A new metric for net extratropical cyclone activity and its insights into surface climate trends

Abstract Extratropical cyclones (ETCs) are ubiquitous features of the global mid- and high-latitudes and are intimately linked with daily weather variations, and play a central role in balancing the Earth’s atmospheric momentum and energy budgets. Our interest in this paper is on how various aspects of cyclonic activity impact on features on the surface, including sea ice distribution and ocean waves. We use the Melbourne University cyclone tracking scheme applied to the ERA5 reanalysis to generate a global database of all ETCs over the period 1979–2022. Among other cyclone characteristics the scheme determines the ‘depth’ of each identified system, this parameter being related to the total kinetic energy of a cyclone and can be regarded as an appropriate measure of ‘intensity’. We argue that the number of cyclones in a given region does not necessarily convey information about their impact, nor does the mean depth of those cyclones. To address this, we have devised a new parameter, the cyclone accumulated depth (CAD), which incorporates both these metrics. We explore the links between the seasonal trends in our cyclone variables and those of various surface features, including sea ice and ocean significant wave height. (Globally the decrease in sea ice extent has continued apace and we explore that for the period December 1979 to August 2025. We find a new record Northern Hemisphere winter low of 12.76 × 10 6 km 2 was set in 2025, while in the Southern Hemisphere in 2023 record lows were set in all four seasons, and the annual mean dropped below 10 × 10 6 km 2 for the first time.) The trend in the CAD parameter appears to be very insightful in this. Our study reveals that over key areas of the globe trends in mean cyclone numbers, intensity and CAD can be quite different, meaning that trends in ‘cyclonic activity’ need to be interpreted cautiously, and may depend critically on what measure is used. We contend that CAD, incorporating both frequency and intensity, is an appropriate measure of such activity.