Commentary on using equivalent latitude in the upper troposphere and lower stratosphere

Abstract

We discuss the use of potential vorticity (PV) based equivalent latitude (EqLat) and potential temperature (<i>&theta;</i>) coordinates in the upper troposphere and lower stratosphere (UTLS) for chemical transport studies. The main objective is to provide a cautionary note on using EqLat-<i>&theta;</i> coordinates for aggregating chemical tracers in the UTLS. Several examples are used to show 3-D distributions of EqLat together with chemical constituents for a range of <i>&theta;</i>. We show that the use of PV-<i>&theta;</i> coordinates may not be suitable for several reasons when tropospheric processes are an important part of a study. Due to the different static stability structures between the stratosphere and troposphere, the use of <i>&theta;</i> as a vertical coordinate does not provide equal representations of the UT and LS. Since the <i>&theta;</i> surfaces in the troposphere often intersect the surface of the Earth, the <i>&theta;</i> variable does not work well distinguishing the UT from the boundary layer when used globally as a vertical coordinate. We further discuss the duality of PV/EqLat as a tracer versus as a coordinate variable. Using an example, we show that while PV/EqLat serves well as a transport tracer in the UTLS region, it may conceal the chemical structure associated with wave breaking when used as a coordinate to average chemical tracers. Overall, when choosing these coordinates, considerations need to be made not only based on the time scale of PV being a conservative tracer, but also the specific research questions to be addressed.