The Final Warming of the Southern Stratosphere from a Lagrangian Perspective

During winter, the stratosphere above Antarctica is characterized by a strong circumpolar, westerly (cyclonic) circulation known as the Stratospheric Polar Vortex (SPV). The transition to the summer circulation in spring is referred to as the final warming, during which the westerly SPV breaks down. Final warmings in the southern hemisphere are broadly similar each year. However, during the final warming of 1979, the transition from winter to summer was accelerated in mid-October during a period when perturbing waves were very active and the vertical energy flux intensified.

We present a novel algorithm for the visualization, analysis and verification of transport and mixing processes in three-dimensional atmospheric flows. This algorithm is based on the methodology of 3D Lagrangian descriptors (LDs) or function M. These tools of Lagrangian analysis are based on the integration along trajectories of bounded positive scalars, which express intrinsic geometrical or physical properties of the trajectories. We illustrate the power of the algorithm by using reanalysis data the evolution of the flow above Antarctica during a period of rapid changes in the southern late winter and spring of 1979. The dataset was obtained from ERA-Interim Reanalysis data provided by the ECMWF.

Application of the tool presented leads to insights into the evolution of Lagrangian coherent structures and the stratospheric/tropospheric connections. Our results confirm that the onset of final stratospheric warming of 1979 is characterized by an initial decay of the vortex in the upper stratosphere where the circulation weakens, albeit it remains strong at lower heights. We also capture the anticyclonic circulation that develops during October preferentially above the southern part of Australia. The results are also indicative of features related to invariant manifolds that can act as deep vertical barriers to transport between vortices.