Speaker
Description
Energetic particle precipitation (EPP) is a key driver of chemical and dynamical processes in the middle atmosphere,influencing ozone concentrations, radiative heating, and large-scale circulation patterns. Investigating EPP impacts in observations and historical simulations is complicated by the sporadic nature of EPP events, sparse and noisy datasets, and the presence of internal atmospheric variability and other forcings that can obscure EPP-driven signals. Trying to overcome these limitations, we performed long-term idealized time-slice simulations with the fully coupled chemistry-climate model SOCOLv4 under different CO2 concentrations and EPP forcing scenarios, contrasting the strong EPP forcing year 2003, which culminated in the Halloween storms, with the weak EPP forcing reference year 2008. In all simulations, EPP increased polar NOx in the mesosphere and upper stratosphere, mainly via low- and medium-energy electrons, with minor proton contributions. Continuous and persistent EPP forcing also caused substantial global and inter-hemispheric transport, persisting beyond the events, spreading NOx to lower altitudes and latitudes,resulting in almost world-wide statistically significant ozone changes. Despite radiative cooling induced by ozone-loss, the polar stratosphere shows a temperature dipole of positive stratosphere and negative upper stratosphere/lower mesosphere temperature anomalies, strongest in Northern Hemisphere winter, and cooling in the tropical stratosphere. This temperature pattern weakens the stratospheric meridional temperature gradient, alters planetary wave propagation, weakens the polar vortex, and triggers surface responses resembling shifts in the Southern Annular Mode and North Atlantic Oscillation. Less frequent EPP forcing scenario, in contrast, resulted in the strongest temperature anomalies in the Southern Hemisphere, reversing sign in late winter and early spring, with negative polar stratosphere and positive mesospheric temperature anomalies, strengthening the polar vortex. Under 4×CO2, the Southern Hemisphere shows amplified NOx descent, while the Northern Hemisphere response is weaker, reflecting hemisphere-dependent and, likely, model-dependent anomalies. While this study confirms the robust atmospheric and surface effects of EPP and their modulation by climate change, it also highlights a strong variety of EPP impacts even in terms of the sign of the response depending on many factors. The results also indicate that the time-slice setup could not fully isolate the polar EPP effects conventionally considered in the previous literature. Conversely, the simulations show that the EPP signal in ozone can be present globally in the stratosphere, because of the long lifetime of NOx, which provides an additional modulation to the related thermodynamical response of the polar middle atmosphere and its connection to the surface climate. This raises a question to which extent this mechanism is manifested under non-idealized realistic conditions, given that there are multiyear periods of enhanced EPP related to the maximum phase of the 11-year solar cycle.
