Lagrangian coherent structures to examine mixing in the stratosphere

Abstract. The study of mixing in the stratosphere is important for understanding the transport of chemical species and the dynamics of the atmosphere. How best to quantify this mixing is not settled, however. In recent years, Lagrangian coherent structures (LCSs) have emerged as a valuable tool for examining mixing in fluid flows, and in this work, we present a stratospheric mixing metric based on the LCS framework. We identify LCSs associated with the transport of air masses and quantify the amount of mixing between different regions of the atmosphere in the Whole Atmosphere Community Climate Model (WACCM). Our results show that LCSs provide a powerful approach to analyze mixing in the stratosphere and can be used to identify regions of high and low mixing as well as to study the dynamics of the atmosphere. The results are compared with those obtained by two other tools to quantify mixing: the commonly used effective diffusivity and the recently introduced isentropic eddy diffusivity. We find qualitative agreement between these metrics for much of the stratosphere, although there are regions where they clearly disagree. A significant advantage of the LCS mixing metric is that it reflects Lagrangian transport in physical latitude rather than the equivalent latitude coordinate needed to calculate effective diffusivity, and we discuss other advantages and disadvantages of these methods.