Following a winter of extreme cold, it’s hard to process the mild start to the 2015 winter season. Meteorologists predict this year’s El Niño could be the strongest in the past 50 years.
El Niño is a weather condition referring to a weak, warm current generally materializing in December along the coast of Ecuador and Peru. Spanish for “the Christ child,” the phenomenon was first observed by South American fishermen in the 1600s, who noticed some years brought unusually warm water to their coast, typically around
Christmas. The condition is characterized by prolonged warming in the Pacific Ocean sea surface temperatures, when compared with the average temperatures. The trade winds, which normally blow west toward Indonesia, relax in the central and western Pacific, allowing normally cool, nutrient-rich waters off of South America to significantly warm. As this warm water spreads eastward, the hot, humid air moves with it causing changes to the position of the jet stream winds. The result affects weather not only in North and South America, but as far away as Africa and Antarctica.
Typically, this anomaly happens at irregular intervals of two to seven years and can last anywhere from nine months to two years, with the average interval length being five years. When this warming occurs for seven to nine months, it is classified as El Niño “conditions;” when its duration is longer, it is called an El Niño “episode.”
The above table shows the historical El Niño periods since 1900, with the years highlighted in red described as “major” El Niño events. According to NOAA’s data analysis, the ten strongest El Niños of the past century show they are occurring more frequently and becoming progressively warmer.
Pacific Decadal Oscillation (“PDO”)
Although El Niño occurs cyclically and independent of any long-term warming trend, there is observational evidence to suggest that rising global temperatures may be linked to stronger, more frequent El Niño episodes. These higher temperatures tend to produce more extreme weather events.
Another factor affecting weather trends is the Pacific Decadal Oscillation, which refers to the special pattern of the sea surface temperature anomalies in the Northern Pacific Ocean. A positive PDO indicates the eastern Pacific Ocean is warmer than the central and western. Observations have been made as to the correspondence between the presence of strong El Niño years and a prolonged warm period, such as in 1982-83 and 1997-98.
The National Center for Atmospheric Research’s Kevin Trenberth notes that the effects of El Niño tend to be augmented by the presence of positive or warm PDO. Some scientists say the current El Niño may impact the length of time the PDO remains positive. Though there are differing opinions from scientists as to whether the current positive PDO is representative of a multi-decadal shift to warmer sea surface temperatures along the North American Pacific Coast. Gerald Meehl, also from the National Center for Atmospheric Research, notes that positive PDO index may undo the perceived break in global warming experienced over the past decade.
The predicted winter will start off rather mild across most of the country, including the Pacific Northwest through the Northern Plains. Later in the season, several weeks of classic winter weather over the eastern third of the country is expected. This period will likely feature several winter storms from the south central Plains to the Northeast, with a higher than typical threat for significant snow and mixed precipitation deep into the South. (www.theweathernetwork.com)
The vast majority of past El Niño winters featured an active storm track across the southern United States, with a turn up the East Coast to New England. Above-average precipitation from Southern California to Florida and up the East Coast to Maine is likely to occur. This will likely also include a higher threat for severe weather near the Gulf Coast, including Florida.
A unique feature of the upcoming winter, as compared to other strong El Niño winters of the past, is the expected persistence of the warmer than normal ocean-water temperatures south of Alaska. Meteorologists have referred to this feature as “the Blob” and it has been a key contributor to the dominant weather pattern across North America for the past two years. This pattern has been associated with extended periods of warm and dry weather in the West and two of the coldest winters in recent memory in the Great Lakes and Northeast. If “the Blob” does persist through the upcoming winter, then the threat for a cold conclusion to winter in the East will increase. (www.theweathernetwork.com) “The Blob” has been cited as the most significant factor in bringing the bitterly cold winters of 2014 and 2015.
“We just don’t know exactly yet whether or not we’re going to see the pattern turn cold and snowy,” AccuWeather Expert Long-Range Forecaster Paul Pastelok said. “…There is an opportunity that [the weather] could change on us as we get into February and early March.” (www.necn.com)