Polar mesospheric ozone loss initiates downward coupling of solar signal in the Northern Hemisphere
Nature Communications volume 16, Article number: 748 (2025 ) Cite this article
Solar driven energetic particle precipitation (EPP) is an important factor in polar atmospheric ozone balance and has been linked to ground-level regional climate variability. However, the linking mechanism has remained ambiguous. The observed and simulated ground-level changes start well before the processes from the main candidate, the so-called EPP-indirect effect, would start. Here we show that initial reduction of polar mesospheric ozone and the resulting change in atmospheric heating rapidly couples to dynamics, transferring the signal downwards, shifting the tropospheric jet polewards. This pathway is not constrained to the polar vortex. Rather, a subtropical route initiated by a changing wind shear plays a key role. Our results show that the signal propagates downwards in timescales consistent with observed tropospheric level climatic changes linked to EPP. This pathway, from mesospheric ozone to regional climate, is independent of the EPP-indirect effect, and solves the long-standing mechanism problem for EPP effects on climate.
Energetic particle precipitation (EPP) is natural solar forcing into the atmosphere that consists of protons and electrons from the Sun and the Earth’s magnetosphere. These charged particles are a known source of ionisation in the polar atmosphere, where the ionisation leads to production of odd nitrogen (NOx) and odd hydrogen (HOx)1,2. Both NOx and HOx influence ozone balance through catalytic loss cycles3. A number of model simulations4,5,6,7 and meteorological reanalysis studies8,9,10,11,12 have suggested that there could be further implications to the dynamical state of the atmosphere. Changes in atmospheric circulation have been reported all the way to surface level, influencing regional climate variability and seasonal weather conditions4,5,13. As we strive towards improved seasonal and climate predictions on regional scales14,15,16,17, we need a better understanding of all sources of natural variability on both annual and decadal scales18,19. As part of this, energetic particle forcing is now included as a recommended input for chemistry-climate simulations20 accounting for solar activity. EPP influences were, for the first time, captured in the Coupled Model Intercomparison Project Phase 6 (CMIP6) exercise, informing the Intergovernmental Panel on Climate Change Sixth Assessment Report. Thus it is critical that we understand what the implications of solar activity via EPP are on the atmosphere and climate system. The big remaining open question is: What links the well understood upper atmospheric chemical changes from EPP to regional scale dynamics and climate in the troposphere on solar cycle timescales. The main candidate thus far has been the EPP “indirect effect”: During the polar winter, EPP-produced NOx is transported inside the polar vortex from higher altitudes down to the stratosphere21,22,23 where ozone loss is initiated24,25. However, the transport from lower-thermospheric and mesospheric altitudes, where EPP ionisation is most common, down to the stratosphere takes several months22. As a result, the main ozone loss by the EPP-indirect effect takes place during polar spring. This contradicts tropospheric temperature analyses showing changes starting during the winter season4,13. Furthermore, springtime stratospheric temperature responses that are highly correlated with wintertime EPP activity levels have been found to be inconsistent with those expected from in situ ozone changes8, and careful assessment is required to account for dynamical variability3. Considering these pieces of evidence together we hypothesize that early winter chemical-dynamical coupling starts in the mesosphere and plays a major role in transferring any EPP signals downwards.
