Resolving weather fronts increases the large-scale circulation response
to Gulf Stream SST anomalies in variable-resolution CESM2 simulations
Abstract
Canonical understanding based on general circulation models (GCMs) is
that the atmospheric circulation response to midlatitude sea-surface
temperature (SST) anomalies is weak compared to the larger influence of
tropical SST anomalies. However, the horizontal resolution of modern
GCMs, ranging from roughly 300 km to 25 km, is too coarse to fully
resolve mesoscale atmospheric processes such as weather fronts. Here, we
investigate the large-scale atmospheric circulation response to
idealized Gulf Stream SST anomalies in Community Atmosphere Model (CAM6)
simulations with 14-km regional grid refinement over the North Atlantic,
and compare it to the response in simulations with 28-km regional
refinement and uniform 111-km resolution. The highest resolution
simulations show a large positive response of the wintertime North
Atlantic Oscillation (NAO) to positive SST anomalies in the Gulf Stream,
a 0.8-standard-deviation anomaly in the seasonal-mean NAO for 2°C SST
anomalies. The lower-resolution simulations show a weaker response with
a different spatial structure. The enhanced large-scale circulation
response results from an increase in resolved vertical motions with
resolution and an associated increase in the influence of SST anomalies
on transient-eddy heat and momentum fluxes in the free troposphere. In
response to positive SST anomalies, these processes lead to a stronger
North Atlantic jet that varies less in latitude, as is characteristic of
positive NAO anomalies. Our results suggest that the atmosphere responds
differently to midlatitude SST anomalies in higher-resolution models and
that regional refinement in key regions offers a potential pathway to
improve multi-year regional climate predictions based on midlatitude
SSTs.