Retaining Short-term Variability Reduces Biases in Wind Stress
Overriding Simulations
Abstract
Positive feedbacks in climate processes can obscure the initial drivers
of climate phenomena. Some recent global climate model (GCM) studies
partially circumvent this issue by controlling the wind stress felt by
the surface ocean such that the atmosphere and ocean become mechanically
decoupled. Most mechanical decoupling studies have chosen to override
wind stress with an annual climatology. In this study we introduce an
alternative method of interannually variable overriding which maintains
higher frequency momentum forcing of the surface ocean. Using CESM1, we
then compare the size of the residuals between these two methods of
overriding with a freely evolving control integration. We find that the
simple act of overriding with a climatology, as has been done in
previous studies, creates sea surface temperature (SST) residuals
throughout the global oceans on the order of
$0.5-1^\circ$C. This is substantially larger than
residuals from the interannually variable overriding case introduced
here. We attribute the SST biases in the climatological overriding case
to the lack of synoptic and subseasonal variability, which creates too
shallow of a mixed layer throughout the global surface ocean. This
shoaling of the mixed layer reduces the effective heat capacity of the
surface ocean and prevents anomalous heat from mixing into the
thermocline. These results have implications for the reevaluation of
past studies which have used climatological wind stress overriding and
for avoiding these biases in future wind stress overriding studies by
using interannually varying wind stress fields.