Resolution Dependence of Atmosphere--Ocean Interactions and Water Mass
Transformation in the North Atlantic
Abstract
Water mass transformation (WMT) in the North Atlantic plays a key role
in driving the Atlantic Meridional Overturning Circulation (AMOC) and
its variability. Here, we analyze subpolar North Atlantic WMT in high-
and low-resolution versions of the Community Earth System Model version
1 (CESM1) and investigate whether differences in resolution and
climatological WMT impact low-frequency AMOC variability and the
atmospheric response to this variability. We find that high-resolution
simulations reproduce the WMT found in a reanalysis-forced
high-resolution ocean simulation more accurately than low-resolution
simulations. We also find that the low-resolution simulations, including
one forced with the same atmospheric reanalysis data, have larger biases
in surface heat fluxes, sea-surface temperatures, and salinities
compared to the high-resolution simulations. Despite these major
climatological differences, the mechanisms of low-frequency AMOC
variability are similar in the high- and low-resolution versions of
CESM1. The Labrador Sea WMT plays a major role in driving AMOC
variability, and a similar North Atlantic Oscillation-like sea-level
pressure pattern leads AMOC changes. However, the high-resolution
simulation shows a pronounced atmospheric response to the AMOC
variability not found in the low-resolution version. The consistent role
of Labrador Sea WMT in low-frequency AMOC variability across high- and
low-resolution coupled simulations, including a simulation which
accurately reproduces the WMT found in an atmospheric-reanalysis-forced
high-resolution ocean simulation, suggests that the mechanisms may be
similar in nature.