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I've started a wiki entry:

that explains this index. It's a famous way to tell if an El Niño is happening.

## Comments

The NOAA data is at 2.5 degree intervals, so you get 5N, 2.5N, 0, 2.5S, 5S. Are the 5N and 5S counted as in or out?

`The NOAA data is at 2.5 degree intervals, so you get 5N, 2.5N, 0, 2.5S, 5S. Are the 5N and 5S counted as in or out?`

Ugh, I can't tell! At least, not yet. All I see is that

I don't know whether they count grid points that lie right on the boundary of a rectangle as part of that rectangle!

`> Are the 5N and 5S counted as in or out? Ugh, I can't tell! At least, not yet. All I see is that > The Niño 3.4 region spans a swath from 5°N to 5°S latitude and 120°W to 170°W longitude. I don't know whether they count grid points that lie right on the boundary of a rectangle as part of that rectangle!`