Seasonal Cycle of Explosive Growth of Extratropical Storms

Extratropical cyclones are responsible for severe and hazardous weather in the midlatitudes. They transport heat, momentum and moisture between latitudes and play important roles in the general circulation. Here, we present a new methodology for studying 6 hourly reanalysis data, based on spectral analysis is space and time, and determine the climatological properties of growing and decaying weather systems in six growth rate bins and two frequency bands. We focus on the seasonal variability of Northern and Southern hemisphere storm track modes for 20-year periods over the last 70 years. Leading Empirical Orthogonal Functions (EOFs) and storm tracks based on 850 hPa meridional winds and streamfunctions are determined for each frequency band and growth rate bin and compared with conventional EOFs and storm tracks that are based on all (growing and decaying) disturbances.

In the Northern hemisphere, results show slow‑growing weather systems exhibit familiar EOF patterns with peak amplitudes across the North Pacific and North America–Atlantic storm track regions near 45–50°N in both frequency bands. In the Southern hemisphere, EOF structures of slow growing modes are similarly focused near 45^oS across the Southern Ocean. In contrast, in both hemispheres moderate and rapidly intensifying systems show a systematic equatorward shift in their dominant structures, highlighting the sensitivity of storm‑track latitude to cyclone growth characteristics.

The observed equatorward displacement of explosive storms in both hemispheres is related to diabatic effects such as convection, latent heating and surface moisture fluxes. These are more prevalent in the subtropical regions and include effects such as the transition of tropical cyclones into explosive extratropical cyclones. During extratropical transition, tropical cyclones inject large amounts of diabatic heating in the midlatitude flow triggering downstream Rossby wave trains, and the rapid deepening of new storms that are strongly linked to intensified rainfall.

Our findings reveal how changes in the life‑cycle characteristics of mid‑latitude cyclones influence storm track structure and rainfall distribution. By linking changes in explosive storm development to long‑term shifts in rainfall, this study strengthens our understanding of the mechanisms driving extreme events, including intense precipitation and prolonged drought. The approach provides a valuable framework for diagnosing mid‑latitude storm behaviour and how associated rainfall may evolve under climate change, with important implications for future climate risk.