How are natural variability and anthropogenic forcings affecting the variability and trends in the Brewer-Dobson circulation and downward ozone flux?

布鲁尔-多布森环流(BDC)是气候的一个关键因素，因为它决定了对流层顶上方臭氧(O3)、水蒸气和气溶胶的输送和寿命，这些都对地球的辐射收支产生了重大影响。BDC的加强将影响对流层上层和平流层下层(UTLS)的痕量气体收支，因此可能对气候产生至关重要的后果。最近，BDC的增强被证明可以调节向下的臭氧通量，而这反过来又对气候和人类健康产生影响。因此，了解BDC在季节到十年的时间尺度上的变异性是可靠地检测和确定自然变异性和人类强迫趋势的先决条件。然而，在当前的气候模拟中，BDC的可变性往往不能可靠地反映出来，这使得与实测值的比较成为疑问。我们的目标是评估BDC的自然可变性和长期人为强迫趋势对UTLS示踪气体分布和气候的影响，并分析导致模式观测差异的动力学机制。该项目将现有的诊断工具、模拟与再分析驱动的拉格朗日输送模式和化学-气候耦合模式结合在一起，并结合现有的观测资料，研究北冰洋边缘地带的变化及其对UTLS臭氧的相关影响。为了实现这些主要目标，工作计划有三个工作包：(1)调查生物圈中心的自然变异性和人为诱导的趋势，(2)了解所涉及的动力机制，以及(3)评估生物圈中心变化对向下的臭氧通量的影响。臭氧和空气平均年龄)将用于研究拉格朗日输送模式(CLAMS)和化学气候模式(EMAC)模拟中的变率和BDC的长期变化。这种观测与模拟的比较是揭示模型准确捕捉BDC变异性的能力的先决条件。然后使用回归分析可以将BDC和UTLSO_3分布中的变异性和长期趋势归因于不同的气候变异性模式。为了研究其动力机制，我们提出了三种类型的敏感性实验。这些实验将能够揭示不同强迫引起的BDC变化的动力学机制，以及模式观测的差异。最后，将使用EMAC输出数据驱动的CLAMS长期模拟来评估BDC变化对向下的臭氧通量的影响以及对气候的相关影响。向下的臭氧通量将使用预算方法来量化，以模拟最低平流层的臭氧。回归分析将臭氧通量的变化归因于对气候和空气质量产生严重影响的不同气候变化模式。

The Brewer-Dobson circulation (BDC) is a key element of climate as it determines the transport and lifetime of ozone (O3), water vapor and aerosol above the tropopause, which significantly affect the Earth’s radiation budget. A strengthening BDC will impact the trace gas budgets in the upper troposphere and lower stratosphere (UTLS) and, thus, may have crucial consequences for climate. Recently, a strengthening BDC has been shown to modulate the downward O3 flux which, in turn, impacts on both climate and human health. Thus, understanding the BDC variability on seasonal to decadal time scales is a prerequisite for a reliable detection and attribution of the natural variability and anthropogenically-forced trends. However, BDC variability is often not reliably represented in current climate simulations, casting comparisons with measurements into doubt.We aim to assess the impact of natural variability and long-term anthropogenically-forced trends in the BDC on the UTLS trace gas distribution and on climate, and to analyze the dynamical mechanisms, leading to model-observation differences. The project combines established diagnostic tools, simulations with a reanalysis-driven Lagrangian transport model and a coupled chemistry-climate model together with available observations for investigating BDC changes and the related impacts on UTLS O3. To achieve these major goals, the work plan has three work-packages: (1) Investigations of the natural variability and anthropogenically-induced trends in the BDC, (2) Understanding dynamical mechanisms involved, and (3) Evaluation of the impact of BDC changes on the downward O3 flux.Available multi-year time series of observations (incl. O3 and mean age of air) will be used for investigating the variability and long-term BDC changes in the simulations with the Lagrangian transport model (CLaMS) and chemistry-climate model (EMAC). This comparison of observations with simulations is a prerequisite for disclosing the models’ ability to accurately capture the BDC variability. The use of regression analysis will then enable an attribution of variability and long-term trends in the BDC and in UTLS O3 distributions to different modes of climate variability.To investigate the dynamical mechanisms, we propose three types of sensitivity experiments in addition to the available simulations. These experiments will be conducted in a way that they will enable to disclose the dynamical mechanisms involved in the BDC changes induced by different forcings, and model-observation discrepancies.Finally, the impact of BDC changes on downward O3 flux and related effects on climate will be assessed using the long-term simulation with CLaMS driven by the EMAC output data. The downward O3 flux will be quantified using a budget approach to model O3 in the lowermost stratosphere. Regression analysis will attribute the variability in O3 flux to different modes of climate variability that critically impact the climate and air quality.