Chemistry-Climate interactions in the Coupled Atmosphere-Chemistry-Ocean Model SOCOL-MPIOM

Muthers, Stefan (2014). Chemistry-Climate interactions in the Coupled Atmosphere-Chemistry-Ocean Model SOCOL-MPIOM (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Understanding natural variations in the climate system and the influence of different components of the climate system on these variations is an important prerequisite to interpret past changes in the climate system and projections for the future. In this thesis an atmosphere-ocean-chemistry-climate model (AOCCM) is developed by coupling a chemistry-climate model (CCM) to an ocean model, which is then used to analyse the influence of the atmospheric chemistry in various case studies. A particular focus is on interactions between the atmospheric chemistry and the external forcings, motivated by the questions whether the atmospheric chemistry reinforces or balances changes in the climate system. Furthermore, the role of solar variations and volcanic activity for past and future climates is addressed in simulations for the past 400 and the upcoming 100 years. These simulations are forced by a state-of-the-art spectral solar forcing re-construction with a large amplitude and the results are compared against proxy based temperature reconstructions.
The introductory chapter gives an overview of the climate system and the components and processes relevant for this thesis. Furthermore, the current state of knowledge of the past (and future) solar and volcanic activity as well as the principles of the stratospheric ozone chemistry and their interactions with the climate system are summarised.
In Chapter 2 the model SOCOL-MPIOM is presented and the characteristic of the AOCCM is evaluated using a number of pre-industrial control simulations and transient experiments for the period 1600-2000 AD. A pre-industrial time-slice simulation with interactive chemistry is used to analyse the characteristics of the coupled model, which is compared to a second control simulation forced by the same boundary conditions, but without interactive chemistry. By comparing both simulations, we find an overall minor influence of the interactive chemistry on the climate state and the variability. Temperatures differ in the mesosphere and the higher stratosphere. These changes are related to a parametrisation for UV absorption by oxygen and ozone and to diurnal variations in the ozone concentrations in the mesosphere, both are included in the model with interactive chemistry only. The influence of these temperature differences on the dynamics is small and limited to the stratosphere. Furthermore, SOCOL-MPIOM is used in transient simulation for the period 1600-2000 AD, forced by two spectral solar irradiance (SSI) reconstructions with medium and large centennial scale variations, respectively. The influence of the solar forcing is obvious in the pre-industrial temperature variations, although the differences between the two forcings is not always detectable. Overall, the Northern Hemispheric scale temperature variations are within the uncertainty range of proxy based temperature reconstructions, but the spatial patterns for the Maunder and the Dalton Minimum suggest an overestimation in many regions. In the industrial period the simulations undergo a pronounced and globally uniform increase of surface air temperature. In comparison to observations the temperature trends are overestimated by about a factor of two. In sensitivity experiments, the relative importance of the major greenhouse gases (GHGs), the solar forcing, stratospheric and tropospheric aerosols, as well as the simulated ozone changes are assessed. Furthermore, climate sensitivity experiments are performed to estimate the climate sensitivity of SOCOL-MPIOM. In summary, the simulated temperature trends from 1850 onwards can be understood by a combination of the GHGs induced warming with additional positive contributions from the comparable large solar forcing used and the simulated ozone changes. All of them are amplified by the comparable high climate sensitivity of the model (transient climate response: 2.2 K). This study is published as a discussion paper in Climate of the Past Discussions and is currently under review for publication in Climate of the Past.
Chapter 3 presents a study published in the Journal of Geophysical Research, in which we use a configuration of SOCOL-MPIOM without interactive chemistry to analyse the role of different ozone climatologies in the dynamic response to strong tropical volcanic eruptions. Ozone climatologies are commonly used in models without interactive chemistry to consider seasonal variations in the ozone concentrations in the radiation schemes. Here, we compare a climatology with stronger and weaker meridional ozone gradients. Ensemble simulations were conducted with a single strong volcanic eruption in the beginning and compared to a set of ensemble control simulations without volcanic eruptions, for each climatology separately. With larger meridional gradients in stratospheric ozone, the northern polar vortex is stronger in the background state and the eruption leads to an additional intensification. This intensification results in a significant increase in the coupling of wind anomalies between stratosphere and troposphere and a highly significant positive phase of the North Atlantic Oscillation (NAO) in the first winter after the eruption. With weaker meridional ozone gradients the response is qualitatively similar but weaker and not significant. The comparison of the number of coupling events in the ensemble simulations reveals indications for non-linear interactions between the ozone gradient and the perturbation by the volcanic aerosols.
Following from these results Chapter 4 focuses on the ozone changes after a strong volcanic eruption in the model with interactive chemistry. Ozone changes are in general influenced by two different processes. Firstly, stratospheric dynamics and chemical reactions rates are affected by the warming in the tropical lower stratosphere, which we summarise as dynamical processes. Secondly, the volcanic aerosols provide surfaces for a number of heterogeneous chemical reactions in the aerosol clouds, that modify the chemical ozone balance of the stratosphere. In idealised simulations the importance of these two processes as well as the combination of both for the ozone changes and the dynamics is addressed by a number of ensemble simulations. Furthermore, the influence of the eruption strength and the climate state, i.e. a present day atmosphere vs. a pre-industrial atmosphere, is simulated. The two climate states differ in their amount of GHGs and ozone depleting substances in the atmosphere. We find that dynamical processes result in rapid changes in the ozone concentrations in the tropics and mid latitudes, almost independent of the climate state. The dynamical mechanism has the largest effect on the dynamics with the intensification of the polar vortex and the following changes in the tropospheric circulation of the northern high latitudes. Significant influences of the second mechanism, heterogeneous chemical reactions are only found in the present day climate state, with a general reduction of the ozone concentrations that is amplified in the high latitudes during polar night and spring. The reaction of the chemistry is slower in comparison to the dynamic mechanism, but longer lasting. With larger eruption strength the amplitude and the duration of the ozone depletion increases. The ozone changes lead to a slight but significant weakening of the polar vortex in mid winter and a slight intensification in spring. The results of this study are currently under review for publication in the Journal of Geophysical Research.
The appendix of this thesis covers three publications addressing mainly the role of solar variability. In Chapter A.1, published in the Geophysical Research Letters, the influence of a possible grand solar minima within the 21th century is simulated under the RCP 4.5 scenario. Two different reduction scenarios are compared to simulations without reduced solar forcing. With solar minima the temperatures at the end of the 21th century increase by 1.61◦C, 1.75◦C for a strong and weak minimum, respectively, in comparison to 1.96◦C without solar minimum. Furthermore, a significant delay of the stratospheric ozone recovery is found with solar minimum, in particular in the tropics and subtropics. Finally, sensitivity experiments for the Dalton Minimum (DM) are presented in Chapter A.2 and A.3, published in Atmospheric Chemistry and Physics and Climate of the Past, separately for the stratospheric dynamics and chemistry and dynamical changes in the troposphere. Here, we explore the relative importance of the top-down (UV variability only), bottom-up (visible and near infrared variability only), volcanic aerosols, and energetic particle precipitation (EPP) for climate change in the period 1780-1840. In the stratosphere, UV variability significantly reduces the ozone concentrations and cools the middle atmosphere, but the influence on the dynamics is weak. Dynamical changes are mainly a result of the volcanic aerosols, which heat the lower tropical stratosphere and intensify the polar vortices in both hemispheres. Volcanic aerosols further have opposing effects on stratospheric ozone. Increased stratospheric water vapour concentrations enhance the catalytic ozone destruction by HOx whereas heterogeneous chemical reactions increase ozone in the tropical stratosphere. Variation in the visible spectrum and EPP have only very small influence on the stratosphere during the DM. In the troposphere, however, the volcanic eruptions together with variations in the visible and near infrared are the driver of the temperature variability, whereas the influence of the top-down mechanism is low. Volcanic eruptions, furthermore, significantly affect the dynamics, visible in a widening of the Hadley cell and the winter warming pattern in the northern high latitudes.

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