Atmosphere, more than ocean, might drive Atlantic climate variation

by Elizabeth Goldbaum
Friday, January 8, 2016

Hurricane Isabel was photographed from the International Space Station in 2003, during the most recent warm phase of the Atlantic Multidecadal Oscillation (AMO). The AMO, which seems to affect the frequency of tropical storms, may be more affected by atmospheric circulation than by the ocean. Credit: NASA.

Atmospheric, not oceanic, circulation may be the main driver of climate variations over the North Atlantic Ocean, potentially complicating future hurricane and drought predictions, according to the authors of a new study.

The climate variations — in air temperatures, precipitation patterns and cloud formation — result from the Atlantic Multidecadal Oscillation, or AMO, a phenomenon in which ocean surface temperatures swing by up to half a degree Celsius. The AMO affects the sea surface between the equator and Greenland, and warm and cool phases can last 20 to 40 years.

“These slow changes in the Atlantic have an impact on the climate on land around the Atlantic, where people are living,” says Amy Clement, an atmospheric scientist at the University of Miami and lead author of the study, published in Science. For example, “the hurricane frequency and the number of storms seem to co-vary with the AMO,” Clement says, and rainfall patterns in Florida and throughout much of the northeastern United States appear to be influenced by the AMO.

Similar to El Niño, the AMO accompanies natural changes in sea-surface temperature. Ocean circulation is a critical driver of El Niño, leading many scientists to assume the ocean is also driving the AMO, Clement says. However, when her team ran climate simulations over the North Atlantic with and without ocean circulation, they found that both yielded similar variations in droughts and hurricanes, among other weather events. That suggested that the atmosphere, rather than the ocean, is driving the AMO in the models.

“We know the climate is going to warm in the future, with or without oceanic circulation,” Clement says. “But fluctuations about that warming trend, which matter for multiyear and multidecadal changes in rainfall patterns, could be harder to predict” if her team’s simulations are accurate, she says, because the more-chaotic atmosphere, instead of the more-predictable ocean, is driving the changes on those timescales.

But not everyone agrees that the atmosphere is solely responsible for driving the AMO. Tom Delworth, a research scientist at NOAA’s Geophysical Fluid Dynamics Laboratory who was not involved in the study, says the atmosphere probably contributes to the AMO on short timescales, but that the ocean is a key driver of decadal-scale changes.

“The ocean is sluggish — it takes a lot of energy to move the ocean and change the ocean,” Delworth says. So, he adds, ocean circulation doesn’t usually respond quickly to shifting winds. But on longer timescales, it does respond to atmospheric circulation changes.

Delworth also notes an inconsistency between the new modeling and physical observations. Changes in the North Atlantic Oscillation, or NAO — the dominant pattern of wind variability over the Atlantic — determine how much energy cycles between the ocean and atmosphere, which adjusts the ocean’s thermostat, Delworth says. In the new study’s models, when the NAO is “positive” — with stronger westerly winds in subpolar regions of the North Atlantic and stronger trade winds in the tropics — winds remove heat from the ocean surface and cause it to cool, he says. “However, what we see in reality on long timescales is the opposite relationship in the Atlantic,” Delworth says. The NAO was strongly positive at times in the 1990s and early 2000s, and that led to strong warming of the North Atlantic, attesting to the influence of ocean circulation, he notes.

Gerard McCarthy, a research scientist at the National Oceanography Center in England who was not involved in the study, also says that ocean circulation plays a role in the AMO. In his view, the NAO controls the temporal pattern of the AMO while ocean circulation drives its spatial pattern in the North Atlantic. To capture the ocean’s role in setting the AMO’s spatial pattern, the study’s models need additional high-resolution data to accurately portray circulation patterns over small regions of the ocean. This will help the simulations better account for the shape of the Gulf Stream, which flows north from Florida and eastward to Europe, among other features, McCarthy says.

Clement says it’s possible that her team’s models are overlooking a role for ocean circulation in explaining the multidecadal fluctuations of the AMO, and that higher-resolution ocean data could help resolve this role if it exists. If you can simulate the ocean at high resolution, then you can start to see smaller-scale features in its circulation, like eddies, and perhaps then “the ocean may start to have more of an impact” in the model, she says.

Understanding how the climate system, particularly in the Atlantic, evolves today is important for predicting how it’s going to change in the future, Delworth says. And the reason researchers are so interested in the AMO is because “there are real impacts of it,” Clement says. Storms are twice as frequent during warm years of the AMO compared to cold years, she says. If the steadier ocean takes a backseat to the atmosphere, “we don’t have a lot of ability to predict that switch.”

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