Convectively coupled Kelvin waves are synoptic-scale, eastward-propagating tropical disturbances that play an important role in the overall convective activity in the Intertropical Convergence Zone (ITCZ). Kelvin waves contribute a substantial fraction of precipitation in the Pacific basin especially during boreal spring and summer as suggested by the wavenumber-frequency local spectra over this region. This study aims to characterize the mechanisms that excite Kelvin waves in the Pacific basin using daily precipitation from TRMM and wind fields and geopotential from MERRA2 and ERA-Interim reanalyses. Fourier-filtering and empirical orthogonal functions are used to isolate the modes of synoptic-scale convection associated with Kelvin waves. Regressions of TRMM-based Kelvin waves against wind fields are calculated in order to characterize the associated extratropical mechanisms and evolution at different lags by day.
Our results suggest that eastward-propagating extratropical waves from both hemispheres can force Kelvin waves. In boreal spring, extratropical Rossby disturbances from the northwest Pacific propagate into the tropics and intensify Kelvin waves in the East Pacific, mainly at 5°N but as far south as the Equator if the cold tongue is muted and replaced by warm water. These Kelvin waves are associated with local circulation effects off the west coast of South America because of interaction with the Andes. During boreal summer, extratropical Rossby disturbances propagating from the southwest Pacific appear linked to the initiation of Kelvin waves in the Central Pacific that then travel into the East Pacific following the ITCZ axis. These Kelvin waves can also be forced by another train of Rossby waves in far southeast Pacific. Therefore, extratropical Rossby waves play an important role in exciting Kelvin waves in the Central and East Pacific when interacting with the upper-level troughs; however, our results also suggest that extratropical Rossby waves (i.e. upper-level ridges) can weaken Kelvin waves in the East Pacific.