Observations of a Convectively Coupled Kelvin Wave in the Eastern Pacific ITCZ

Abstract

A case study of a convectively coupled Kelvin wave in the eastern Pacific intertropical convergence zone (ITCZ) is presented, as observed during the 1997 Pan American Climate Studies (PACS) Tropical Eastern Pacific Process Study (TEPPS). The large-scale convective envelope associated with this disturbance, with a zonal scale of approximately 1000?2000 km, propagates eastward at 15 m s?1 along the mean convective axis of the ITCZ. This envelope consists of many smaller-scale, westward-moving convective elements, with zonal scales on the order of 100?500 km. As the convectively coupled Kelvin wave disturbance propagates eastward, it exerts a strong control on local convection. Radar and vertical profiler data collected aboard the NOAA R/V Ronald H. Brown during the wave passage show that convection deepens rapidly as the Kelvin wave approaches from the west, progressing from isolated, shallow cumuli to organized deep convective features within just 12 h. Initially, rainfall in the vicinity of the ship consists of a significant deep convective fraction, but as the large-scale envelope departs to the east, stratiform precipitation becomes dominant. Radiosonde data collected during the Kelvin wave passage reveal dynamical perturbations in the troposphere and lower stratosphere that are consistent with linear equatorial Kelvin wave theory. The TEPPS radiosonde data also compare remarkably well with the vertical structure of a typical eastern Pacific Kelvin wave disturbance in the ECMWF reanalysis dataset, based on a 15-yr linear regression analysis. When this analysis is expanded to include all global grid points, it is shown that Kelvin waves in the eastern Pacific ITCZ have a dynamical structure that is nearly symmetric with respect to the equator, as would be expected based on linear Kelvin wave theory. However, the convective signal associated with these symmetric dynamical perturbations is itself primarily asymmetric with respect to the equator. The deepest convection is located significantly to the north of the equator, in the region of warmest sea surface temperatures. These observations present a somewhat different perspective on the dynamics of convectively coupled Kelvin waves, in that the symmetric dynamical fields and asymmetric convection interact to sustain the simultaneous eastward propagation of both fields.