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Thursday, November 28, 2013

Stop Assuming so Much

When I mentioned that the AMO and PDO are defined oscillations best used for weather not climate I feel the deer in the highlights look.  The AMO and PDO are effects.  Something causes them on longer time scales.  If all you are concerned with is weather patterns, they are fine, but if you are trying to predict climate, without even knowing what time scale is best or climate, you don't just assume things are fixed oscillations. 

Ocean heat transport is the "cause" of the oscillations.  The chart above is the percentage ocean by latitude.  65N has the least ocean and most land so it is a ocean heat transport "choke" point.  The Thermohalide circulation and Coriolis effect along with equator to pole temperature gradients pump energy poleward.  The choke point limits that transport amplifying the impact of ocean heat content in that region. 

This chart compares the 30N-70N ERSSTv3b ocean surface temperature with the BEST land only data for the same region.  The BEST data is scaled by a factor of 0.24 in order to match the trends of both data sets. 

Fans of the AMO will have noticed how similar the 30N-70N SST looks.  This compares the Kaplan AMO with the 30N-70N SST and the yellow curve is the difference between the two. 

Surprise, surprise, the difference bears a remarkable resemblance to the Pacific Decadal Oscillation which has to be scaled since it has been defined as a weather oscillation based on the Aleutian Low.

They are not perfect fits, but the AMO combined with the PDO properly scaled pretty much replicate the 30N-70N SST.  If you have the 30N-60N SST though, why do you need to replicated it with couple of combined weather pseudo-oscillations? 

Since the AMO and PDO are the results of changes in ocean heat transport, not the causes of ocean heat transport it is a little twisted to consider either "causing" climate to do anything.  They do impact weather which really should be considered a different subject. 

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