Global Oscillation Discussion Thread

[weatherwiz]

I thought it would be cool to have a thread devoted to discussions involving all the global indicies...a place where people can ask questions, or just discuss how they work, etc.  

 

Anyways, one thing I've gotten away from the past few years is really digging deep into the many global oscillations we have and trying to understand them better and do some work with them and I would like to get back into that.  

 

My first question has to do with the NAO.  I've been wondering these past few days, if the NAO is defined using differences in SLP, why do we always look at 500mb maps?  Wouldn't looking at SLP anomalies give a better idea?

 

For example, take a look at December of 2008.  Using Hurrell's PC-Based index (from what I gather, the PC-Based value is probably a bit more reliable than the station based value) you get a NAO which is on the positive side (0.61) while Osborne's NAO value is fairly negative (-1.30), along with the CPC's value of -0.35.  Below are the anomaly maps:

 

 

 

Seeing how Osborne/CPC both have this has a negative NAO and Hurrell, positive.  Looking at the 500mb map could be a bit confusing determining the signal, but when looking at the SLP anomaly map, you can actually see a bit more clearly, that this is certainly a more positive NAO as the low overland is slightly stronger than normal, as is the high pressure in the North Atlantic.  

[dendrite]

This thread is more suited for Met101.

[CoastalWx]

I almost never look at the surface anomalies. The H5 look tells me all I need to know since I can get an idea of what it looks like.

[weatherwiz]

I almost never look at the surface anomalies. The H5 look tells me all I need to know since I can get an idea of what it looks like.

 

Are there instances though where looking at the SLP anomalies would give you a more clear idea, especially when the signal may be on the weaker side, such as the example above or when you have cases where the CPC version gives a (positive or negative) and Hurrell's version gives the opposite.  I know Will loves to use Hurrell's version more than the CPC and that is enough to get me to use Hurrell's version instead but I hate those cases where the CPC's version is like positive and you'll have Hurrell's and even Osborne's version showing an opposite sign.