Reducing Numerical Diffusion in Dynamical Coupling between Atmosphere
and Ocean in Community Earth System Model (CESM), version 1.2.1
Abstract
Climate models contain atmospheric and oceanic components that are
coupled together to simulate the thermodynamic and dynamic processes
during air-sea interactions. Community Earth System Model (CESM, version
1.2.1) is a state-of-the-art coupled model that is widely used and
participates in Coupled Model Intercomparison Projects. Community
Atmospheric Model (CAM), the atmospheric component of CESM, is based on
the finite-volume dynamic core, which utilizes staggered Arakawa-D
grids. However, the dynamics-physics (D-P) coupling in CAM causes the
prognostic winds of the dynamic core be interpolated onto non-staggered
locations, which affects the wind structure for computing the air-sea
interaction and dynamical coupling. In this study we propose a new
scheme that eliminates the extra interpolation during D-P coupling for
the atmosphere-ocean interaction. By numerical experiments and
comparative study of the new scheme, we show that it improves the
simulated climatology in key regions including eastern-boundary
upwelling regions and Southern Oceans. In turn, existing problems of the
model, such as warm SST biases, are reduced. The new scheme contain code
changes in CAM and the coupler, and they are provided as open-source
files. Similar approaches can also be adopted in coupled models that
utilize the atmospheric components with on staggered dynamics and
physics, such as spectral-element method based CAM.