Resolving mesoscale frontal processes increases the large-scale circulation response to Gulf Stream SST anomalies

Robert Jnglin Wills

University of Washington and NCAR

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Tuesday, Feb. 28, 2023, 2:00 pm MT

Abstract

Canonical understanding based on general circulation models (GCMs) is that the large-scale circulation responds only weakly to extratropical sea-surface temperature (SST) anomalies, compared to the larger influence of tropical SST anomalies. However, the horizontal resolution of modern GCMs, which ranges from roughly 200 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 a variable resolution version of the Community Atmospheric Model (CAM6), with regional grid refinement of 14 km over the North Atlantic, and compare it to versions with 28-km regional grid refinement and global 111-km resolution. The high-resolution simulations show a large positive response of the wintertime North Atlantic Oscillation (NAO) to positive SST anomalies in the Gulf Stream, a 1-standard-deviation NAO anomaly for 2°C SST anomalies. The lower-resolution simulations show a much weaker response, and in some cases, a different spatial structure of the response. The enhanced large-scale circulation response results from an increase in resolved vertical motions with resolution, which enables SST forcing to have a larger influence on transient-eddy heat and momentum fluxes. In response to positive SST anomalies, these processes contribute to a stronger North Atlantic jet that varies less in latitude, as is characteristic of the positive phase of the NAO. Our results suggest that the atmospheric circulation response to extratropical SST anomalies is 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 thereby improving multi-year climate predictions.

SPEAKER BIO: Robert is a climate scientist studying the physical basis of climate variability and climate change, specifically the mechanisms governing changes in the large-scale circulations of the atmosphere and ocean. He obtained his Ph.D. from Caltech in 2016, studying planetary-scale Rossby waves and their role in the hydrological cycle in a changing climate under the supervision of Prof. Tapio Schneider. He is currently a research scientist at the University of Washington and an affiliate scientist at NCAR. From April 2023, he will be an assistant professor of climate dynamics at ETH Zurich.

Seminar Contact: psl.seminars@noaa.gov