A historic storm occurred over Alaska this past weekend as typhoon Nuri merged with an extra tropical system and became a perfect storm. With it also came the chance for more extreme weather for the United States in the form of a small polar vortex event that flooded much of eastern North America with frigid temperatures. But how can we have such cold outbreaks in our warming world?
Superstorm Nuri in the Bering Sea. Image: NASA
Polar vortex and global warming
The polar vortex became a household term after last winter’s bitterly cold temperatures over much of the eastern United States. Here in Washington I recall wearing so many layers that it took several minutes to get ready to face the cold. It was deemed the coldest air in twenty years, and, although it broke some records, it wasn’t the coldest outbreak in history. Did it disprove global warming or did global warming actually influence it?
One theory that has been developed during the past decade holds that the reduction in ice cover in the Arctic has caused more extreme undulations in the jet stream, a fast moving current of air high in the atmosphere that separates cold air masses to the north from warmer air masses to the south. Arctic amplification, the rapid warming of the Arctic, lowers temperature gradient between the high latitudes and the tropics, weakening the jet stream and allowing for more “waviness.”
This is a possible factor that may have played a role in January 2014’s cold wave. Larger undulations in the jet stream mean bigger dips and bigger crests. In the case of the polar vortex, a particularly deep trough allowed for very cold air masses to travel much further south than normal.
Polar vortex and the Bering Sea Bomb
The “Bering Sea bomb” was a record-breaking low pressure system that affected parts of western Alaska. It formed from the merging of Typhoon Nuri with an extra-tropical system from Siberia, adding steep temperature gradients, the fuel for storms in the mid latitudes. It reached a lowest pressure of 924mb over the Aleutian islands, beating the previously-recorded 925mb for the North Pacific set in 1977. A powerful system such as the Bering Sea Bomb causes a cascade of effects on the rest of the atmosphere, especially the jet stream. It is akin to throwing a large rock into a pond. The bigger the rock, the bigger the resulting wave.
The effect of the intensity of the Bering Sea Bomb is to cause a strong bend in the jet stream pattern. This trough acts as a slide, of sorts, through which cold air from the arctic is able to reach deep into states to the east of the Rockies. In doing so, a portion of the polar vortex, a semi-permanent low pressure system in the high latitudes, broke off, sliding south with temperatures 20-40 degrees below normal.
Attribution of cold waves and severe snow storms
Image: NOAA
Does climate change due to human activities influence the occurrence of cold outbreaks or intense snow storms?
Although meanders in the jet stream and associated cold waves might be increasing for parts of the Northern Hemisphere due to arctic amplification, colder-than-average winters and years (such as 2014), are not breaking as many records as previous years since record-keeping began in 1880. For example, new attribution research for parts of Europe also show how the frequency in cold extremes is likely to be lower due to climate change. On the other hand, the risk of seeing more record-breaking warm years is increasing. In 2012, a great fraction of the country saw the warmest average temperature on record and California, along with the rest of the planet, are on pace to have warmest year on record in 2014.
Image: NOAA
Severe snow storms present a more complex exercise in attribution due to the lengthy list of variables required, but trends show an increasing number of storms and a northward shift in storm track. Further, the increase in atmospheric moisture can play a role in the amount of precipitation experienced.
Still, this does not mean the individual, record-breaking storms are connected to climate change. Attribution studies have shown storms such as last year’s South Dakota blizzard in September do not have the footprint of climate change, and, in fact, could become less likely as the planet warms.
Image: NOAA
Several inches of snow fell over the upper Midwest and the Great Plains ahead of the polar vortex. Cold air will reach the east coast later in the week, making it feel like January before Thanksgiving for 200 million people.