Observations of a convectively coupled Kelvin wave

A large-scale, organized tropical convective disturbance propagated rapidly eastward across the eastern Pacific during the 1997 Tropical Eastern Pacific Process Study (TEPPS). In this study, evidence is presented that this eastward-propagating disturbance during TEPPS is a convectively coupled Kelvin wave. The convection associated with this wave passed directly over the TEPPS observational platform aboard the NOAA ship Ron Brown, which was stationed in the eastern Pacific ITCZ during August 1997. Both the remote and in-situ data collected during this Kelvin wave passage are analyzed in this study.

A convectively coupled Kelvin wave is a large-scale, eastward-propagating tropical convective disturbance which is coherently linked to shallow water-like perturbations in the dynamical fields of the troposphere and lower stratosphere. Radiosonde data collected during the TEPPS Kelvin wave passage show remarkable similarities to the theoretical structure of a linear Kelvin wave. Temperature and zonal wind anomalies propagate downward in time in the stratosphere and upper troposphere, as would be expected for a Kelvin wave observed as it passes over a fixed point. The vertical structure of temperature and zonal wind also match quite well with the vertical structure of a composite Kelvin wave derived from ECMWF reanalysis data.

High-resolution GOES-9 data show that the eastward-propagating Kelvin wave convective envelope during TEPPS consists of many smaller-scale, westward-propagating convective elements. Radar observations taken aboard the Ron Brown suggest that the Kelvin wave envelope is much more convectively active on its eastern end than its western end, which is dominated by stratiform rainfall. NOAA vertical profiler data are consistent with the radar data, with a strong convective component seen in the first 24 hours of the Kelvin wave convective signal, and a clear stratiform signal for the latter 24 hours. Surface meteorological data show a buildup of specific humidity for about two days prior to the convective event, and a rapid decline during the event itself. Recovery takes approximately 2-3 days. Surface wind is fairly weak and steadily easterly before the convective envelope passes by; during the convection the wind is stronger, more gusty, and westerly/southwesterly in direction. After the convection passes to the east of the ship, the surface wind turns back to easterly. This wind signal agrees with the theoretical zonal wind structure of a shallow water Kelvin wave.