ENSO Index: The El-Nino Southern Oscillation is the primary driver of short-term climate variance. It is most strongly characterized by fluctuations in sea surface temperatures across the tropical Pacific Ocean. A warm pool anomaly in the east Pacific (El Nino) is indicated by a positive ENSO index. A negative value indicates La Nina conditions, characterized by a cool pool in the eastern Pacific and warm pool in the western Pacific.

El Nino winters tend to be warmer and drier on average in the Pacific Northwest. La Nina winters tends to be cooler and wetter. Read More Here

The Arctic Oscillation (AO) Index:

 Good Description of AO Here (click link)