Impacts of background field removal on CMIP5 projected changes in Pacific winter cyclone activity

Cyclones are responsible for much of the high impact weather in the extratropics, thus how they will change under global warming is of great concern. Several studies have used the multimodel climate simulations conducted under Phase 5 of the Coupled Model Intercomparison Project (CMIP5) to examine such changes. One study suggested that the frequency of strong cyclones is projected to decrease over the North Pacific, while another concluded that this frequency will increase. A single tracking algorithm has been used to derive cyclone statistics from 23 CMIP5 simulations using two different definitions of cyclones: cyclones as minima in total sea level pressure (SLP) or cyclones as minima in SLP perturbations about a large scale, low-frequency background. When cyclones are defined by total SLP, the frequency of deep cyclones over the Pacific is projected to increase, while if cyclones are defined as perturbations, this frequency is projected to decrease. These differences are shown to be due to a projected deepening of the climatological mean Aleutian low. In view of these results, it is important to critically assess how cyclones should be defined. Preliminary results suggest that among CMIP5 simulations, over the Pacific, both the projected changes in the frequency of high wind events and mean available potential energy are better correlated with the projected changes in the frequency of cyclones defined as perturbations. It is concluded that more research should be done to quantify and understand the impacts of the different definitions of cyclones.