ENSO Impact on Kelvin Waves and Associated Tropical Convection

Abstract

Abstract The impact of El Niño–Southern Oscillation (ENSO) on atmospheric Kelvin waves and associated tropical convection is investigated using the ECMWF Re-Analysis, NOAA outgoing longwave radiation (OLR), and the analysis technique introduced in a previous study. It is found that the phase of ENSO has a substantial impact on Kelvin waves and associated convection over the equatorial central-eastern Pacific. El Niño (La Niña) events enhance (suppress) variability of the upper-tropospheric Kelvin wave and the associated convection there, in both extended boreal winter and summer. The mechanism of the impact is through changes in the ENSO-related thermal conditions and the ambient flow. In El Niño years, because of SST increase in the equatorial central-eastern Pacific, variability of eastward-moving convection, which is mainly associated with Kelvin waves, intensifies in the region. In addition, owing to the weakening of the equatorial eastern Pacific westerly duct in the upper troposphere in El Niño years, Kelvin waves amplify there. In La Niña years, the opposite occurs. However, the stronger westerly duct in La Niña winters allows more NH extratropical Rossby wave activity to propagate equatorward and force Kelvin waves around 200 hPa, partially offsetting the in situ weakening effect of the stronger westerlies on the waves. In general, in El Niño years Kelvin waves are more convectively and vertically coupled and propagate more upward into the lower stratosphere over the central-eastern Pacific. The ENSO impact in other regions is not clear, although in winter over the eastern Indian and western Pacific Oceans Kelvin waves and their associated convection are slightly weaker in El Niño than in La Niña years.