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

Climate models have long-standing difficulties simulating the South Pacific Convergence Zone (SPCZ) and its variability. For example, the default Zhang-McFarlane (ZM) convection scheme in the Community Atmosphere Model version 5 (CAM5) produces too much light precipitation and too little heavy precipitation in the SPCZ, with this bias toward light precipitation even more pronounced in the SPCZ than in the tropics as a whole. Here, we show that implementing a recently developed convection scheme in the CAM5 yields significant improvements in the simulated SPCZ during austral summer and discuss the 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 state-of-the-art reanalyses. This shift toward heavier, more variable rainfall increases both the magnitude and altitude of diabatic heating associated with convective precipitation, intensifying lower tropospheric convergence and increasing the influence of convection on the upper-level circulation. Increased diabatic production of potential vorticity in the upper troposphere intensifies the distortion effect exerted by convection on transient Rossby waves that pass through the SPCZ. Weaker distortion effects in simulations using the ZM scheme allow waves to propagate continuously through the region rather than dissipating locally, further reducing updrafts and weakening convection in the SPCZ. Our results outline a dynamical framework for evaluating model representations of tropical-extratropical interactions within the SPCZ and clarify why convective parameterizations that produce "top-heavy" profiles of deep convective heating better represent the SPCZ and its variability. The South Pacific convergence zone (SPCZ), a band of strong rainfall that stretches diagonally across the South Pacific from northwest to southeast, is difficult for climate models to simulate well. Here, we suggest that much of this difficulty stems from underestimating both how much heavy rainfall is produced in the SPCZ and how high above the surface this rainfall forms. The SPCZ has previously been described as a "graveyard" for weather systems. Our hypothesis casts the SPCZ more as a toll collector and suggests that the vertical location of the collection point is key. Simulated weather systems that produce heavier rainfall as they move through the SPCZ region release energy higher in the atmosphere, providing the SPCZ with the means to maintain itself. A model that releases this energy lower in the atmosphere by producing too much light rain allows many weather systems to bypass the toll, weakening the simulated SPCZ and drawing it equatorward in search of the energy it needs. Biases in simulated precipitation rate affect diabatic heating and the upper-level response to transient Rossby waves An improved deep convection parameterization reduces biases in the South Pacific Convergence Zone (SPCZ), especially for heavy rainfall More realistic upper-level heating strengthens feedbacks between waves and convection, blocking propagation of wave energy locally