[ncl-talk] Issue with calculating Arctic Oscillation Index

[Xueke Li]

Dear NCL users,

I'd like to calculate the Arctic Oscillation Index using NCL followed
the method of NOAA CPC (
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/history/method.shtml).
I found the script (http://www.ncl.ucar.edu/Applications/Scripts/eof_4.ncl)
that fit for purpose. However, after calculating the eof, there is one line
that reverses the eof in order to *special* match sign of CPC as noted
(shown below).

*eof = eofunc_Wrap(wx, neof, optEOF) *
*eof = -1*eof ; *special* match sign of CPC*

I don't quite understand the reason to reverse eof since other eof
calculations of geopotential height do not have this procedure. Could
someone explain the logic here?

Thank you in advance.

Regards,
Xueke
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