Wave-Convection Interactions Amplify Convective Parameterization Biases in the South Pacific Convergence Zone

Climate models have long struggled to realistically simulate the South Pacific Convergence Zone (SPCZ) and its variability. For example, the default Zhang-McFarlane (ZM) convection in the Community Atmosphere Model version 5 (CAM5) produces too much light precipitation and too little heavy precipitation in the SPCZ, with this bias even more pronounced in the SPCZ region than in the broader tropics. In this presentation, we show that implementing a recently developed convection scheme in the CAM5 yields significant improvements in the simulated SPCZ during austral summer and describe the main reasons behind these improvements. In addition to intensifying both mean rainfall and its variability in the SPCZ, the new scheme produces a larger heavy rainfall fraction that is more consistent with observations and a state-of-the-art reanalysis. This shift toward heavier, more variable rainfall amounts is linked to increases in both the magnitude and altitude of diabatic heating associated with convective precipitation, thereby intensifying lower tropospheric convergence along the SPCZ axis and increasing the extent to which convection influences the upper-level circulation. Increased diabatic production of potential vorticity in the upper troposphere increases the distortion effect exerted by convection on transient Rossby waves passing through the SPCZ region. The much weaker distortion effects in simulations using the ZM scheme mean that waves are more likely to propagate continuously through the region rather than dissipate locally, thereby reducing updrafts and weakening convection within the SPCZ. Our results outline a dynamical framework for evaluating model representations of tropical-extratropical interactions within the SPCZ region and clarify why convective parameterizations that produce a more realistic top-heavy profile of deep convective heating are beneficial to representing the SPCZ and its variability.