Sorry, time flies, you now need to go back 4 years to get the real cool pool, not the couple of years I suggested. It's ben a gradually 'warming' of the pool, as my rather repetitive posts over the last 2 years have pointed out.
Not September, but an anomaly map I saved from the height of the cool pool when forecasters were queuing up to say it was a likely a permanent change, north Atlantic drift / conveyor belt changes and all that..
Different time of year I'll grant you, but the general anomaly pattern persisted month after month for so long I began to doubt my inclinations that it was temporary, due to preceding circulation patterns that had become somewhat self perpetuating.
Answering my own question, my own feeling is that the Atlantic has been exceptionally quiet for some considerable time. (Hence the diabolical lack of surf, though a change in some recent days) This has probably helped the surface waters become particularly anomalously warm, and may have prevented warm water being pushed into the Arctic during the late Summer early autumn.