On the use of equivalent latitude in the extratropical UTLS region

Equivalent latitude (EQL) is widely used in the stratospheric chemistry and dynamics community as a Lagrangian coordinate. It has been demonstrated that the use of EQL is especially effective in the analyses of polar processes, and it serves to separate the dynamical variability and the chemical processing, both of which contribute to the chemical distribution in and around the polar vortex. With the growing interest in the upper troposphere and lower stratosphere (UTLS), there is a need of a similar Lagrangian coordinate for the tropopause region. EQL has been considred as such a coordinate. We present an initial analysis of EQL in the extratropical UTLS to assess the potential problem of its applications in this region. Comparisons of alternative choices of Lagrangian coordinates will be discussed.