By Robert A. Vella
It may seem paradoxical to laypeople that we would have severe cold weather spells in wintertime given that the world is rapidly warming up due to manmade climate change; and, climate change deniers are quick to exploit this paradox for political reasons. But, it is true. Global warming is increasing the incidence of extreme weather events of every kind from prolonged droughts and powerful storms to deadly heat waves and brutal cold snaps. The following details the basic science behind the phenomenon popularly, though inaccurately, known as the “polar vortex.” The real polar vortex is something else altogether. More on that later.
First, we need to examine the general atmospheric circulation of the Earth. The tropical regions around our planet’s equator receive the most sunlight. It heats up the land surface and oceans causing the air to rise high into the atmosphere. This warm air then moves towards the north and south poles through a series of circulation patterns called Hadley Cells. What the Earth is doing here is simple physics – transferring heat from hot spots to cold spots to maintain temperature equilibrium. As this moving air releases its heat, it dramatically cools and descends towards the surface. At the poles, it forms a stable dome of cold air on top of the Arctic Ocean and the Antarctic continent. See figures 1 and 2:
Where these Hadley Cells interact at the tropopause (i.e. the interface boundary between the troposphere and stratosphere), high-speed jet streams traverse across the globe from west to east which direct the prevailing weather systems in the mid-latitudes (i.e. temperate and subtropical zones). There are two in each hemisphere, a weaker subtropical jet and a stronger polar jet. These jet streams result from temperature gradients between the different air masses of each cell. See figures 3 and 4:
Figure 5 shows a typical weather pattern over the United States:
Climate change is warming the polar regions at a faster rate because of melting glaciers and sea ice which decreases albedo (i.e. reflectivity) and increases the heat absorption of sunlight, and because of the increasing transfer of heat through oceanic circulation and evaporation. As the poles warm, especially in the Arctic, the temperature gradient between interacting Hadley Cells decreases which causes the jet streams to slow down and meander (i.e. greater amplitude) in a more north-south direction (i.e. latitudinal) instead of the usual west-east direction (i.e. longitudinal). During the northern winter, this effect can pull down cold Arctic air masses much further south than normal and cause them to linger longer over specific areas. It also can produce the opposite effect resulting in abnormally warm periods or even allowing tropical storms to venture further north such as “superstorm” Sandy in 2012. See figures 6 and 7:
Research shows that over the past several decades, the jet stream has weakened. There’s also evidence that as it wobbles, it can get stuck out of kilter, which can lead to more persistent weather extremes, including heat waves, cold snaps, droughts and flooding.
Scientists say there is strong evidence that human-caused global warming has altered the strength and path of the powerful winds.
Research into three centuries of European tree ring data by Valerie Trouet of the University of Arizona found evidence of significant changes in the jet stream starting in the 1960s. The recent deviations exceeded normal variations in the past, suggesting a connection to the changing climate. The result: more extreme drought, flooding and heat waves.
Rutgers University climate scientist Jennifer Francis has found “robust relationships” between Arctic warming and a wavier jet stream.
Melting sea ice speeds up the warming of the Arctic because open water absorbs more heat. And that, in turn, leads to even more sea ice melting. In that vicious cycle, Arctic temperatures are rising twice as fast as the global average. And that’s reducing the temperature contrast that’s one of the jet stream’s main engines, Francis said.
More extreme and persistent swings in the jet stream may also be shaping a North American winter weather pattern that’s been common the past few years—a warm and dry West, especially California, and cold waves in the Eastern U.S.
“We think it takes two ingredients to make this pattern so robust. A lot of warm water off the West Coast, and warm conditions and declining sea ice in the western Arctic around Alaska. Both pump a big ridge in the jet stream along the West Coast,” she said. Then, a few thousand miles east, the jet stream dives south, bringing polar air to the East Coast.
Another study last summer by climate scientist Michael Mann and co-authors also found significant evidence for links between the rapidly warming Arctic and a jet stream slowdown.
Finally, the real polar vortex describes the cyclonic (i.e. counter-clockwise rotation in the northern hemisphere) low-pressure system above the dome of cold polar air near the surface. Climate change is splitting the usual single system into multiple low-pressure systems which can then migrate independently over a larger area.
A study in 2001 found that stratospheric circulation can have anomalous effects on weather regimes. In the same year, researchers found a statistical correlation between weak polar vortex and outbreaks of severe cold in the Northern Hemisphere. In later years, scientists identified interactions with Arctic sea ice decline, reduced snow cover, evapotranspiration patterns, NAO anomalies or weather anomalies which are linked to the polar vortex and jet stream configuration. However, because the specific observations are considered short-term observations (starting c. 13 years ago) there is considerable uncertainty in the conclusions. Climatology observations require several decades to definitively distinguish natural variability from climate trends.
The general assumption is that reduced snow cover and sea ice reflect less sunlight and therefore evaporation and transpiration increases, which in turn alters the pressure and temperature gradient of the polar vortex, causing it to weaken or collapse. This becomes apparent when the jet stream amplitude increases (meanders) over the northern hemisphere, causing Rossby waves to propagate farther to the south or north, which in turn transports warmer air to the north pole and polar air into lower latitudes. The jet stream amplitude increases with a weaker polar vortex, hence increases the chance for weather systems to become blocked. A blocking event in 2012 emerged when a high-pressure over Greenland steered Hurricane Sandy into the northern Mid-Atlantic states.