Sunday, 8 May 2016

Arctic melt

New research has proposed a critical connection between sharp declines in Arctic sea ice and changes in the atmosphere - not only affecting ice melt in Greenland but also weather patterns all over the North Atlantic.

The studies centre on an atmospheric phenomenon known as ‘blocking’ - when high pressure systems remain stationary in one place for long periods of time (days or even weeks), causing weather conditions to stay relatively stable for as long as the block remains in place.

These can occur when there’s a change or disturbance in the jet stream, causing the flow of air in the atmosphere to form a kind of eddy, according to Jennifer Francis, a research professor and climate expert at Rutgers University.

Blocking events over Greenland are particularly interesting to climate scientists because of their potential to drive temperatures up and increase melting on the ice sheet.

“When they do happen, and they kind of set up in just the right spot, they bring a lot of warm, moist air from the North Atlantic up over Greenland, and that helps contribute to increased cloudiness and warming of the surface,” says Francis. “When that happens, especially in the summer, we tend to see these melt events occur.”

Two recent studies have suggested that there’s been a recent increase in the frequency of melt-triggering blocking events over Greenland - and that it’s likely been fueled by climate change-driven losses of Arctic sea ice.

A paper set to be published tomorrow (9 May 2016) in the ‘International Journal of Climatology’ reveals an increase in the frequency of these blocking events over Greenland since the 1980s.

A team of researchers led by the University of Sheffield’s Edward Hanna used a global meteorological dataset relying on historical records to measure the frequency and strength of high pressure systems over Greenland back to the year 1851. Previous analyses had only extended the record back to 1948, so the new study is able to place recent blocking events in a much larger historical context.

When the researchers analysed the data, they found that the increase in blocking frequency over the past 30 years is particularly pronounced in the summer, the time of year when blocking events are likely to have the biggest impact on ice melt.

What’s been causing this is a big source of interest for climate scientists hoping to gain a better understanding of the events affecting the vulnerable Greenland ice sheet.

In the new paper, Hanna and his colleagues suggested that declines in Arctic sea ice might be playing a role  -  and it’s a theory that’s heavily supported by another paper just out in the ‘Journal of Climate’. That study used both observational data and computer simulations to investigate the connection between sea ice declines and atmospheric changes in the Arctic.

Diminishing Arctic sea ice driven by climate change-induced warming is a well-established trend. Im April scientists reported that the maximum extent of Arctic sea ice over lastwinter had reached a record low for the second year in succession.

Interestingly, what happens on the surface of the ocean also has the potential to seriously influence activity in the atmosphere, adds Francis, a co-author on the study.

“When there’s less sea ice, obviously it’s a much darker surface that’s exposed to the sun - and especially in the late spring, early summer when the sun is really strong, that open water that would normally have ice on it absorbs a lot more of the sun’s heat.”

As a result, the surface of the ocean warms up and that extra heat is also transferred into the atmosphere. When this happens, lower layers of the atmosphere warm and expand, pushing up on higher layers of the atmosphere and causing the jetstream to bulge, as a result of the physics behind airflow in the atmosphere.

Since warm air takes up more space than cooler air, and the equator is the warmest part of Earth, the atmosphere is generally thickest there. This creates a kind of downhill ‘slope’ from the equator to the poles over which air flows.

Because the Earth is spinning so quickly, however, airflow ends up being pushed toward the east. The result is the jetstream - a current of air that generally flows from west to east around the world but also tends to meander north and south in wavy lines as it goes along.

If the Arctic warms more quickly than the rest of Earth, however, the downhill slope between the equator and the poles becomes less steep and this can weaken the jetstream’s flow, making it more susceptible to twists and turns.

So, as sea ice disappears and the atmosphere in the Arctic warms and expands, it can make airflow in the jetstream more likely to loop and bulge, causing the kinds of swirling eddies that result in blocking events.

Both the researchers’ observations and their model simulations strongly supported the idea that sea ice declines are a major factor in the frequency of blocking events over Greenland. When the researchers made changes only to sea ice in their model, they found an immediate connection to increased blocking, which in turn has led to increased surface melt on the ice sheet.

The findings may help explain some of the unusual weather patterns that have been seen in both Europe and North America in recent years. High pressure systems over Greenland can have the effect of blocking polar jet stream flow over part of the North Atlantic, causing the jet stream to split into branches and bringing about all kinds of severe weather events as a result.

In 2007, a high pressure blocking system over Greenland caused a split in the polar jet stream over the northern part of the North Atlantic, which ultimately resulted in extreme rainfall and flooding in the UK.

“To a certain extent, those conditions were repeated in summer of 2012, when we had a record wet summer in the UK,” adds Hanna. “And of course that was the year of the record high blocking conditions and extreme high temperatures and surface melt in Greenland.” 

So in this way, climate-induced changes in Greenland are not just causing problems locally - they’re creating mayhem throughout the North Atlantic, an example of the far-reaching influences of climate change and the interconnected nature of oceanic and atmospheric conditions around the world.

Greenland, of course, remains of prime concern thanks to its potentially devastating contributions to future sea-level rise. And, unfortunately, as more sea ice melts away, conditions are only likely to worsen, Francis said.

“It won’t be increased every single year, but the trends should continue downward — which isn’t good, because as we lose that ice that’s sitting on land on the Greenland island, it goes straight into the ocean and of course is one of the main contributors to sea-level rise,” she said. “That’s an effect that is felt all over the world.”

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