Arctic cyclones have become more intense and longer-lived over the past seven decades

Arctic cyclones are a critical component of atmospheric circulation, impacting weather patterns and climate change. Their variability spans various timescales, with mid-latitude origins and Arctic generation both contributing. These cyclones significantly influence Arctic climate through temperature, humidity, and wind changes, cloud formation, precipitation, and surface fluxes. Strong cyclones, defined as those with central sea level pressures (SLPs) below 970 hPa, have been increasingly observed in recent decades, causing extreme weather events like rapid sea ice loss, winter heat waves, atmospheric rivers, and heavy precipitation. These events have profound environmental and socioeconomic consequences and potentially accelerate Arctic climate change. Prior research, utilizing reanalysis datasets, showed inconsistencies in long-term changes in Arctic cyclone activity, with varying sensitivities to cyclone activity parameters, algorithms, datasets, and periods. This study aims to resolve these inconsistencies by performing an integrated assessment using multiple reanalysis datasets covering more than seven decades, an improved cyclone identification and tracking algorithm, and a refined integrative cyclone activity metric. The study also investigates the underlying physical processes to provide benchmarks for evaluating models and future projections.

Previous studies suggested intensification of Arctic cyclone activity based on SLP data and upper-air relative vorticity. However, inconsistencies and debates arose from using different reanalysis datasets, parameters, algorithms, and time periods. Studies highlighted sensitivities to cyclone activity descriptors and the datasets used, leading to diverging conclusions on long-term trends. These discrepancies are even more pronounced in model simulations of past periods and future climate change scenarios. To address the conflicting evidence, this research performs a comprehensive analysis over an extended period, aiming to identify robust signals of Arctic cyclone change.

This study utilized three reanalysis datasets: NCEP-NCAR, ERA5, and JRA-55, covering the period from the 1950s to 2021. An improved cyclone identification and tracking algorithm was employed, combining SLP minimum requirements with thresholds for minimum SLP gradient, isobar closure, maximum propagation distance, minimum lifetime, and primary cyclone center selection. A key improvement involves calculating mean SLP gradients within a defined radius of cyclone centers, enabling universal application across different spatial resolutions. The study then derived monthly energy-based cyclone activity index (EnCAI) values, incorporating both potential and kinetic energy. EnCAI aggregates cyclone count, intensity, and duration, providing an integrated measure of cyclone activity. The study analyzed cyclone counts and durations across various cyclone intensity categories, and examined the spatial distribution of strong cyclone frequency, identifying regions with increased cyclone activity. Finally, to understand the mechanisms driving the observed intensification, the study analyzed baroclinic instability (using maximum Eady Growth Rate, EGR), tropospheric jet streams and waves, and the stratospheric vortex. ERA5 data was used for mechanistic analysis due to its high spatial resolution and consistency with other datasets.

The analysis of Arctic cyclone activity revealed a robust intensification over the past seven decades. The EnCAI, averaged across the three reanalysis datasets, showed a statistically significant upward trend, indicating increased cyclone energy. Examination of cyclone counts and durations across intensity categories revealed a long-term shift of the maximum cyclone counts from weaker to stronger cyclones. Strong cyclones, with central SLPs below 980 hPa in winter and 990 hPa in summer, showed a pronounced increase in both count and duration. The count of these strong cyclones increased by roughly 30% in winter and 35% in summer from the 1950s to the 2010s. Spatial analysis showed increased frequencies of strong cyclones across the Arctic, most notably over the North Atlantic Arctic during winter and the central Arctic during summer. The driving mechanisms for the intensification included enhanced lower tropospheric baroclinicity, amplified winter jet stream waves, and a strengthened summer tropospheric vortex. In winter, a deepened tropospheric trough over the subpolar North Atlantic steered cyclones northeastward, intensified by stratospheric influences. In summer, a strengthened tropospheric vortex over the central Arctic, coupled with a downward-intruding synoptic lower stratospheric Arctic vortex (S-SAV), played a crucial role in strengthening cyclones. The study differentiated between the Polar Stratospheric Vortex (P-SPV) influencing winter cyclones and the S-SAV affecting summer cyclones.

The findings address the research question by demonstrating a clear intensification of Arctic cyclone activity, resolving prior inconsistencies. The increased frequency and intensity of strong cyclones have significant implications for the Arctic climate system and mid-latitude weather patterns. The shift towards stronger cyclones suggests a more impactful influence on sea ice, ocean dynamics, and weather extremes. The enhanced baroclinicity and altered atmospheric circulation patterns highlight complex interactions within the climate system. The observed changes are more intricate than the simple Arctic amplification hypothesis. The role of the stratosphere, particularly the contrasting roles of the P-SPV in winter and the S-SAV in summer, provides novel insights into Arctic cyclone intensification mechanisms.

This study provides robust evidence of intensifying Arctic cyclone activity over seven decades, driven by enhanced baroclinicity and altered atmospheric circulation dynamics. The increased frequency and duration of strong cyclones have important implications for Arctic climate and weather extremes. Future research should explore the projected changes in Arctic cyclone activity under future climate scenarios, utilizing advanced climate models and focusing on the complex interplay between near-surface baroclinicity, atmospheric circulation changes, and stratospheric-tropospheric interactions. An Arctic cyclone intercomparison project is needed to evaluate uncertainties in model projections.

The study employed a single cyclone identification and tracking algorithm, although this improved algorithm is universally applicable across different resolutions. A comparison with other algorithms is needed to assess potential uncertainties. The study relied on reanalysis datasets, which contain inherent data uncertainties. Future work should incorporate high-resolution observational data, to further reduce uncertainties. The study primarily focused on the changes in cyclone activity; further research could integrate changes in atmospheric moisture transport and precipitation to better understand the complete impact of Arctic cyclones.