I find it interesting that this El Nino struggled to get going and is pretty much vanishing before really getting anywhere, I wonder if its the atmosphere not really playing ball and wanting to go back into a more La Nina state or after that massive kelvin wave last spring there was nothing for a few months and the momentum was lost until the autumn, or a combination of both.
The the strength of the +ve PDO I think can be explained mostly by the kelvin waves. Please excuse my rather amateur annotations.
Looking back at last year the PDO was only weakly negative prior to January which is when we saw that huge kelvin wave appear, (think it appeared just before January). Following that the PDO gradually became quite strongly positive by May. In the early summer when the kelvin wave finished it was followed by a brief lull, which saw the PDO values drop during June, July & August before a further round of kelvin waves appeared during the late summer onwards and another sharp increase in the PDO was seen and we had the weak El Nino forming.
The large values of the PDO to me suggest that we should be seeing a strong ENSO event but whatever seems to be watering down the effects of the El Nino isn't necessarily having the same impact on the PDO, possibly?
I'm not an expert and I don't know the full answers but thats just my humble thoughts and thought i'd put my two pence worth in. But if this is true then I doubt we are seeing a longterm trend emerge and it could just be a blip in an otherwise prolonged -ve state and once these kelvin waves weaken and stop we may well see the PDO swing back to a -ve state, knockers post suggests the atmosphere is starting to tip that way again, bit early to jump to conclusions though.