Effects of energetic electron Precipitation In a Changing climate

Variable space weather injects energetic electron precipitation (EEP) into the upper atmosphere at high latitudes. EEP produces nitrogen oxides (NOx), which have a long chemical lifetime in the wintertime during polar night, and descend to stratospheric altitudes via meridional residual circulation. It has a significant effect on polar stratospheric ozone, that can potentially affect atmospheric dynamics down to the surface. Outstanding question is how the coupling between particle precipitation and the atmosphere is influenced by long-term changes in climate. Anthropogenic climate change is cooling the atmosphere above our weather system due to the less longwave radiation escaping the troposphere. Furthermore, global ozone experienced a dramatic decrease at the latter half of the twentieth century due to the emissions of ozone depleting substances (CFC emissions). Stratospheric ozone levels are now slowly recovering after the Montreal Protocol banned the use of CFCs. Recovery to 1960s level is estimated to happen sometime after 2050. Furthermore, cooler stratosphere is slowing ozone destroying chemical reactions, leading to net positive ozone production in stronger greenhouse gas emission scenarios. This is called the super recovery of ozone. This project will advance our understanding on atmospheric effects of EEP and its changes on longer timescales. We will use two different chemistry-climate models and reanalysis data to study how chemical and dynamical effect of EEP is modulated by the change of background atmosphere. This will help us answering if particle precipitation impact in the atmosphere is different under climate change or due to man-made CFC emissions. First modeling results show that nitrogen oxides (NOx) produced by EEP in the Antarctic upper atmosphere descend faster to stratospheric altitudes during winter in the stronger greenhouse gas scenarios by the end of the 21st century. Faster transport is a consequence of accelerated meridional residual circulation in the Antarctic mesosphere due to climate change. This enhanced stratospheric NOx leads to a strong seasonal ozone depletion. This EEP effect resists the overall ozone increase by greenhouse gas cooling in the stratosphere during the 21st century, and will potentially prevent a super recovery of ozone in the Antarctic upper stratosphere. Modeling results from the CFC era also show that EEP has been a significant modulator of reactive chlorine in the Antarctic stratosphere, thus impacting the ozone depletion by CFC emissions and partly the magnitude of the Antarctic ozone hole. Stratospheric chlorine has acted as a buffer of ozone depletion by EEP via limiting the NOx and HOx catalytic cycle efficiencies. With the Montreal Protocol and the declining levels of CFC chemicals, we can expect more efficient chemical ozone depletion by EEP in the future. Furthermore, EEP direct mesospheric temperature modulation via ozone depletion has been obtained in climate simulation during sudden stratospheric warming (SSW) event in January 2010. By adding EEP to the WACCM climate model, mesospheric SSW imprint was shown to be substantially weaker compared to control run. In another study mesospheric impact of EEP was noted to be too weak in the WACCM climate model compared to observations, which further highlights the potential importance of EEP on mesospheric dynamics.

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Anthropogenic climate change is not just limited to the lower atmosphere but have notable implications to the middle atmosphere as well. As a consequence, the stratosphere is cooled due to the less longwave radiation escaping the troposphere. Recent results provide an indication that dynamics in the stratosphere may have significant implications to the troposphere, especially in the high latitudes. While effect of solar variability on the surface is thought to be fairly small due to the direct radiative forcing changes in solar cycle timescales, substantial uncertainties still exist when considering indirect effect through the stratospheric forcing. Especially energetic electron precipitation (EEP) effect to middle atmosphere dynamics and its potential element of influencing high latitudinal climate is still not well verified. This project will try to unravel the uncertainties of EEP effect in the middle atmosphere and its changes in longer timescales. Recent research indicates that the relation between the particle forcing and the circulation changes in the lower atmosphere became significant since 1970s onwards. So far no thorough or consistent explanation has been given to this behavior, although the mechanism how EEP can induce circulation change in the high latitudinal stratosphere via chemical ozone depletion is reasonably well established. We will use two different chemistry-climate models (WACCM and SOCOL) to study how chemical and dynamical effect of EEP is modulated by the change of background atmosphere. In essence, is the effect different in the preindustrial and modern atmosphere. Additionally we will use volcanic atmosphere to study if these effects vary also after major volcanic eruption.