Separating Forced and Internal Contributions to Future Northern Hemisphere Storm Track Changes and Associated Precipitation Impacts

Abstract. Storm tracks — the preferred pathways of extratropical cyclones in the midlatitudes — are projected to shift poleward, migrate upward in the atmosphere, and change in intensity under anthropogenic climate change. These changes have important consequences for weather and climate variability across the Northern Hemisphere (NH) midlatitudes. Here we investigate future changes in NH storm track strength and position, their driving physical mechanisms, and their impacts on precipitation, using large ensemble (LE) simulations from three CMIP6 models alongside ERA5 reanalysis data. Robust projections are developed under two scenarios (SSP2-4.5 and SSP5-8.5) for both boreal winter (DJF) and summer (JJA) at end-of-century (2070–2100) relative to the present day (1984–2014). The LE approach enables a rigorous characterisation of internal variability and the separation of the forced response from sampling noise. Key results include: (i) a poleward and upward shift of winter storm tracks, driven primarily by tropical upper-tropospheric warming that enhances upper-level baroclinicity, increasing precipitation poleward of ~45° N; (ii) a weakening of summer storm tracks associated with reduced static stability in the upper troposphere, leading to decreased precipitation across the midlatitudes; and (iii) substantial spread among ensemble members, particularly in DJF under SSP2-4.5, highlighting the prominent role of internal variability in shaping projected changes. The contribution of internal variability is reduced under SSP5-8.5 and during JJA, where the externally forced signal dominates. Inter-model differences, linked primarily to differing equilibrium climate sensitivities, emphasise the importance of multi-model LE frameworks for robust climate impact assessment.