El Nino is a natural phenomenon in which changing ocean temperatures occur alongside changes in atmospheric circulation and rainfall. It’s irregular; every 2-7 years ocean waters near the tropical Pacific cycle between warm El Nino or cold La Nina conditions. Though the tropical Pacific may seem far away, the resulting changes in ciruclation do impact Alaska and other Northern Hemisphere locations. The cycling pattern is also known as the El Nino-Southern Oscillation (ENSO), or the ENSO Cycle.
Jon Gottschalck, Acting Chief, Operational Prediction Branch at NOAA’s National Weather Service and National Centers for Environmental Prediction Climate Prediction Center, addressed why people around the world should care.
Changes in ocean temperature cause changes in rainfall. “Changes in tropical rainfall along the equator can produce very strong changes in heating profiles or distributions throughout the atmosphere and these changes in heating throughout the atmosphere produce basically influences to the mid-latitude airflow or circulation across the Pacific and North America and globally for that matter,” said Gottschalck. It changes the strength and direction “Of jet streams and also overall patterns of longwave troughs and ridges or areas in the upper atmosphere of low or high pressure and we call these sort of impacts- which are typically lagged- teleconnections.” Gottschalck emphasized that “These teleconnections are why folks in Alaska and again around the world need to pay attention towards changes in ocean temperatures and tropical rainfall associated with the ENSO cycle.”
Bands of powerful winds called jet streams occur in the troposphere, the layer of our atmosphere closest to earth, where areas of high and low pressure meet. The temperature difference between the pressure fronts helps define the strength of jet stream winds. The jet stream in the northern hemisphere, or Polar Jet Stream, happens where cold polar air interacts with the warmer air originating in the tropics and pushing north through the mid-latitudes. It is that interaction which influences weather trends in North America and beyond. El Nino changes alter the nature of the jet stream, and an altered jet stream can amplify extreme weather events and change the origin, persistence, and severity of regional weather.
Pressure patterns described as a positive Arctic Oscillation have a swift west-east moving Polar jet stream (zonal flow). They oppose those described as a negative Arctic Oscillation with a slower more meandering Polar jet stream (meridional flow) featuring radical high-pressure ridges stabbing south and low-pressure troughs stabbing north. Severe storm fronts often form where the pressure systems with their wild temperature gradients rub against one another. And the meridional flow can make weather conditions dominate in one region for a longer duration, potentially inflicting heat spells, drought and increased wildfire probability, extreme summer rainfall leading to flooding, or unseasonal warm or cold.
Strong El Nino events feature a strong zonal jet stream across Southern United States. That tends to keep storms and therefore precipitation heavily active in the Southern U.S. Increased precipitation would be a big help to drought-stricken California this year. While the south is getting rain, Northern U.S. can expect milder-than-average conditions and fewer storms.
When it comes to El Nino events like the one that began winter 2015, it’s truly not easy to forecast specific weather conditions because there is quite a bit of variability from event to event. But scientists like Rick Thoman, National Weather Service Alaska Region Climate Science and Services Manager, still take a stab at it by using past El Ninos as forecast tool.
During strong El Nino winters like this one, Alaska is typically warmer than average. In 2014 Thoman explained Alaska is particularly tough to forecast because mountains are always a complicating factor; we need to talk regionally. With a strong El Nino expect to see warmer than average temperatures in South and Southeast Alaska. Thoman stated “Actual sea surface temperatures and the large scale Northern hemisphere response to those have changed over the past 60 years, especially during the winter season.” There may be significant Bering and Chucki Sea coastal storms. One might expect a somewhat drier North Alaska, while South and Southeast coastal areas might anticipate greater than normal precipitation.
Why more precipitation? Duane Waliser, Chief Scientist, Earth Science and Technology Directorate, NASA Jet Propulsion Laboratory, spoke about atmospheric rivers at the 2015 American Geophysical Union Fall Meeting. Atmospheric rivers are narrow stripes or regions of concentrated moisture in the atmosphere which transport water vapor from tropical ocean areas toward land, making them responsible for i.e. coastal rainfalls along California’s coast. They are common and useful, and make landfall on virtually every continent and also in high latitudes. El Nino tends to make atmospheric rivers stronger: they carry and deposit higher precipitation amounts to coastal areas. That enhances rainfall characteristics to the western coast of the U.S. It remains to be seen whether this winter’s El Nino can be a ‘drought-buster’ for California after four years of drought.
Laura Nielsen 2016
Frontier Scientists: presenting scientific discovery in the Arctic and beyond