[ncl-talk] Issue with calculating Arctic Oscillation Index

[Dennis Shea]

*http://meteora.ucsd.edu/~pierce/eof/eofs.html*
<http://meteora.ucsd.edu/~pierce/eof/eofs.html>
"Be aware that the EOF/PC combination can have BOTH signs switched and
still be correct.  It's just the product of the two that has a determined
sign."

<goog_503595139>
*https://www.ncl.ucar.edu/Applications/eof.shtml*
<https://www.ncl.ucar.edu/Applications/eof.shtml>
See section:* EOF: Standard Empirical Orthogonal Analysis*
*Note on signs of EOF analysis *(conributed by Andrew Dawson, U. East
Anglia)

On Fri, May 1, 2020 at 11:42 AM Xueke Li via ncl-talk <ncl-talk at ucar.edu>
wrote:

> 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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