- Temperature patterns across the major population zones of the Northern Hemisphere are tracking in a similar fashion to last winter. Highly impressive November cold is already being experienced across much of the central/eastern US, while Europe is experiencing a warm start to the winter season. Are there any indications that the upcoming winter could be similar to a year ago?
- The Polar Vortex is one of the dominant drivers of winter weather patterns across the Northern Hemisphere. Research performed by scientists Dr. Judah Cohen and Dr. Jason Furtado at AER (Atmospheric Environmental Research) in Boston, MA has determined several key indicators for the behavior of the Polar Vortex in winter. Snow cover across Siberia during October and jet stream patterns in November, particularly across Asia and the Pacific are the strongest predictors of the mid-winter Polar Vortex.
- This year, October Siberia snow and November jet stream patterns are indicating an increased likelihood of colder than normal temperatures. The research by Cohen and Furtado indicates the heightened likelihood of colder than normal conditions across the central/eastern U.S./Canada, northern East Asia and continental Europe east into western Russia. Earthrisk Technologies, a weather company specializing in commodities, is indicating the strong likelihood of colder than normal conditions this winter across the US.
- A more detailed discussion of the long-lead indicators for the upcoming winter follow below. Topics include defining the Polar Vortex, how meteorologist measure the strength of the Polar Vortex via the Arctic Oscillation, what specifically has happened with Siberia snow and the jet stream this autumn, and wrapping up with a summary of indications for the upcoming winter.
- The Polar Vortex: Winter temperatures and storm track are strongly influenced by the behavior of the Polar Vortex. The Polar Vortex is a very deep column of pure Arctic air that extends from the surface into the stratosphere above the Arctic. If the Polar Vortex is strong and stable, most of the Northern Hemisphere population centers will experience a warmer than normal winter as cold air is bottled up over the Arctic and the storm track is shifted north of normal. In contrast, a cold winter in the Northern Hemisphere population centers features a Polar Vortex that is frequently stretched and shifted off the North Pole, driving cold out of the Arctic into the mid-latitudes. The graphic below highlights the Polar Vortex on January 9, 2014 – the coldest stretch of temperatures in early January of last winter across many areas of the central/eastern U.S. The Polar Vortex was elongated and resulted in ‘cross-polar flow’ from Siberia directly into central Canada and the US. Meanwhile, mild maritime Atlantic air poured into Europe resulting in a parade of storm systems bringing stormy but an overall warm bias last winter.
- Arctic Oscillation (AO) measures the Polar Vortex strength: The Arctic Oscillation (AO) is an index which measures the strength and behavior of the Polar Vortex. The AO is positive when a strong Polar Vortex ‘bottles up’ cold air over the Arctic and limits cold air flowage into the mid-latitudes. In a positive AO regime, the Polar Vortex features stronger than normal winds in the stratosphere around the North Pole and the vortex is arranged in a circular fashion. Conversely, the AO is negative when cold air is flowing out of the Arctic into the mid-latitudes, indicating an unstable Polar Vortex. In severe -AO regimes, the Polar Vortex can split into two features or be displaced well off the North Pole.
- The current situation across the Northern Hemisphere is a great example of a negative AO regime and an unstable Polar Vortex. Colder than normal air has been pushed out of the Polar Regions into North America and a large part of Asia, while warmer than normal conditions reside over the Arctic. This can be seen in the map below highlighted the temperature departure from normal on November 17. Below the map, a time series of the AO is depicted from July through November. The current -AO period is the third instance of a strongly -AO since October 1st with very few days of +AO over the last 45 days.
- Siberia snowcover: One of the first indicators of a winter dominated by an unstable Polar Vortex (negative AO) is how snow advances across Siberia in October. Snow cover that expands rapidly to above normal levels is highly correlated with a negative AO winter and an unstable Polar Vortex. Last month, Siberia snow cover was the second highest on record for October, only trailing 1976, which resulting in the coldest winter on record for the continental US. Snow cover across Eurasia was a very impressive 4 million square kilometers above the long term average (roughly equal to half the land area of continental US). Hence, with very expansive snow cover last month, it is not a surprise that the month of October was already featuring an unstable Polar Vortex and negative AO.
- Snow cover/jet stream handoff: Expansive snow cover across Eurasia in October, particularly towards China/Mongolia as well as into western Russia, sets the stage for a more amplified jet stream in November. Cold air builds over the Siberia snowpack and pushes the jet stream south over Asia. When the jet stream dips south across Asia, it responds by amplifying north (ridging) across the North Pacific towards Alaska and the Arctic. Ridging of the jet stream into the Arctic in November disrupts the formation process of the Polar Vortex, setting the stage for a unstable Polar Vortex for the following winter which favors cold air flowage out of the Arctic and a negative AO.
- Super Typhoon Nuri impacts the Polar Vortex: The map above illustrates how the extensive October 2014 Siberia snow cover starting to impact the Polar Vortex by the first week of November. A large Pacific ridge developed in response to the October Siberia snow cover which started the process of destabilizing the Polar Vortex, elongated from Siberia to Canada. The Polar Vortex stability was further impacted by Super Typhoon Nuri which played a role in strengthening the ridge over the North Pacific. Nuri transitioned into a record strong North Pacific extratropical storm system on November 9th reaching a central pressure of 924 mb – equivalent in pressure level to a borderline Category 5 hurricane! The record breaking low pressure system further built the ridge over the North Pacific to even more intense levels. As a result the Polar Vortex alignment resulted in the ‘Siberian express’ of cold air flowing out of Siberia, across the North Pole and into the lower 48 in the middle third of November.
Indications for the upcoming winter: The evolution of autumn variables used to predict the Polar Vortex are all aligned in the direction of a cold bias this winter across much of the Northern Hemisphere population centers. While impossible to determine the duration and exact timing of cold air outbreaks in mid-November for the duration of the winter, leading research is currently pointing towards a colder than normal outlook this winter. Determining which population zones of the Northern Hemisphere are most significantly impacted is also hard to tell at this time. However, the cold and snowy start for the US parts of Asia lean more towards these population centers seeing significant cold. In contrast, the Atlantic jet stream pattern has not been conducive for cold air flowing west out of Siberia into central and western Europe to this point. The current global snow cover map, highlighted below, shows expansive snow cover in place across the US while Europe is largely without snow.