Impacts of Model Horizontal Resolution on Mean Sea-Surface Temperature
Biases in the Community Earth System Model
Abstract
Impacts of model horizontal resolution on sea-surface temperature (SST)
biases are studied using high-resolution (HR) and low-resolution (LR)
simulations with the Community Earth System Model (CESM) where the
nominal resolutions are 0.1° for ocean and sea-ice and 0.25° for
atmosphere and land in HR, and 1° for all component models in LR,
respectively. Results show that, except within eastern boundary
upwelling systems, SST is warmer in HR than LR. Globally averaged
surface ocean heat budget analysis indicates that 1°C warmer global-mean
SST in HR is mainly attributable to stronger nonlocal vertical mixing
and shortwave heat flux, with the former prevailing over the latter in
eddy-active regions. In the tropics, nonlocal vertical mixing is
slightly more important than shortwave heat flux for the warmer SST in
HR. Further analysis shows that the stronger nonlocal mixing in HR can
be attributed to differences in both the surface heat flux and shape
function strength used in the parameterization. In addition, the shape
function shows a nonlinear relationship with surface heat flux in HR and
LR, modulated by the eddy-induced vertical heat transport. The stronger
shortwave heat flux in HR, on the other hand, is mainly caused by fewer
clouds in the tropics. Finally, investigation of ocean advection reveals
that the improved western boundary currents in HR also contribute to the
reduction of SST biases in eddy-active regions.