Photo: Cape Eleuthera Institute
WASHINGTON, DC—New research reveals temperatures in the deep sea fluctuate more than scientists previously thought and a warming trend is now detectable at the bottom of the ocean.
In a new study in American Geophysical Union’s (AGU) journal Geophysical Research Letters, researchers analyzed a decade of hourly temperature recordings from moorings anchored at four depths in the Atlantic Ocean’s Argentine Basin off the coast of Uruguay. The depths represent a range around the average ocean depth of 3,682 meters (12,080 feet), with the shallowest at 1,360 meters (4,460 feet) and the deepest at 4,757 meters (15,600 feet).
They found all sites exhibited a warming trend of 0.02 to 0.04 degrees Celsius per decade between 2009 and 2019 – a significant warming trend in the deep sea where temperature fluctuations are typically measured in thousandths of a degree. According to the study authors, this increase is consistent with warming trends in the shallow ocean associated with anthropogenic climate change, but more research is needed to understand what is driving rising temperatures in the deep ocean.
“In years past, everybody used to assume the deep ocean was quiescent. There was no motion. There were no changes,” said Chris Meinen, an oceanographer at the NOAA Atlantic Oceanographic and Meteorological Laboratory and lead author of the new study. “But each time we go look we find that the ocean is more complex than we thought.”
Researchers today are monitoring the top 2,000 meters (6,560 feet) of the ocean more closely than ever before, in large part due to an international program called the Global Ocean Observing System. Devices called Argo floats that sink and rise in the upper ocean, bobbing along in ocean currents, provide a rich trove of continuous data on temperature and salinity.
The deep sea, however, is notoriously difficult to access and expensive to study. Scientists typically take its temperature using ships that lower an instrument to the seafloor just once every ten years. This means scientists’ understanding of the day-to-day changes in the bottom half of the ocean lag far behind their knowledge of the surface.
Mr. Meinen is part of a team at NOAA carrying out a rare long-term study at the bottom of the ocean, but until recently, the team thought the four devices they had moored at the bottom of the Argentine Basin were just collecting information on ocean currents. Then Mr. Meinen saw a study by the University of Rhode Island showcasing a feature of the device he had been completely unaware of. A temperature sensor was built into the instrument’s pressure sensor used to study currents and had been incidentally collecting temperature data for the entirety of their study. All they had to do was analyze the data they already had.
“So we went back and we calibrated all of our hourly data from these instruments and put together what is essentially a continuous 10-year-long hourly record of temperature one meter off the seafloor,” Mr. Meinen said.
The researchers found at the two shallower depths of 1,360 and 3,535 meters (4,460 feet and 11,600 feet), temperatures fluctuated roughly monthly by up to a degree Celsius. At depths below 4,500 meters (14,760 feet), temperature fluctuations were more minute, but changes followed an annual pattern, indicating seasons still have a measurable impact far below the ocean surface. The average temperature at all four locations went up over the course of the decade, but the increase of about 0.02 degrees Celsius per decade was only statistically significant at depths of over 4,500 meters.
According to the authors, these results demonstrate that scientists need to take the temperature of the deep ocean at least once a year to account for these fluctuations and pick up on meaningful long-term trends.