Tropopause definition? as used for the ASEP

[PTP]

Here's an oddball question.

I've always thought of the tropopause as the boundary between the troposphere and the stratosphere. Where the air ceases to cool with height.

Now often the ASEP will have the blue line for the Tropopause at heights that appear random. Sometimes it's across the minus 40 C temperatures, while the air temp is decreasing above it.

What do the people who make the ASEPs use to define the Tropopause?

Thanks,
FTP