Response of Convectively Coupled Kelvin Waves to Surface Temperature Forcing in Aquaplanet Simulations

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 to represent the current climate as well as warmer (+4 K) and cooler (−4 K) climates. 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 −4 K, while the phase speed of KWs is approximately that of the first baroclinic mode KW in +4 K. Meanwhile, the coupling between the two baroclinic modes weakens with warming. We hypothesize that in −4 K, as the first and second baroclinic modes are strongly coupled, KWs destabilize by positive EAPE generation within the second baroclinic mode and propagate more slowly, following the second baroclinic mode KW phase speed. In +4 K, as the first and second baroclinic modes decouple, KWs are damped by negative EAPE generation within the first baroclinic mode and propagate faster, following the first baroclinic mode KW phase speed.