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

Brewer-Dobson环流是气候的一个关键要素，因为它决定对流层顶上方臭氧（O3）、水蒸气和气溶胶的输送和寿命，而臭氧、水蒸气和气溶胶又对地球的辐射收支产生重大影响。加强BDC将影响对流层上部和平流层下部（UTLS）的微量气体收支，因此可能对气候产生重要影响。最近，加强BDC已被证明调制向下的O3通量，这反过来又影响气候和人类健康。因此，了解BDC的季节性到十年的时间尺度上的变化是一个可靠的检测和归因的自然变率和气候强迫的趋势的先决条件。然而，BDC的变化往往是不可靠的表示在目前的气候模拟，铸造与测量比较成doubt.We的目标是评估的自然变率和长期的气候变化趋势的影响，在BDC上的UTLS示踪气体分布和气候，并分析动力机制，导致模式观测差异。该项目结合了现有的诊断工具，再分析驱动的拉格朗日传输模型和耦合化学-气候模型的模拟，以及调查BDC变化和对UTLS O3的相关影响的可用观测。为了实现这些主要目标，工作计划有三个工作包：（1）调查BDC的自然变率和气候变化引起的趋势，（2）了解所涉及的动力机制，（3）评估BDC变化对向下O3通量的影响。O3和空气的平均年龄）将被用于调查的变率和长期BDC的变化与拉格朗日传输模式（CLaMS）和化学气候模式（EMAC）的模拟。这种观测与模拟的比较是揭示模型准确捕捉BDC变化能力的先决条件。回归分析的使用，然后将使一个属性的变化和长期趋势的BDC和UTLS O3分布的不同模式的气候variable.To调查的动力机制，我们提出了三种类型的敏感性实验，除了可用的模拟。这些实验的进行方式，他们将能够揭示在BDC的变化所涉及的动力机制，不同的强迫，模式观测的差异，最后，BDC的变化向下O3通量的影响和气候的相关影响将使用长期模拟评估与CLaMS驱动的EMAC输出数据。向下的O3通量将使用预算方法来量化，以模拟最低平流层中的O3。回归分析将O3通量的变化归因于对气候和空气质量产生重大影响的不同气候变率模式。

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.