El Niño on the doorstep?

Not since 2009 has El Niño held sway over our weather. Three meteorologists from Oregon foresee its return this coming winter.

How its arrival might influence our weather over the next few months was the subject of the Oregon Chapter of the American Meteorological Society's 22nd annual winter weather forecast conference held Saturday, October 25, at the Oregon Museum of Science and Industry in Portland.

Liana Ramirez of the National Weather Service in Portland; Rod Hill meteorologist for KGW-TV; and Kyle Dittmer, hydrologist/meteorologist at Columbia River Inter-Tribal Fish Commission, all began their talks by describing changes currently going on in the tropical Pacific Ocean.

The El Niño/La Niña Southern Oscillation (ENSO) is the most important coupled ocean-atmosphere phenomenon to cause global climate variability. Essentially, it is the see-saw pattern of reversing surface air pressure between the eastern and western tropical Pacific.

An El Niño is in effect when persistent westerly winds push warm ocean water into the eastern Pacific. It's a La Niña if strong easterly trade winds move the warmer water into the western Pacific.

The Multivariate ENSO Index (MEI) is a numerical indicator that combines the effects of air pressure, wind, air and water temperature and cloud cover. When the MEI is between minus-0.5 and 0.5 the ENSO is said to be neutral. It's an El Niño when the number is greater than 0.5; a La Niña when the index is less than minus-0.5.

It turns out that winters here in the Pacific Northwest are more strongly correlated to the ENSO than anywhere else in the country. During an El Niño, warmer and drier conditions normally prevail. It's cooler and wetter when La Niña pays a visit.

"El Niño" is a Spanish term that refers to the Christ child, or the male child. The name was chosen because the periodic warming of the eastern Pacific near South America often coincides with the arrival of Christmas. La Niña, the opposite of El Niño, represents the cool ENSO phase and literally means the female child.

Last winter, from November through March, it was significantly colder and somewhat wetter than usual here in Sisters, and was punctuated by three noteworthy events: the fierce windstorm on December 1 that blew down numerous trees; a severe arctic outbreak that sent the thermometer plummeting to minus-28 degrees Fahrenheit on December 8; and a three-foot snowfall in February. Yet, the Central Oregon Cascade snowpack was just over half of normal. The ENSO signal was neutral.

Currently the MEI is at 0.5, right on the threshold of an El Niño. Ramirez thinks there's a 60 to 65 percent chance of developing a weak El Niño that should remain frail throughout the winter. She's calling for warmer and drier-than-normal conditions for the period December through February. Hill is also buying into the feeble El Niño scenario with warmer temperatures and normal precipitation levels from November through January. Dittmer foresees near-normal temperatures and slightly on the dry side for the same period.

Hill thinks he sees a weather trend that has been going on for the past 10 to 12 years in northwestern Oregon regardless of the ENSO phase.

"It seems to me that the heaviest annual precipitation amounts that have normally fallen in the mid-winter months have shifted. Now we are seeing somewhat wetter autumns and springs and drier winters," he said.

Excursions of frigid air from the far north into our area are somewhat less likely during an El Niño. If the expected anemic El Niño does develop this winter, slightly warmer and drier-to-normal conditions should result. But if the male child gains strength as the season progresses, don't be too surprised if we find ourselves beneath an upper-level ridge for extended periods of time in December and January, and longing for any form of precipitation.