SAM and IOD

LaNeige

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Aug 20, 2010
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After a CC warning over in the Prediction thread I thought I'd start a new one to answer BlueHue question asked below:

LWT doesn't determine moisture directly
Its more a way of displaying polar air.

Yeah so what determines the moisture and precip potential and why are these late winter polar outburts so dry when they pass over the alps ? (I hope we are still on topic if not happy to move somewhere else )

they are not always typically dry. September has massive potential for large moisture readings.
The last few changes have been dry cause
1. the pressure was too high
2. Lacked a moisture infeed.


Donza may be on to it with the lack of moisture feed. The dryness may be partly due to the Indian ocean.

Current IOD (Indian Ocean Dipole, positive values mean dryer for Oz., negative mean wetter - usually):

click on IOD index time series on the menu
http://www.bom.gov.au/climate/enso/indices.shtml

IOD in the positive like now comes from cooler tropical waters off Sumatra to the NW of Australia. These cooler tropical waters apparently lead to less mositure feeding into fronts over the SE. (I'm not sure whether a correlation has been demonstrated), in fact it is believed the South East is probably the part of Oz. most affected buy these water temps.

IMO of all the variables that we have some understanding of, a combination of negative SAM (which generally means more fronts pushing north out of the southern ocean) and negative IOD is the best setup for good snowfalls. I really should do some data analysis on it one day and see if there is anything to that).

We have a negative SAM at the moment but a weakly positive IOD so not sure what this means for snow in the next few weeks. Probably more of what we have already seen in the last few weeks, some colder air masses coming north with fronts that are fairly dry due to lack of moisture infeed.

I've had a bit of look at the influence of SAM, IOD and ENSO on max snow depth and I've found exactly what BH suggests, that negative SAM and IOD have the highest correlation with larger maximum snow depths as shown below.

This figure is a bit simplistic plotting the combined additive SAM and IOD indices for a given winter against snow depths. However, it does show a fairly strong trend with obvious scatter.

snowdepthvssamiod.png
 
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Gerg

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Well, yes ... noting that SAM and AAO are exactly the same thing, back in April we had:

Peak depth (cm) = 1090 - 30 x AAO + 1.8 x SOI - 9.4 x IOD - 0.45 x Calendar Year

...based on a series of correlations. That actually turned out pretty close this year. Then in July we had the correlation pane:

Spencers_correlations.png


Nice to know you all noticed...

Which AAO/IODs are you using, winter average? Adding SAM(=AAO) to IOD for your correlation is OK, but assumes their weights are equal, which they aren't. AAO appears to have about 3 times the effect IOD does once you do the multiple regression.
 
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LaNeige

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Ah ... I didn't really get into that thread so I missed all of that.

The plot I posted was simply to demonstrate the correlations that BlueHue noted in a non-parametric sense. You are right that some form of modelling should be done if you want to use the model for predictions but that wasn't the point of the plot.
 

Gerg

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Brissy
Sorry LaNeige; I'm a grumpy old... Did you do any more with correlation to the year-to-year change in SAM that you were looking at earlier?
 
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