This is another view of how the biannual modulation creates the extra shoulder features on the QBO oscillations

![QBO features](

Note the red arrows which indicate the biannual accentuation on the quasi-biennial oscillations. Those appear at periods of half a year.

There may also be some confusion here. The three "exact" frequencies that seem operable are as follows

* biannual = twice a year (also known as semi-annual)
* annual = once a year
* biennial = once every two years

I have seen the influence of each of these frequencies but with different emphasis depending on the physical behavior we are looking at.

It may be useful to make a chart to keep track of these factors.