> And yes the University of Reading animation is only for roughly 6 years and it would definitely be better to have that for 60 years, but however in that animation the phenomenom really looks exact biannual.
I know that earlier you said that biannual can mean either twice per year or every two years, so you will have to be less ambiguous, otherwise we can not be certain what period you are referring to. Sorry.
There is really no doubt that the twice yearly frequency is exact as the sun crosses the equator exactly twice per year. However the exact once every two year period is less certain. I think I have found evidence [here](http://forum.azimuthproject.org/discussion/1480/tidal-records-and-enso/?Focus=12629#Comment_12629), but that is inferred from tidal gauge records.