Arctic Oscillation keeps La Nina from working its snow magic
It’s halftime at the 2010-2011 skiing and riding season. Winter is in the locker room, analyzing the first half and planning its second-half strategy. From the opening kickoff, winter’s star quarterback, La Nina, made some big plays and piled up the points.
But what looked to be a rout has become a closer contest. Late in the first half, winter was caught off guard and burned with a trick play – the Pineapple Express – cutting into its lead. The Unseasonables also made some defen- sive adjustments, setting up a high- pressure system over the West Coast, which kept La Nina contained.
When hype for the big game began last summer, the consensus was that La Nina would have the hot hand in this matchup. The quarterback’s ability to heave deep powder bombs to streaking storm fronts would lead skiers and riders to an epic season.
But the team has a diva on the roster. Many analysts may have underestimated or overlooked the impact of Arctic Oscillation, the mercurial player known as “AO,” on the team’s performance.
La Nina is part of the El Nino Southern Oscillation (ENSO), a persis- tent phenomenon that has a profound effect on our climate. But there are other atmospheric oscillations influencing local weather patterns that behave independently of El Nino and La Nina.
The Arctic Oscillation shifts among positive, neutral and negative phases. AO is so temperamental, meteorologists can’t predict it beyond 10 days. When AO is positive, atmospheric pressure is below normal in the Arctic and above normal in the midlatitudes. A positive AO keeps Arctic air where it belongs.
AO is negative now. While La Nina increases our chances for a productive skiing and riding season, it’s not guaranteed with a negative AO.
La Nina is associated with cooler temperatures and increased precipitation in the Pacific Northwest. The weather has been unusually warm this week, so what gives? Historical data shows that La Nina’s cooler-than- normal temperatures don’t arrive in full until the second half of winter.
History also shows that a La Nina coupled with a negative AO brings wet but near-normal temperature conditions to the Pacific Northwest from November through January, exactly what has happened so far this winter.
Such data raises hopes for the late-season mountain storms that bring powder days in March. But there’s a catch. For that to happen, NWS data also shows that we need a positive AO from February through April. AO was positive in the second half of winters in 2007-08 and 2008-09 – seasons with legendary late-powder days.
Last winter, a season remembered for a dearth of snow, AO was negative. Chances are, winter’s second half will be drier than normal unless AO gets in a better mood. Meteorologists don’t know how AO will feel after Feb. 7.
Powder hounds can find cold comfort in the fact that an AO index on the Spokane National Weather Service website shows that AO rose from negative to flirt with positive briefly in early January, before returning to its negative phase.
Throughout the first half, AO brooded and complained. Winter capitalized on its negative energy. Now AO needs to step up and play a different role. As the team leader, La Nina needs to give AO a halftime pep talk at that will inspire it to a more positive outlook.
Bill Jennings can be reached at firstname.lastname@example.org