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久地上升到平流层及其向极地和向下再循环的过程，它在决定整个中层大气的化学成分方面发挥着关键作用，对地球系统产生了深远的影响。它负责化学物质在平流层内的长时间和远距离传输，特别是那些涉及臭氧化学的化学物质。它的上升分支在热带对流层顶附近引起局部冷却，控制上升到平流层的空气的脱水，从而控制平流层水汽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发挥着核心作用。此外，通过它对平流层水蒸气、二氧化碳和臭氧的分布的影响，它影响平流层的辐射加热，进而影响平流层的动力学，并最终通过与对流层的动力耦合影响地面气候。来自数值气候模型的越来越多的证据表明，来自布鲁尔-多布森环流的热带上升流正在增强，并将在气候变暖的情况下继续这样做。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型中的不确定性的稳健程度。它将主要通过检查一种名为势涡度的动力量来实现这一点，势涡度是一种测量气团自转的指标，它考虑了局部密度分层和地球背景自转。位涡随纬度的变化，主要是由于行星的不同旋转，使得一种波动--罗斯比波--向上传播到大气中。这些海浪可以破裂，就像水波在海滩上破裂一样，在它们发生破坏的地方，它们会导致局部动量沉积，迫使气团向极地移动，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，仍然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，背景位涡分布往往会组织成高度不均匀的分布，梯度非常陡峭的区域被梯度非常弱的区域隔开。众所周知，由于水平分辨率有限，这些不均匀现象很难在数值模式中表现出来。因此，建议的工作将更详细地考虑波在这种非均匀位涡分布上传播和破裂的性质，当不均匀不能很好地表示时，这种传播和破裂是如何影响的，以及这反过来如何影响布鲁尔-多布森环流。

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.