Upward- and Downward-Propagating Kelvin Waves

Propagating Kelvin waves were decomposed into upward and downward components using ERA-I reanalysis. The dataset anomalies were filtered by using an algorithm similar to Wheeler and Kiladis, which they applied to separate tropical waves into eastward and westward bands. We found that downward-moving Kelvin waves tilt to the east with height with maximum amplitude around the tropopause, and the signal decays rapidly in the troposphere near 400 hPa. Its footprint is also remarkable in the mesosphere. The downward propagating Kelvin wave, which transfers momentum upward seems to emanate from the stratosphere and not from the middle troposphere as commonly mentioned, but the associated energy is moving upward from the middle troposphere.

On the other hand, upward-moving Kelvin wave signals tilt to the west with height up to 200 hPa where they sharply tilt vertically. Upward Kelvin waves increase their tilt when they pass through the tropopause, while downward Kelvin waves show no change in the tilt, which is inconsistent with gravity wave dynamics. We also found that Kelvin wave conserve amplitude when they pass through the tropopause. This result invalidates the rigid lid approximation.