Lifetime and longitudinal variability of equatorial Kelvin waves around the tropical tropopause region

Abstract

Using the European Centre for Medium‐Range Weather Forecasts 40 year reanalysis data for zonal wind fields, we investigated the characteristic variability and mean lifecycle of Kelvin waves around the tropical tropopause layer and upper troposphere. The distributions of Kelvin wave activity, mean squared amplitude, and number of passing cases closely resemble each other. Kelvin wave disappearance locations relative to appearance longitudes were examined at 200 and 100 hPa. At 200 hPa Kelvin waves mostly appear in the western hemisphere. At 100 hPa the highest‐frequency region is centered in the eastern hemisphere. On average, eastward propagation extending over ∼80° in longitude occurs at both levels examined. Mean lifecycle and its relation to background conditions were analyzed with a composite method. In a typical example at 200 hPa, Kelvin waves appear over the Indian Ocean and western Pacific, propagate eastward through westerly basic winds, decrease over South America, and then accelerate eastward. Their vertical structure has divergence in the upper and middle troposphere and weak convergence in the lower troposphere, tilting eastward with height. At 100 hPa, Kelvin waves typically propagate eastward in easterly basic winds and then decrease over a region between the Indian Ocean and western Pacific Ocean. Kelvin wave signals continuing over South America suggest that some waves first recognized in the western hemisphere at 200 hPa propagate upward and eastward near South America, reaching 100 hPa in the eastern hemisphere. Temperature anomalies in both hemispheres show the boomerang‐like vertical structure while associated with eastward‐propagating convections, which suggests at least some waves are convectively coupled.