A global network of currents transports tens of millions of cubic meters (billions of gallons) of water throughout the world’s oceans every day. This system, known as the Atlantic Meridional Overturning Circulation, or AMOC, is crucial for circulating heat on Earth and regulating weather patterns, especially in the Northern Hemisphere.
Some research has suggested that AMOC will weaken as a result of climate change and that the phenomenon may have already begun. Such a weakening would reduce the flow of water—and therefore heat—through the oceans, contributing to possibly catastrophic changes in the global climate.
“The evidence we have right now is that the Florida Current has been stable for 40 years.”
But according to a new study in Nature Communications, direct evidence of that slowdown has yet to be seen in the Florida Current, a flow of water between Florida and the Bahamas that makes up part of AMOC. The new study presents a reanalysis of data that slightly changes scientists’ understanding of the extent to which AMOC has already weakened, reducing that trend by about 40%.
“This doesn’t mean that there is no long-term decline in the AMOC,” said Denis Volkov, a physical oceanographer at the University of Miami and NOAA and lead author of the new study. “But the evidence we have right now is that the Florida Current has been stable for 40 years.”
Revisiting Voltages
The new study focuses on motion-induced voltages measured on a submarine cable. Previous research using tide gauge data showed that the Florida Current slowed throughout the 1900s, and one 2023 study, which used data from the telecommunications cable, determined the current had slowed by about 4% over the past 40 years.
The decline noticed by previous research represented a very slight slowdown that was difficult to detect given the strong variability of the current, said Jaime Palter, a physical oceanographer at the University of Rhode Island who was not involved in the new study.
Still, the studies revealing a slowdown piqued Volkov’s interest, so he and his colleagues took a second look at the cable data. They noticed two interesting qualities in the data: First, the declining trend was seen only after a 17-month data gap between October 1998 and March 2000, during which a change in the voltage data processing was implemented. Second, observational data from cruises, as well as satellite data, did not show any significant trends, Volkov said. Both factors spurred the team to consider whether there was an issue with the data processing.
Earth’s geomagnetic field changes daily, shifting in both strength and orientation as Earth’s liquid metal core churns. These changes subtly alter voltage measurements captured by the undersea cable.
After digging into the data processing protocols, the team discovered that as the cable data record lengthened, it required a correction to account for secular (long-term) changes in Earth’s geomagnetic field, especially when using the data to study long-term processes. The need for a correction became apparent only over time, which is “why long-term monitoring is so important,” Volkov said.
The newly corrected data show a remarkably stable Florida Current, Volkov said. The data “basically eliminated the trend completely.”
The study was well done, said Lisa Beal, a physical oceanographer at the University of Miami who was not involved in the new study. Beal was a coauthor on the 2023 study that identified a 4% slowdown in the Florida Current. The new study “convincingly shows that the trend in the Florida Current cable transport is actually spurious, owing to a 20-year error in the processing, which is really unfortunate,” she said.
Adding the corrected data to data collected by the RAPID array—a group of instruments in the Atlantic that have measured global currents hourly since 2004—reduced estimates of AMOC’s weakening by about 40%.
AMOC Assessments
The new paper’s correction changes only slightly the scientific consensus about the Florida Current, Palter said. “It’s a great correction that they’ve done, but it doesn’t change too much about my overall impression about the ocean circulation there,” she said. “The decline in the Florida Current was already thought to be pretty subtle—the new paper shows it’s been even steadier than we thought over the last 40 years.”
“The ocean does not give up her secrets easily.”
Even with the corrected data, RAPID array instruments still indicate that AMOC as a whole has declined. However, with just a few decades of robust data, Palter said, there’s still a lot of uncertainty about the AMOC decline. “We won’t know until we have more data collected.”
“The ocean does not give up her secrets easily,” Beal said.
Amid this uncertainty, the new study “highlights the importance of collecting and analyzing long-term measurements of the ocean,” wrote Magdalena Andres, a physical oceanographer at the Woods Hole Oceanographic Institution who was not involved in the study, in an email. “In situ observations are essential!”
Beal expects there is much more to be learned about the Florida Current. Scientists may need to look beyond bulk transport and investigate the specific characteristics of the water moving through the Straits of Florida, she said. For example, most of the flow in the Florida Current is wind driven rather than driven by AMOC. It’s possible there is a weakening of AMOC at the same time that wind-driven flow is strengthening, she said.
And despite the uncertainty, society may still want to take indications of a weakening AMOC seriously, Palter said. An AMOC slowdown is “what the models are telling us is likely to happen, and observations suggest a possibility that it’s already underway,” she said. “So let’s plan for it.”
—Grace van Deelen (@GVD__), Staff Writer
Citation: van Deelen, G. (2024), The Florida Current may be slowing down, but not by much, Eos, 105, https://doi.org/10.1029/2024EO240445. Published on 7 October 2024.
Text © 2024. AGU. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
Related
