CDO

Dear colleagues,

Would like to know please how to plot fast Fourier transforms (FFT) from a netcdf file (.nc) that I downloaded from the c3s (sea surface height, temperature, salinity and currents). I tried several times and I couldn’t, because I get the following message:

"This operator needs fields with complex numbers!"

In advance I apologize sincerely if I posted my question in the wrong place. Any answer will be appreciated.

Thank you very much.