This past week, the American Geophysical Union (AGU) 2015 annual meeting was abuzz with discussion of El Niño 3.4 and the Arctic Oscillation. No wonder. These indicators are clues to how major competing factors may play out this winter in the northern hemisphere.
The autumn conditions (October to November) rank as the third largest El Niño since 1950. (Between June and September, El Niño logged the second strongest conditions since 1950.)
Other news released during the AGU meeting was just as remarkable. The latest Arctic Report Card monitoring year (October 2014 to September 2015) broke the hottest Arctic land temperature annual average since 1900.
Five key indicators to watch this winter
Will El Niño stay strong and bring drought relief for California? Will cold outbreaks penetrate into the continental U.S.?
To help answer these and other questions, scientists are paying close attention to five key indicators—and to how these powerful forces interact, evolve, and help shape this winter’s weather.
1. The MEI—Will El Niño dominate the early part of the winter?
The multivariate El Niño/Southern Oscillation (ENSO) Index (MEI) is based on the six observed variables over the tropical Pacific: sea-level pressure, zonal and meridional components of the surface wind, sea surface temperature, surface air temperature, and total cloudiness fraction of the sky. The red (positive values) are El Niño and the blue (negative values) are La Niña. Source: NOAA/ESRL
2. Precipitation patterns
Will the El Niño precipitation patterns be typical for the winter in the U.S?
The wintertime pattern during El Niño conditions typically bring precipitation to the southern region of the continental U.S. The Northern U.S. states, in contrast, can be unseasonaby mild and drier. Figure Source: NOAA
3. Arctic temperature
How do the temperatures north of 60 degrees latitude influence the sea ice during the summer and the stratospheric polar vortex in the following winter?
A record-breaking polar year for the Arctic is an indicator of how the region north of 60 degrees latitude is warming faster than the global average. Graphic: Dan Pisut/NOAA
4. Arctic Oscillation Index
A positive Arctic Oscillation (AO) number indicates a pattern with strong winds that tend to circulate counterclockwise around the pole at 55 degrees North latitude. These strong winds tend to keep the cold air in the Arctic. A negative AO number indicates weaker winds that are more likely to become distorted and allow colder air to penetrate into lower latitudes.
Daily Arctic Oscillation (AO) Index. Graphic: NOAA Climate Prediction Center (CPC), September 2015 through December 20, 2015.
5. 500 hPa geopotential height contours
Weather tends to follow the direction of winds around 5.5 km above sea level, or at the 500 hPa geopotential height. This indicates the upper level pattern, such as discussed in this synoptic weather discussion. Temperature at 850 hPa, around 1.5 km above sea level, indicates frontal zones. These charts can be used to see how much of the cold air is contained near the North Pole or if the pattern leads to cold outbreaks to lower latitudes.
Northern hemisphere geopotential height at 500 hPa (contours) and temperature at 850 hPa (color shading). A hectopascal (hPa) is a unit of pressure that is equivalent to millibar. Graphic: ECMWF
I’m dreaming of a warm Christmas
Unseasonably warm December conditions are dashing records and creating fun antics and some unnerving impacts on inhabitants across the Eastern U.S., as reported by my colleague Erika Spanger-Siegfried. All signs point to the El Niño pattern continuing to dominate the end of December with the potential for delivering some rain or snow to drought-stricken California and a high probability of a warmer than normal Christmas in the U.S. northeast.
However, there may be a need to keep those winter coats in the front of the closet. Some scientific indicators suggest a January 2016 weakening of the stratospheric polar vortex, increasing the chance for severe late winter weather over the continents. For a recent history and updated overview of how the stratospheric polar vortex and tropospheric polar jet stream can influence winter extremes check out this update by my colleagues Astrid Caldas and Matt Heid.