> 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!