Examining Changes in North Atlantic Extratropical Cyclones with Climate Change

Extratropical cyclones are major contributors to our everyday weather and while they can often contribute to societal and economic distress, many areas, such as the Mediterranean region, are also reliant on these systems to sustain livelihoods. Thus, it is vital to understand how these systems will change with our changing climate. Many studies examining changes in the frequency and intensity of extratropical cyclones with climate change to date have been conducted using relatively coarse resolutions, which may not fully be resolving diabatic processes, thus underestimating the impacts of global warming on extratropical cyclones. Furthermore, while several studies have investigated climate change impacts on the most intense storms, few studies have examined changes in cyclones across various intensity regimes.

This study provides a unique, comprehensive analysis investigating the changes in the location, frequency, intensity, and storm-scale dynamics of North Atlantic extratropical cyclones with warming. High-resolution simulations spanning ten winter seasons over the North Atlantic conducted using the Weather Research and Forecasting (WRF) model with a horizontal grid spacing of 20-km were analyzed. Using temperature changes derived from a subset of CMIP5 models using the RCP 8.5 scenario, a pseudo-global warming approach was taken to simulate the same 10 winter seasons in a warmer climate. Extratropical cyclones were then tracked as minima in sea-level pressure (SLP) in both the present-day and future simulations using the MAP Climatology of Mid-latitude Storminess (MCMS) tracking algorithm. Cyclones were categorized as strong, moderate, or weak based on 5th and 95th percentile thresholds of minimum SLP perturbation and storm-relative composites of various fields were generated to explore changes in storm dynamics.

Consistent with previous studies, an overall reduction in extratropical cyclone activity is seen over the North Atlantic storm track. Enhanced activity, however, is evident in the northeast North Atlantic and immediately off the United States East Coast. While shifts in storm intensity are statistically significant in some instances, the changes are relatively small and thus, may not hold practical significance. However, due to the potential for compensating processes, little alteration in storm strength does not necessarily imply that the dynamics of storms will remain the same with warming.

Storm relative composites of both strong and moderate storms show an increase in 6-hourly precipitation, contributing to enhanced latent heat release, which in turn contributes to the strengthening of the 900-700 hPa layer average potential vorticity (PV). Without the compensating effect of additional latent heat release enhancing the diabatic PV, strong and moderate storms would be even weaker in the future, indicating that storms are more diabatically driven in a future climate. These results suggest that while storms within all strength regimes show no substantial change in intensity, the dynamics of these systems are changing and the impacts associated with these 
systems, such as strong near-surface winds and heavy precipitation, are becoming more severe with warming.