Structure of AGCM-Simulated Convectively Coupled Kelvin Waves and Sensitivity to Convective Parameterization

A study of the convectively coupled Kelvin wave (CCKW) properties from a series of atmospheric general circulation model experiments over observed sea surface temperatures is presented. The simulations are performed with two different convection schemes (a mass flux scheme and a moisture convergence scheme) using a range of convective triggers, which inhibit convection in different ways. Increasing the strength of the convective trigger leads to significantly slower and more intense CCKW activity in both convection schemes. With the most stringent trigger in the mass flux scheme, the waves have realistic speed and variance, and also exhibit clear shallow-to-deep-to-stratiform phase tilts in the vertical, as in observations. While adding a moisture trigger results in vertical phase tilts in the mass flux scheme, the moisture convergence scheme CCKWs show no such phase tilts even with a stringent convective trigger. The changes in phase speed in the simulations are interpreted using the concept of “gross moist stability” (GMS). Inhibition of convection results in a more unstable tropical atmosphere in the time mean, and convection is shallower on average as well. Both of these effects lead to a smaller GMS, which leads to slower propagation of the waves as expected from theoretical studies. Effects such as changes in radiative heating, atmospheric humidity, and vertical velocity following the wave have a relatively small effect on the GMS as compared with the time mean state determined by the convection scheme.