This has nothing to do with volcanoes. Their cooling effect is short-lived -- and that is IF they managed to inject considerable aerosols high into the stratosphere. The volcano in Iceland was too weak to do that, for example. In fact, Pinatubo was the last volcano to have a notable effect on temperature, and that only lasted 18 months. Far weaker, high latitude volcanoes are not the cause of this ridge-trough pattern.