From Maine to Delaware, the US coastal Northeast is warming faster than most of North America, and a new study shows why. He links the outsized warming to unusually rapidly rising temperatures in the North Atlantic Ocean and changes in wind patterns that now tend to send warmth to the US coast instead of the other way around. The survey appears this week in the magazine Nature Climate Change.
As a result of the changes, not only are Northeast winters getting warmer, as long predicted by climate models, but also significant and rapid summer warming. “Some of the largest population centers in the US are suffering the greatest degree of warming,” said the lead author Ambarish Karmalkar, professor of geosciences at the University of Massachusetts at Amherst. “This warming is driven by both equally rapid trends in the Atlantic Ocean and changes in atmospheric circulation patterns.”
Ocean waters around the world are warming, thus contributing to warming on land. But beyond that, several recent studies show that the Atlantic Meridional Overturning Circulation (AMOC), a system of long-range Atlantic currents, is slowing due to climate change. The AMOC is a type of conveyor belt that carries warm, salty water from the tropics north to Greenland, where it cools and sinks. This cold water then flows back south in the form of deep water currents. But as the climate warms and the glaciers in Greenland melt and send fresh water into the ocean, the conveyor stops. One consequence is even greater warming of the ocean off the northeast coast. This explains the spikes in ocean temperatures in the Mid-Atlantic Bight and Gulf of Maine regions, the authors say.
A link between the AMOC and warming along the coast is the North Atlantic Oscillation (NAO), a cyclical weather phenomenon that governs the strength and position of winds blowing from the United States over the Atlantic and into the Europe. There is a degree of natural variability in the NAO, which affects storm tracks, the jet stream, and seasonal temperatures in the Northeast. The study shows that the NAO in recent decades has tended to settle into a pattern that enhances the flow of ocean air toward the east coast. Because the Northwest Atlantic is warming in part due to the slowing of the AMOC, this is accelerating the warming of cities like Boston, Providence, RI, and New York.
While scientists already know about the unusual rate of warming in the region, computer models trying to simulate the forces at play have so far failed to reproduce the pattern. Radley Hortonclimatologist at Columbia University Lamont-Doherty Earth Observatory and co-author of the new study, says rising Atlantic temperatures and changed wind patterns are the missing links.
“Climate models remain our best tool for projecting future climate risks in the populous and exposed US coastal Northeast,” he said. “But the inability of these models to reproduce the observed summer pattern of enhanced warming of coastal lands relative to inland regions points to a potential blind spot.”
The research suggests that scientists should use a new generation of high-resolution climate models that could more accurately capture changes in regional climate, the authors say. “Our research suggests that without improved high-resolution data, regional climate estimates, which inform the ability to plan for the future, may understate warming in this densely populated region,” said Karmalkar.
