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