Dynamics of convectively coupled Kelvin waves in the eastern Pacific ITCZ

Convectively coupled Kelvin waves are large-scale, eastward-propagating tropical convective disturbances which are coherently linked to shallow water-like perturbations in the dynamical fields of the troposphere and lower stratosphere. These waves are often observed in the eastern Pacific ITCZ. In this study, we analyze the convective and dynamical structures of Kelvin waves in the eastern Pacific ITCZ during Northern Hemisphere summer (JJA) using composites calculated from outgoing longwave radiation (OLR) and ECMWF reanalysis data.

The convective and dynamical signals of convectively coupled Kelvin waves propagate eastward together at approximately 17 m s-1. The upper tropospheric and lower stratospheric dynamical signals are symmetric with respect to the equator, as would be expected for a theoretical linear Kelvin wave, while the convective signal is centered to the north of the equator, localized within the eastern Pacific ITCZ. The tropospheric and lower stratospheric vertical structures of these Kelvin waves also agree well with linear theory, with temperature and zonal wind phase lines tilting eastward with height in the stratosphere, and westward with height in the troposphere, below about 250 mb. These structures match the expected linear Kelvin wave response to an upper tropospheric convective energy source.

Prior to the onset of the convective signal associated with the composite convectively coupled Kelvin wave, a strong wavetrain propagates equatorward from the Southern Hemisphere jet into the subtropics. Kelvin wave convection appears to be initiated in a region of poleward, divergent upper tropospheric flow associated with this equatorward-propagating energy source.

Kelvin wave activity is shown to be modulated by lower frequency convective variability, such as is associated with ENSO and the Madden-Julian Oscillation (MJO). During a warm ENSO event, the overall increase in Kelvin wave variance occurs in approximately the same location as increased total convection; however, the peak in Kelvin wave variance is shifted significantly to the east of the peak in total convection. This suggests that increased convection in the eastern Pacific during a warm ENSO event may trigger an increase in Kelvin wave initiation, with maximum amplitude to the east of the maximum total convection. During the convective phase of the summer MJO, increased Kelvin wave activity is located well to the east of the western Pacific MJO convection, extending into the central and eastern Pacific. This suggests that MJO convection either triggers eastward-propagating Kelvin waves itself or sets up a large-scale basic state flow field that is conducive to Kelvin wave initiation to its east.