People who are looking forward to a warm and vital spring in Madison might just wonder about this unusually wintry and snowy March.
While no one can explain with certainty what caused this long winter, climate scientists turn their eyes to the melting ice in the Arctic. The Arctic sea ice loss, they suspect, may be to at least partly blame for such extreme weather patterns.
Steve Vavrus, a senior scientist in the Nelson Institute Center for Climate Research at the University of Wisconsin-Madison, suggests that the increase in ice melting caused by warming in the Arctic may weaken the jet stream winds, an essential factor driving weather systems. This in turn, could result in extreme weather, such as cold spells, heat waves, drought, and flooding, in the mid-latitudes of the northern hemisphere.
“The jet stream behavior we saw here last month [March] when it was so cold and snowy was a lot like what we are hypothesizing,” Vavrus said.
Jet streams are narrow bands of fast-flowing winds that blow from the west to east several miles up in the atmosphere. Like any other wind, they are formed out of the pressure difference when two masses of air run into each other.
Typically, the cold air in the Arctic contracts into a thin column of air while that of the warmer mid-latitudes expands to a thicker air column. When these two columns meet, their different temperatures and densities cause the pressure difference and create jet streams. The more pronounced the pressure difference, the more forcefully the jet streams tend to blow.
Normally the temperature varies greatly between the Arctic and the mid-latitudes, which results in a big pressure difference that ensures strong jet stream winds. As they flow above the earth’s surface, the westerly winds push the weather systems around the globe, avoiding long-lasting weather patterns in any one place.
However, the ice loss in the warming Arctic appears to be slowing the speed of the jet stream winds.
Vavrus’s studies show that the Arctic has warmed nearly twice as fast as the entire northern hemisphere during the past few decades, largely because of a phenomenon called “Arctic amplification”, a feedback loop between the sunlight and ocean ice.
Initially, the ocean surface is covered with bright ice that reflects much of the sunlight back into space. As the climate warms, the ice melts leaving the darker water beneath exposed to the sunlight. The dark water absorbs more solar energy and heats the sea as well as the air, which in turn melts more ice and exposes more dark water to the sunlight. As a result of the melting and warming feedback loop, the Arctic ice coverage hit a record low in 2012.
“The ice is a very good isolator that blocks the seawater and the air,” said Vavrus. As the ice melts, the air can pull more heat from the seawater, especially in winter when the seawater is much warmer than the air. The Arctic air warms up and forms a thicker column. This reduces the pressure difference with the air column of the mid-latitudes and weakens the jet streams.
The weakened jet streams move in a slower and more meandering way. “It’s easier to think of it as a river. When the river is flowing quickly, it tends to move straight in one direction whereas when it slows down, it tends to go this or that way,” Vavrus explained.
The meandering jet streams allow weather patterns to hang around longer in one place, instead of promptly pushing them forward. Moreover, as the winds flow in bigger U-shaped patterns, they bring about new weather systems as well. For example, in this past winter, the winds didn’t blow directly from the North Pacific Ocean to the United States, carrying warmer ocean air as they used to, but instead climbed up to Canada and brought the frigid Arctic air down, leading to a lingering chilly March.
This also partly explains the big heat and drought in the summer of 2012, Vavrus said. Rather than flowing in a straight line, the winds carried the cool ocean air up to Canada and then dipped down around the United States, leaving the heat waves unrelieved.
“What is most favorable for these extreme weather systems is a persistent and wavy pattern of air,” Vavrus said.
Along with Jennifer Francis, a scientist at Rutgers University, Vavrus published their findings in 2012 in Geophysical Research Letters and aroused media attention at a news conference in March 2013.
Vavrus proposed a three-year funding from the National Science Foundation to carry on an in-depth study of this hypothesis.
“Right now it’s just the preliminary analysis,” said Vavrus, “ We suggest one potential impact of the Arctic amplification on the weather patterns in the mid-latitudes. But it needs to be proved yet.”