Coupling between the First and Second Baroclinic Modes within Convectively Coupled Kelvin Waves

A recent study by the authors showed that convectively coupled Kelvin waves (KWs) weaken and accelerate with surface warming in aquaplanet simulations. They hypothesized that the weakening and acceleration of KWs are due to the weaker coupling between the first and second baroclinic modes. The current study tests this hypothesis by investigating the mechanisms that couple the two baroclinic modes within KWs, focusing on how the second-baroclinic-mode temperature anomalies trigger the first-baroclinic-mode diabatic heating anomalies. Simulation results suggest that the second-baroclinic-mode temperature anomalies perturb the subgrid-scale convective updraft mass flux, which enhances the grid-scale vertical motion and the moistening of the lower troposphere. When moisture in the lower troposphere reaches maximum, first-baroclinic-mode diabatic heating also reaches maximum. The coupling between the two baroclinic modes weakens with warming due to the change in the second-baroclinic-mode structure, which presents upward-shifted peaks and weaker variability right on top of the boundary layer. Because the second-baroclinic-mode structure has a weaker control on temperature variability in the lower troposphere, the lower tropospheric moistening weakens, and thus, the deep convection cannot be triggered effectively. These results highlight the lower tropospheric moistening as a critical aspect of the coupling mechanism. Meanwhile, changes in the melting level and the second-baroclinic-mode structure are key aspects that determine the response of KWs to surface warming.