problem with the latitude in a *nc file

Dear ncl community,

I want to plot a contour from a *nc file. But I found there is a problem with the latitude, i.e.
the north and south is opposite, e.g. 90 S should be 90 N, 20 S should be  20 N, etc.

Since I got the nc file from others, so I can't create the correct *nc file.

Is there any way to fix the problem in ncl script? or how to make change to nc file?

Thank you very much,

Xiaoyan