Exploring Stratospheric Mixing using Lagrangian Coherent Structures

The stratospheric circulation shapes the distribution of trace gases in the stratosphere, including ozone, water vapor, and ozone-depleting substances. The study of mixing in the stratosphere is crucial for understanding the transport of chemical species and the dynamics of the atmosphere. Adiabatic mixing, for instance, displaces tracers across large spatial scales and facilitates the exchange of midlatitude and tropical air. Accurately quantifying mixing is vital for comprehending stratospheric transport trends in a changing climate.

Lagrangian Coherent Structures (LCSs), which act as transport barriers by serving as key material surfaces, play a critical role in shaping global transport. These finite-time structures hold particular relevance in time-dependent flows. They determine the deformation of the fluid and the evolution of any advective tracer field.

This presentation introduces the density of Lagrangian Coherent Structures as an innovative metric for characterizing stratospheric mixing. Using the Community Earth System Model 1 Whole Atmosphere Community Climate Model (WACCM), we compare this metric with two alternative measures: the widely used effective diffusivity and the novel tracer-based isentropic eddy diffusivity [Gupta et al. JGR: Atmospheres (2023)]. Although effective diffusivity is widely used, its computation is resource-intensive, yielding results in the somewhat ambiguous equivalent latitude coordinate. In contrast, the density of LCSs does not alleviate the computational challenge, but it is a Lagrangian metric that yields results in physical latitude. Findings obtained through this approach exhibit remarkable consistency with prior studies, establishing a direct correlation between LCS characteristics and measures of mixing/diffusivity.