Many Northwest residents have heard enough about El Nino and La Nina to know these events have quite the bearing on the region’s weather. Both these features, which technically refer to the presence of warm or cold water anomalies in the eastern Pacific Ocean, are the two opposites of the El Nino-Southern Oscillation, or ENSO.
These two opposing events occur because of a complicated cascade of interactions between the ocean and atmosphere. These interactions are often reinforcing on one other, driving ocean temperatures to be as much as a degree or more above or below average, an immense amount of additional or diminished heat when one considers how vast the Pacific is.
Last century, researchers began studying how this event affects the various parts of the world, including the U.S. West Coast. The primary method for how ENSO’s influence is translated from the Pacific to different regions is how these oceanic heat imbalances change the jet stream, the current of air high in the atmosphere that is responsible for creating, moving, and destroying our weather systems.
Typically, the influence of ENSO events are most felt in winter. In an El Nino winter, the warm water anomaly, the jet stream has a tendency to lift north above the region, leaving us warm and dry. But during a La Nina winter, the jet stream is more centered over the Pacific, often passing near or over our region. As the jet stream is the highway for our cold season storms, this brings more active weather to our region.
So what’s in store this winter? Unfortunately, just determining if it will be an El Nino or La Nina year would be like seeing just the flop of a poker game: It narrows the possibilities of what hands may be held, but there is still missing and forthcoming information that could change the game.
That said, oceanic temperatures in the east Pacific are already about a degree below normal (La Nina) with other atmospheric variables agreeing with developing La Nina conditions. Medium-range weather models capable of modeling the cascade of interactions that creates ENSO indicate La Nina conditions will be likely through the winter.
When one assesses previous moderate to strong La Nina events and their outcomes on the region, one signal is quite strong and another signal is suggested. The stronger of the two signals suggests the winter months are cooler in a La Nina year owing to the jet stream configuration, allowing for more cold air outbreaks from Canada.
This signal is particularly strong in the Columbia Basin. The other signal is for precipitation. In a La Nina year, moisture is transported from the Pacific into the region on the high altitude jet stream, but much of this precipitation is wrung out on the Cascade Crest, which typically does receive above-normal rain and snow during La Nina.
We do see a similar, but weaker, suggestion that higher elevation areas in our region, like the Idaho mountains, Blue Mountains, and maybe even Steens Mountain in Harney County, receive just slightly above-normal precipitation in moderate to strong La Nina events.
There are many moving pieces to the weather forecast, but ENSO is an important oceanic oscillation that exists on the perfect timescale to influence our year-to-year weather. Given the presence of cold Pacific temperatures that indicate La Nina, and the suggestion by models this year’s La Nina will be both moderately strong and persist through winter, it would suggest the probabilities of a colder-than-average winter are increasing for portions of the region, with slightly less confidence that we may receive slightly more rain and snow, particularly in higher elevations, due to La Nina conditions.