Jim asked:
> "I wonder if the 3-4x frequency modulation of the IOD relative to ENSO might be related to some sort of "shudder" due to the various chokepoints around the straights of Malacca which iirc were adduced as contributing the slowdown of the increased SST in the last decade?"

Th concept of LTE modulation is somewhat counter-intuitive. It's not really a modulation in frequency but a modulation of amplitude level, which is an indirect frequency multiplier in that it introduces harmonics. So when the multiplier is 3 to 4, harmonics are definitely introduced but I am not certain if these change the frequency by 3-4.

In this case, it appears as if there is a North/South aspect to the IOD definition and that may have an impact on the higher multiplying factor. This would contribute likely a shorter bounding waveguide in that dimension, which would create a tighter standing wave and thus higher frequency standing wave harmonics. The definition states: *"Intensity of the IOD is represented by anomalous SST gradient between the western equatorial Indian Ocean (50oE-70oE and 10oS-10oN) and the south eastern equatorial Indian Ocean (90oE-110oE and 10oS-0oN). This gradient is named as Dipole Mode Index (DMI)."*

![iod](http://www.bom.gov.au/climate/influences/images/map-indices.png)


I am thinking along those lines, because the multiplier is also very high for the NAO, and that has a significant North/South aspect to the dipole definition.

![nao](https://www.air-worldwide.com/uploadedImages/Blog/europe_winterstorms_fig1.jpg)

In general, what I am characterizing as the LTE multiplier may require a different vocabulary to describe the resultant behavior. Since there is nothing in the research literature that is even close to this solution, there is no lingo or common understanding to draw from.

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The PNA also has a large LTE multiplier as I mentioned in the last comment. From Wikipedia: *"The positive phase of the PNA pattern features above-average barometric pressure heights in the vicinity of Hawaii and over the inter-mountain region of North America, and below-average heights located south of the Aleutian Islands and over the southeastern United States. The PNA pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream."*

![LTM](https://imagizer.imageshack.com/img923/2324/S4N6mq.png)

This is the tidal forcing for the PNA model and once again it matches that to ENSO
![Forcing](https://imagizer.imageshack.com/img924/4748/ynaTV2.png)

This is the fit on a relatively short training interval, which shows good cross-validation out-of-band. What the strong LTE modulation does is bring the peaks into strong relief
![PNA](https://imagizer.imageshack.com/img921/7110/m6zFyT.png)

The dipoles that are left to do are the arctic (AO) and antarctic (SAM). I looked at the AO before and that has very high frequency content, to the point it almost looks like white noise. I am not sure what kind of success I will have in characterizing it via the model.