I'm developing an algorithm for troughs identification at 500 and 200 hPa and their axes. I want to set as less as possible thresholds. For the troughs identification, I use the geopotential minimum (Zmin) and areas of different signs of meridional wind on both sides (positive / negative). For the position of the axis the southern wind (v = 0). However, I would need some variable that would allow me to discern between tropical troughs and those that are extratropical.
Maybe Potential vorticity? Or using potential temperature levels?