Response of convectively coupled Kelvin waves to surface temperature forcing in aquaplanet simulations

Abstract

This study investigates changes in the propagation and maintenance of convectively coupled Kelvin waves (KWs) in response to surface warming. We use a set of three aquaplanet simulations made with the Community Atmospheric Model version 6 by varying the sea surface temperature boundary conditions, representing the current climate, a warmer (+4K), and a cooler (-4K) climate. Results show that KWs accelerate at the rate of about 7.1%/K and their amplitudes decrease by 4.7%/K. The dampening of KWs with warming is found to be associated with a weakening of the internal thermodynamic feedback between diabatic heating and temperature anomalies that generates KW eddy available potential energy (EAPE). The phase speed of KWs closely matches that of the second baroclinic mode KW in -4K, while the phase speed of KWs is approximately that of the first baroclinic mode KW in +4K. Meanwhile, the coupling between the two baroclinic modes weakens with warming. We hypothesize that in -4K, as the first and second modes are strongly coupled, KWs destabilize by positive EAPE generation within the second mode, and they propagate slower following the second mode KW phase speed. In +4K, as the first and second modes decouple, KWs are damped by negative EAPE generation within the first mode, and they propagate faster following the first mode KW phase speed.