Characteristics of Extratropical Cyclones Under Uniform SST Warming and Increased CO2 Forcing in a Global Storm-Resolving Model

Abstract

Using the GFDL EXperimental System for High-resolution prediction on Earth-to-Local Domains (X-SHiELD), a global storm-resolving atmosphere model with a slab ocean, we examine extratropical cyclone (ETC) responses under three scenarios: 4 K sea surface temperature (SST) warming, increased CO₂ concentration, and their combination. Compared to ERA5, the X-SHiELD control run produces more intense and short-lived ETCs, as well as a stronger seasonal cycle with fewer summer cyclones. Relative to the control run, SST warming reduces cyclone frequency in both the North Atlantic and North Pacific, especially for short-lived winter cyclones, while storm-track displacement is most evident over the North Atlantic. CO₂ forcing shows similar but weaker effects. While the average cyclone central pressure remains nearly unchanged, SST warming increases the variability of cyclone central pressure and intensifies the strongest ETCs, while higher CO₂ concentration reduces the frequency of intense ETCs. The mean 925-hPa wind speeds intensify in the warm sector of a cyclone under both forcings, with the increase in the 95th percentile wind speeds 2–3 times larger than the increase in the mean. Additionally, SST warming enhances precipitation near and north of cyclone centers, while CO₂ slightly suppresses precipitation. These precipitation changes are more closely associated with changes in moisture availability and moisture convergence than with changes in vertical motion. Additionally, the comparison among different experiments shows that precipitation responses to individual forcings are approximately additive while the wind speed response is more nonlinear.