Was it weather or was it climate? The colder winter anomaly in the Eastern United States
For those of us in the eastern part of the United States, the shortest month was long and hard. About the time we recovered from one severe cold front, another moved in. February 2015 continued a trend James Hansen and colleagues refer to in the following statement: “Residents of the eastern two-thirds of the United States and Canada might be surprised that 2014 was the warmest year, as they happened to reside in an area with the largest negative temperature anomaly on the planet, except for a region in Antarctica.” columbia.edu In scientific language “negative temperature anomaly” means colder in an unexpected way. Yes, we know it’s been colder. Is this cold pattern what we usually call weather, or is it a feature of the changing climate?
A number of studies are beginning to show that the cold winter anomaly relates to global warming—specifically the warming of the Arctic. Here is how recent scientific research explains it. scientificamerican, usnews.com
- In the summer, as Arctic sea ice melts, what remains is dark ocean water, which absorbs more heat.
- As winter comes on, an unprecedented degree of heat lingers in the ocean and atmosphere above it, with two important results:
- A warmer Arctic weakens the jet stream—the belt of fast-flowing, west-to-east winds that moves between northern and southern climates.
- The less stable jet stream makes it more likely that cold Arctic air will escape the polar vortex and flow southward.
The jet stream is described by Rutgers climate scientist Jennifer Francis in this way: “Jet streams exist because of differences in air temperature. In the case of the polar jet stream, which is responsible for most of the weather we experience around the middle-latitudes of the northern hemisphere, it’s the cold Arctic butting against warmer areas to the south that drives it. Anything that affects that temperature difference will affect the jet stream.” theconversation.com, usnews.com For a more in-depth explanation see skepticalscience.com.
The polar vortex was described for National Public Radio by Washington Post weather editor Jason Samenow. He said, “We’re talking about a huge sprawling area of circulating cold air originating from the North Pole. It’s a low-pressure center, and typically during the winter months it resides up there. At times, some tentacles of it will slip southward and bring cold air outbreaks into the U.S.” npr.org
Research into the phenomenon is fairly preliminary, in part because extreme Arctic sea ice loss is a fairly recent event. Scientists don’t have the long data sets they need to draw more robust conclusions about the interaction between Arctic warming and cold snaps. time.com
But what about the fear that Arctic warming could alter the Gulf Stream, leading to a new ice age? The Gulf Stream is a fast, intense current along the eastern coast of the United States, a result of the wind pattern acting on most of the North Atlantic Ocean. The combination of the Trade Winds (10°-25°N) blowing to the west and the Westerlies (35°-55°N) blowing to the east cause the North Atlantic Ocean to rotate clockwise. Because of the Earth’s rotation, the poleward flow in the western Atlantic is constrained to a narrow current from Florida to Newfoundland then across the Atlantic to northern Europe. wikipedia.org
An authoritative 2004 report states, “The thawing of sea ice covering the Arctic could disturb or even halt large currents in the Atlantic Ocean. Without the vast heat that these ocean currents deliver–comparable to the power generation of a million nuclear power plants–Europe’s average temperature would likely drop 5 to 10°C (9 to 18°F), and parts of eastern North America would be chilled somewhat less.” nasa.gov/science
Is that happening now, and if so, has it contributed to the chill of 2014 and 2015? Changes in the Gulf Stream are indeed being observed in an abundance of research, with reports being published almost daily.
- Evidence is mounting that the long-feared circulation decline is already well underway. One study shows a 15 to 20 percent weakening that could be the beginning of a further slowdown with great consequences. com, nature.com
- “The force of the Gulf Stream was significantly influenced by the sea ice situation in the Fram Strait in the past 30,000 years.” org. Fram Strait is the gateway to the Arctic Ocean located between Greenland and Svalbard.
- “In recent years, the retreat of sea ice in the Arctic has been linked to cold winters in Eurasia and North America. Now a study by a team from Japan indicates that a northwards shift of the Gulf Stream could trigger planetary waves that cause cold weather in Eurasia.” org
- “The great concern is that the removal of the ice will result in a more turbulent ocean, which will in turn affect what happens to the freshwater and heat within the Arctic and alter the currents linking the Arctic Ocean to the Atlantic Ocean.” com
- “It has been a concern that a layer of Arctic freshwater could impede the Gulf Stream’s Arctic branch. Going back in time—into and through ice ages—such a freshwater lid has been understood to reduce ocean circulation and thus the Gulf Stream’s poleward heat transport.” To make matters more confusing, this study found that today’s decreasing salinity in the Nordic Sea is traceable not to melting Arctic ice but to freshwater entering the Gulf Stream from the south, possibly due to increased precipitation over the North Atlantic Ocean—with that increase also traceable to climate change. com
A careful reader will begin to grasp the complexity of the climate system, a glimpse made more difficult for laypeople by the need to master a precise vocabulary. For example, ordinarily the issue of salinity is associated with the deep ocean current, the Great Ocean Conveyor (GOC). In the GOC, cold, salty, dense water sinks at the Earth’s northern polar region and heads south along the western Atlantic basin. It picks up more cold, salty, dense water in Antarctica and proceeds northward on two paths: one into the Indian Ocean and one into the western Pacific. The two branches of the conveyer warm and rise as they travel northward and continue circulating around the globe, eventually returning to the North Atlantic where the cycle begins again. noaa.gov
The GOC is driven by differences in the density of the sea water, which is controlled by temperature (thermal) and salinity (haline), hence thermohaline circulation, another name for the GOC. The Gulf Stream, on the other hand, is considered to be a surface ocean current. Currents nearer the surface are driven by global wind systems fueled by energy from the sun. education.noaa.gov
In addition to the more stable ocean and wind patterns that influence temperature in our region, there are El Niño and La Niña to consider. At irregular intervals (roughly every 3-6 years), sea surface temperatures in the Pacific Ocean along the equator become warmer or cooler than normal. These anomalies are the hallmark of El Niño and La Niña climate cycles, which can influence weather patterns across the globe. Wikipedia. The 2014-2015 Hansen report states, “Record global temperature in 2014, achieved with little assistance from the tropical ENSO (El Niño Southern Oscillation) cycle, confirms continuing global warming. More warming is expected in coming years and decades as a result of Earth’s large energy imbalance, more energy coming in than going out, and with the help of even a mild El Niño 2015 may be significantly warmer than 2014.“ columbia.edu
A sense of wonder regarding Earth’s intricately interwoven climate system is an almost unavoidable result of examining this array of research—as well as a deeper appreciation for the work of climate scientists. What we can know for now is that a warming world can also be colder in spots. Perhaps the biggest irony about that phenomenon is this: international global warming policy is being most strongly influenced by politicians in the only inhabited region of the planet that has been cooler in all seasons than the 1951-1980 average. (Temperature data columbia.edu)