That doesn't really satisfy me. An index, any index, be it NAO, AO, EPO, PNA or whatever is a description of an observed pattern of measurements. It is not, in and of itself a mechanistic explanation of WHY the observed pattern is occurring, or why it has certain consequences.
And saying that heights in the SW are low because the -PDO is tautological. It's like saying that something is hot because the thermometer has high readings.