The rapid decline of Arctic sea ice during the last couple of decades has spurred climate scientists to study how that meltdown influences the rest of the planet, and a new study suggests that the effects may extend deep into the tropics.
The study, published Monday in the journal Proceedings of the National Academy of Sciences, detected a pattern that links sea ice decline since the late 1990s with more frequent warm cycles in the Central Pacific Ocean. The surges of ocean heating in that region can disrupt the climate, affecting drought, flood and hurricane patterns around the world.
Winds are the link between the melting ice and the tropics. The researchers posit that the Arctic Ocean has warmed up so much in the last 20 years that warm, late-summer sea surface air forms powerful convective towers, rising to the stratosphere. When the air falls back toward the equatorial Pacific, it intensifies prevailing east-to-west trade winds that push warm water toward Asia and Oceana, giving birth to a Central Pacific El Niño, a geographically specific variation of the well-documented Pacific warming and cooling cycle that is a key driver of the global climate.
The study found a secondary effect. The atmospheric roller coaster rebounds back north, and may weaken a weather pattern near Alaska that steers Pacific storms toward the West Coast.
The new research shows that El Niños forming in the Central Pacific started becoming more frequent at the same time that Arctic sea ice extent started its precipitous decline.
Jennifer Francis, a climate researcher with the Woods Hole Research Center who was not involved in the study, said the findings add to, "a growing body of evidence suggesting that the influence of Arctic sea-ice loss on large-scale wind systems reaches farther than thought, beyond the mid-latitudes and into the tropics."
There is still a robust debate among climate scientists when it comes to identifying links between sea ice loss and specific changes in far away weather patterns, and not everyone agrees that the findings of the new study are conclusive. More modeling is needed to clarify the fingerprint of sea ice melt in the tropics, said Penn State climate scientist Michael Mann.
But especially in the context of other similar recent studies, the new research offers more evidence that Arctic sea ice decline sets off a chain of events that affects tropical Pacific winds and sea surface temperatures.
It's hard to imagine that the rapid decline of sea ice, the planet's heat-reflecting polar shield, doesn't change weather patterns in other regions, said Charlie Kennel, former director and professor emeritus at the Scripps Institution of Oceanography and a co-author of the new study.
"There's a definite relationship and a change in tropical climate," Kennel said.
Here's how the connection works. As sea ice dwindles, larger areas of ocean water are exposed to the sun for much longer periods. Ocean water is darker than ice, and absorbs about 93 percent of the incoming heat (ice reflects up to 80 percent back to space).
According to the study, that warmer water creates convection—rising air— that goes to the top of the lower atmosphere, where it has "nowhere to go but south," Kennel said. That movement goes hand-in-hand with recent shifts in weather patterns that have caused extreme cold outbreaks in the central U.S. and deadly flooding in Asia, he added.
In a secondary, complex link, that vertical wave of air is then reflected northward again, possibly weakening the Aleutian low, a persistent weather pattern that usually spins off the southern coast of Alaska, steering storms toward the West Coast. Kennel said the impacts of that weakening aren't yet fully understood, but may be related to the recent dramatic decline of sea ice in the Bering Sea.
A related 2017 study suggested that the chain of events caused by dwindling Arctic sea ice could reduce winter rainfall in California. In a hemisphere-spanning, two-step connection, "sea-ice changes lead to reorganization of tropical convection that in turn triggers an anticyclonic response over the North Pacific, resulting in significant drying over California," the researchers wrote in the journal Nature Communications.
Changing El Niño
Changes to central Pacific El Niños are at the heart of the new research. El Niño is the warm phase of a multiyear temperature cycle in the equatorial Pacific Ocean that can bring flooding rains to some areas, and crop-killing droughts to others. It can even affect the frequency and location of tornado outbreaks, according to the National Oceanic and Atmospheric Administration.
Kennel said the new study identified a step-change in the Pacific Ocean climate starting in 1999, "associated with increased seasonal sea ice loss and increase of trade winds in the Central Pacific." The increase in trade winds is the mechanism that drives the emergence of more frequent El Niño events in the Central Pacific in the 2000s, he said.
Before 1990, there was only one Central Pacific El Niño recorded, but since then, they've become the dominant type of El Niño, the authors wrote in the study. Proxy climate data (including tree rings and mineral measurements in cave deposits), showing El Niño cycles before people started measuring them, suggest that the ratio of Central Pacific to Eastern Pacific El Niños was the highest in 400 years — all signs of a sudden climate shift coinciding with accelerated Arctic sea ice meltdown in the last two decades.
"The case is strengthening that the larger melts of Arctic sea ice are altering the spatial pattern and intensity of midlatitude extreme weather events," the researchers wrote in the new study.
Kennel said it appears the sea ice decline passed a threshold with global consequences about 20 years ago.
"Sea ice reached a kind of tipping point around 1999," he said. "Before that, the seasonal melting from June to October was about the same size (each year). After that, it was much bigger."
Francis, who was one of the first to propose that declining Arctic sea ice could intensify storm and drought extremes in the mid-latitudes, said that "interestingly, the Arctic connection was weak or non-existent during years before sea ice had declined substantially."
She added, "These authors present multiple lines of evidence that all fit the story that, when it comes to influences on mid-latitude weather, the tropics are no longer the only game in town."
Francis said another recent paper found different types of links between Arctic sea ice and the tropics. In the 2019 report in the journal Nature, the researchers wrote that extensive sea ice around Greenland and the Barents Sea favors El Niño conditions the following winter.
Ivana Cvijanovic, a Lawrence Livermore National Lab climate scientist who was not involved in the study but does similar research, said the findings advance scientific understanding of global climate system links.
The way Arctic sea ice decline affects the tropics suggested by the new study is compelling, said Cvijanovic, who co-authored an earlier study of links between the sea ice meltdown and future declines in California winter rains. The research is ambitious, she said, because it tackles two complex questions: the remote impacts of Arctic sea ice loss and the apparent recent intensification of central Pacific El Nino events.
"These questions are typically answered by scientists from different communities, like those studying ENSO and those studying Arctic teleconnections," Cvijanovic said, using the scientific acronym for the El Niño—La Niña cycle. "I feel that this study is a prompting for these different scientific niches to further open up to each other and explore the idea that these processes are most likely all linked anyway."
She added, "What is most interesting and intriguing to me, however, is that this study did not employ a modeling approach to isolate the impacts of sea-ice loss." Instead, the researchers reanalyzed data from observations that represent the "best idea of real world changes, so it is exciting to see that, despite very large noise due to the natural variability, the signals are starting to emerge there too."
Uncertainties
Mann said the ideas in the study are intriguing, but still speculative.
"I would like to see more compelling evidence that these mechanisms can be seen in state-of-the-art coupled climate models," he said. "It's difficult to tease these relationships apart because there are so many interacting mechanisms in the real world."
The jet stream, a belt of fast-moving west to east winds in the mid-latitudes, might have a stronger influence on the tropics than melting sea ice, said James Overland, a climate researcher with the National Oceanic and Atmospheric Administration.
"Yes, there can be an equatorial connection through vertical circulation, but the big deal on trends and year-to-year variability is the wavy jet stream carrying temperatures north and south," Overland said.
The Pacific Ocean climate has a huge range of natural variability, including decadal oscillations with huge shifts of warmer and cooler areas. So trying to find the influence of melting ice in the Arctic is a bit like "digging in the noise," said Mojib Latif, head of the marine meteorology research unit at the GEOMAR Helmholtz-Centre for ocean Research at the University of Kiel (Germany).
But sometimes it takes a little digging to find buried treasure in climate data, said Cvijanovic. And the research does offer some clues about the cause of the surge in climate extremes that threatened life and property during the 2000s. To be more confident of the links would require more data from many more years with Arctic sea ice loss, but that's time that we don't really have, considering the growing urgency of the climate crisis, she said.
"The issue is, when that happens, we may be quite deep into the problem," she said. "So, while we can't wait for too long to conduct the research, we have to be very honest about the possible limitations."
Still, Cvijanovic noted, the new research suggests that Arctic sea ice changes have an important influence on the tropical Pacific" and on El Niño and La Niña cycles.
"If this is true, it would mean that the impacts of Arctic sea-ice loss could literally affect the entire planet," she said. "Arctic sea ice melt is by no means only a polar bear problem."
