Ocean across different speeds at zonal wavenumber 4. Results for fast waves show dry upward-phase signal in the troposphere, while downward-phase Kelvin waves occupy most of the stratosphere. The presence of upward-phase tilted waves in the troposphere suggests that the tropospheric Kelvin wave is not a superposition of the upward and downward components, as one might expect in a normal mode. We found that propagating Kelvin waves in the troposphere obey gravity wave dynamics with geopotential height in phase with the zonal wind, the vertical velocity out of phase with the zonal wind, and the temperature in quadrature with the zonal wind. Both dry and moist tropospheric Kelvin waves show a westward vertical tilt, suggesting that tilt probably cannot be a superposition between baroclinic modes coupled to convective and stratiform heating. In the context of radiating gravity waves, results suggest that faster tropospheric Kelvin waves appear to be associated with higher Brunt-Vaisala frequencies, and waves maintain similar vertical tilt across a wide range of phase speeds.

Automated summary

Stratospheric Kelvin waves are interpreted as plane gravity waves, while tropospheric Kelvin waves are seen as a superposition between baroclinic modes. Through Fourier filtering and wavelet regression analysis, it is found that fast Kelvin waves exhibit dry upward-phase signals in the troposphere and downward-phase waves in the stratosphere. Tropospheric Kelvin waves follow gravity wave dynamics with specific relationships between geopotential height, vertical velocity, temperature, and zonal wind. The presence of westward vertical tilt in both dry and moist tropospheric Kelvin waves suggests that this tilt cannot be a superposition between different baroclinic modes.