Seasonal Variability of Kelvin Wave Phase Speeds During Different Phases of the Madden-Julian Oscillation

Cloud superclusters associated with convectively coupled Kelvin waves occur within the active and suppressed convective envelopes of most Madden–Julian Oscillation (MJO) events. In turn, the MJO modulates the deep convection intensity, vertical cloudiness, and upper-level outflow associated with Kelvin waves. Previous work has also shown that Kelvin waves propagate via a broad range of phase speeds, around 10 m/s to 17 m/s, depending on location in the world and the phase of the MJO (Roundy, 2008). Thus, we examine further the seasonal behavior of Kelvin wave propagation and its interactions with the background environment.

This study investigates the phase speeds of Kelvin waves propagating along the tropics at different Real-time Multivariate MJO (RMM) phases during summer, winter, El Niño, and La Niña. To filter Kelvin waves from other convectively coupled tropical waves and the MJO, this study uses 2D Fourier transform applied to Outgoing Longwave Radiation (OLR) anomalies, as in Wheeler and Kiladis (1999). The days with Kelvin waves passing through each longitude during different RMM phases were separated into summer (JJAS), winter (NDJF), El Niño, and La Niña. The ENSO days depend on the Oceanic Niño Index (ONI).

To calculate the dominant phase speed of the Kelvin waves, we created time-lagged composites of the filtered OLR for each pair of base longitude and RMM phase separated by season. In each OLR composite, we followed the points of strong convective disturbance (negative OLR) and applied linear regression to get their phase speed. This study compares the phase speeds of Kelvin waves as they propagate along the eastern hemisphere versus the western hemisphere, during summer versus winter, and during El Niño versus La Niña. This study also compares the phase speeds of Kelvin waves during the eight different phases of the MJO to observe how the background winds dictate the behavior of Kelvin wave propagation.