Human-caused climate change is slowing the Atlantic Meridional Overturning Circulation (AMOC), a critical ocean current system that acts as a planetary conveyor belt. A study published in Nature Communications, utilizing decades of NASA atmospheric data and climate simulations, projects that this slowdown will shift global weather patterns, increasing the frequency of intense atmospheric rivers along the North American West Coast while reducing them in the Arctic and Greenland.
The Mechanics of a Stalling Ocean Current
The AMOC is a massive system of currents that transports warm water from the tropics to Europe, then returns cooler water south along the seafloor. As greenhouse gas emissions warm the planet, this circulation is losing momentum. Mohima Mimi, a climate dynamics researcher at the University of California, Riverside, and lead author of the study, notes that while the slowdown itself is well-documented, the cascading effects on atmospheric moisture remained unclear until now.
The research reveals that the AMOC does not just influence ocean temperatures; it dictates the behavior of atmospheric rivers (ARs). These are long, narrow regions in the atmosphere that transport concentrated water vapor. Some of these systems carry as much as 15 times the volume of water flowing through the mouth of the Mississippi River.
Did you know? Atmospheric rivers supply up to 50 percent of annual rainfall in the western US. While they are vital for water supply, they are also the primary drivers of flood risk in the region.
Regional Impacts: From California to Antarctica
The study projects a significant redistribution of moisture across the globe as the AMOC weakens. North America’s West Coast, stretching from Baja California to Alaska, faces a projected increase in the frequency and intensity of atmospheric rivers.
Conversely, the Arctic, Greenland, and parts of northern Asia may see a decrease in AR frequency. This occurs because a slower AMOC leads to cooler surface air temperatures in the Northern Hemisphere, which in turn reduces the atmosphere’s capacity to hold moisture.
The consequences extend to the Southern Hemisphere as well. According to the research team, atmospheric rivers play a major role in the stability of polar ice, accounting for 40 to 80 percent of summer meltwater on West Antarctic ice shelves. As the high-altitude westerly jet stream shifts toward the poles, these moisture-laden systems are expected to penetrate higher latitudes, potentially accelerating global sea level rise.
Global Redistribution of Storms
The researchers forecast that the global mean frequency of atmospheric rivers could increase by approximately 50 percent. This is driven by both thermodynamic changes and dynamic shifts in atmospheric circulation caused by the AMOC’s decline.
Areas expected to experience more frequent, rain-heavy atmospheric rivers include:
- The east coast of South America
- Southern Asia
- Western Europe
- Parts of the Pacific
- Regions surrounding Antarctica
While the outlook for many regions involves increased flood risk, the study suggests that the “double-edged sword” of atmospheric rivers offers potential for adaptation. In places like California, restoring natural landscapes to capture more water could help mitigate the impact of persistent, climate-driven droughts.
Frequently Asked Questions
What is the AMOC and why does it matter?
The AMOC is an Atlantic Ocean current system that moves warm water north and cold water south. It is a fundamental component of the global climate, and its slowing could significantly alter weather patterns worldwide.
How does the AMOC affect California’s water supply?
The AMOC influences atmospheric rivers, which provide up to 50 percent of annual rainfall in the western US, especially California. A weakening AMOC is projected to increase the frequency of these rivers along the California coast, creating both greater water availability and higher flood risks.
Will atmospheric rivers increase everywhere?
No. While the study projects an increase in AR frequency for regions like North America’s West Coast and South Asia, other areas, such as the Arctic, Greenland, and northern Australia, may see a decrease in these moisture-carrying systems.
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