Enhanced atmospheric response to Gulf Stream SST anomalies in CAM6
simulations with 1/8-degree regional grid refinement over the North
Atlantic
Abstract
North Atlantic sea-surface temperatures (SSTs) exhibit variability on
seasonal to decadal timescales, providing a potential source of
predictability for the atmospheric circulation and regional climate on
these timescales. Recent work has shown that initialized climate models
have skill in predicting the decadal evolution of North Atlantic SSTs
[1], but this will only help to predict regional climate in the
surrounding continents if models can correctly simulate the atmospheric
response to these SST anomalies. There is growing evidence that models
systematically underestimate the atmospheric response to extratropical
SST anomalies [2], and that this may be rectified by increasing the
atmospheric resolution to resolve mesoscale processes over ocean frontal
zones [3]. Here, we investigate the large-scale atmospheric
circulation response to idealized Gulf Stream SST anomalies in two
configurations of the Community Atmospheric Model (CAM6), one with
1-degree resolution globally and one with regional grid refinement of
1/8-degree over the North Atlantic. The variable resolution
configuration, which resolves mesoscale atmospheric processes, shows a
large negative response of the wintertime North Atlantic Oscillation
(NAO) to a strengthening of the SST gradient across the Gulf Stream (a
2-standard-deviation NAO anomaly for SST anomalies that vary between
±2°C). The response is substantially weaker and has a different spatial
structure in the lower resolution simulations. The large-scale
atmospheric circulation response in the variable resolution simulations
results from mesoscale processes that enhance convection over the Gulf
Stream and lead to latent-heating and divergence anomalies in the upper
troposphere. These results suggest that the atmospheric circulation
response to extratropical SST anomalies may be fundamentally different
at higher resolution. Regional refinement in key regions offers a
potential pathway towards improving simulation of the atmospheric
response to extratropical SST anomalies and thus improving multi-year
regional climate predictions. [1] Yeager, S.G., et al., 2018,
https://doi.org/10.1175/BAMS-D-17-0098.1. [2] Simpson, I.R., et al.,
2018, https://doi.org/10.1175/JCLI-D-18-0168.1. [3] Czaja, A., et
al., 2019, https://doi.org/10.1007/s40641-019-00148-5.