Potential vorticity control of the Brewer-Dobson circulation

布鲁尔-多布森环流是热带对流层空气缓慢而持续地上升到平流层及其向极地和向下的再环流，它在决定整个中层大气的化学成分方面起着关键作用，对地球系统有着深远的影响。它负责平流层内化学物质的长时间和远距离输送，特别是与臭氧化学有关的化学物质。它的上升分支引起热带对流层顶附近的局部降温，控制上升到平流层的空气脱水，从而控制平流层水蒸气的分布和极地平流层云的形成。因此，它在我们对未来50年臭氧恢复的估计中起着核心作用。此外，通过对平流层水汽、二氧化碳和臭氧分布的影响，它影响平流层的辐射加热，而辐射加热反过来影响平流层的动力学，并最终通过与对流层的动力学耦合影响地表气候。越来越多的数值气候模型证据表明，来自布鲁尔-多布森环流的热带上升流正在增强，并且在气候变暖的情况下将继续增强。总的来说，拟议的工作将试图更好地理解导致这一趋势的过程，并确定这一趋势对当前气候模式的不确定性有多强。它将主要通过检查一个被称为潜在涡度的动态量来做到这一点，潜在涡度是考虑到局部密度分层和地球背景旋转的空气包裹旋转的度量。位势涡度随纬度的变化，主要是由于地球自转的差异，使得一种波运动，罗斯比波，向上传播到大气中。这些波浪会破裂，就像海滩上的水波破裂一样，在那里它们会引起局部的动量沉积，这迫使空气包裹向极地移动，以保持它们的角动量。正是这一基本过程推动了布鲁尔-多布森循环。然而，关于这些波在大气中的分布以及它们最终在哪里破裂，还有很多不确定的地方。这在很大程度上取决于背景位涡分布的细节，然而，背景位涡分布往往组织成一个高度不均匀的分布，梯度非常陡的区域被梯度非常弱的区域隔开。由于水平分辨率有限，这些不均匀性在数值模型中难以表示。因此，建议的工作将详细考虑在这种不均匀位涡分布上波的传播和破裂的性质，当不均匀性表现不佳时，这种传播和破裂是如何受到影响的，以及这反过来又是如何影响布鲁尔-多布森环流的。

The Brewer-Dobson circulation, the slow, persistent ascent of tropical tropospheric air into the stratosphere and its poleward and downward recirculation, plays a key role in determining the chemical composition of the entire middle atmosphere with profound effects on the earth system. It is responsible for the long-time and long-range transport of chemical species within the stratosphere, notably those involved in ozone chemistry. Its ascending branch causes local cooling near the tropical tropopause, controlling the dehydration of air ascending into the stratosphere and hence the distribution of stratospheric water vapour and the formation of polar stratospheric clouds. It therefore plays a central role in our estimates for ozone recovery over the next 50 years. Further, through its influence on the distribution of stratospheric water vapour, carbon dioxide, and ozone it influences stratospheric radiative heating, which in turn affects stratospheric dynamics and ultimately, through dynamical coupling with the troposphere, surface climate. There is mounting evidence from numerical climate models that tropical upwelling from the Brewer-Dobson circulation is increasing in strength, and will continue to do so in a warming climate. Broadly speaking, the proposed work will attempt to better understand the processes responsible for this trend and to establish how robust the trend is to uncertainties in current climate models. It will do this mostly by examining a dynamical quantity called the potential vorticity, a measure of the rotation of air parcels that takes into account local density stratification and the background rotation of the Earth. The variation of potential vorticity with latitude, mostly due to the differential rotation of the planet, allows a type of wave motion, Rossby waves, to propagate up into the atmosphere. These waves can break, much like water waves break on a beach, and where they do so they cause a local deposition of momentum, which forces air parcels to move poleward in order to conserve their angular momentum. It is this basic process that drives the Brewer-Dobson circulation. However, much remains uncertain about the distribution of these waves in the atmosphere and where they eventually break. Much depends on the details of the background potential vorticity distribution, which, however, tends to organise itself into a highly inhomogeneous distribution with regions of very steep gradients separated by regions of very weak gradients. These inhomogeneities are notoriously difficult to represent in numerical models because of limited horizontal resolution. The proposed work will therefore consider in some detail the nature of wave propagation and breaking on this inhomogeneous potential vorticity distribution, how such propagation and breaking is affected when the inhomogeneities are poorly represented, and how this in turn affects the Brewer-Dobson circulation.