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. Convectively coupled Kelvin waves are a unique type of tropical weather systems for which the interaction between cumulonimbi and atmospheric circulations plays a crucial role. The convectively coupled Kelvin waves aggregate rain-producing cumulus clouds at the spatial scales of a few to several 1,000-km and move them eastward at approximately 15 m/s. The wave therefore regulates the variability of tropical precipitation on timescales of days to weeks. Our work investigates the changes in the amplitude and speed of the wave in a warmer climate, which has not been studied much despite its implication on the future rainfall variability changes in the tropics. We run a set of water-covered Earth simulations with different surface temperature boundary conditions and examine the key aspects of the waves. We find that the convectively coupled Kelvin waves are weaker and move faster in a warmer climate. Our results also suggest that the mean state changes associated with warming alter the way cumulonimbi and atmospheric circulations interact within the wave at the fundamental level. Convectively coupled Kelvin waves (KWs) weaken and accelerate as the surface warms Internal thermodynamic feedback is the dominant KW maintenance mechanism in our simulations The results suggest a change in the KW dynamics with warming, especially regarding the vertical mode central to the maintenance of the wave