What's also interesting is that the PNA (Pacific-North America) and now [AO (Arctic Oscillation)](https://www.ncdc.noaa.gov/teleconnections/ao/) can be easily fit from a perturbation of the NAO model. These are all northern latitude behaviors as highlighted in orange below.


This is the common tidal forcing for each of these models, with the LTE modulation in the lower panel. The tidal forcing has a strong semi-annual factor, as with the QBO.


The LTE modulation differs subtly between the three, as the multipliers are slightly different for NAO and AO and within ~15% for PNA. They are in sync at the yellow arrows shown in the lower panel. The LTE modulation is dependent on the fundamental spatial wavenumber defining the dipole, which should be different for each of the regions.

These are the fits for each of the time-series


You can see how the NAO and AO are vaguely similar and the the PNA is similar but flipped in polarity. It is known that the QBO has a connection to the polar vortex, so the semi-annual commonality between QBO and AO makes some sense.

The only major index left is the Southern Annular Mode (SAM) index associated with the [Antarctic Oscillation](https://en.wikipedia.org/wiki/Antarctic_oscillation).