Robot data shows steady increase in deep ocean warming

A new study published on September 19 in Geophysical Research Letters shows that using data collected by deep-ocean robots called Deep Argo floats, combined with historical data from research vessels, has increased confidence that parts of the global deep ocean are warming at a rate of 0.0036 to 0.0072°F (0.002 to 0.004°C) each year.

“Ocean warming is the dominant driver of global warming and one of the major drivers of climate change,” said Greg Johnson, an oceanographer at NOAA’s Pacific Marine Environmental Lab and lead author of the study.

“This study confirms previously reported deep-ocean warming and reduces uncertainties in global ocean heat uptake in waters less than 2,000 meters deep, a key area of ​​the ocean for predicting sea-level rise and extreme weather events.”

The new research also provides more detailed information about the geographic patterns of deep ocean warming, which can help scientists better understand changes in the global ocean conveyor belt, called the Global Southern Overturning Circulation, also key to predicting weather and climate change.

Research shows that the deepest ocean waters off Antarctica are a warming hotspot. These bottom waters carry the warming northward along the ocean conveyor belt. Another warming hotspot is the deep ocean waters off Greenland, which no longer receive large amounts of cold water descending from the ocean surface because of increased atmospheric warming and the cooling of these surface waters by melting ice.

More detailed information about deep ocean warming can help improve climate models used to prepare society for future changes in ocean and air temperatures that lead to rising sea levels, precipitation, the frequency and intensity of tropical cyclones, and their impacts on humans and the environment.

Scientists began observing this warming trend in the deep ocean off Antarctica about 20 years ago without the help of Deep Argo data, Johnson said. But the magnitude of the warming trend was quite uncertain because of the few measurements available before. The new Deep Argo data have helped reduce the uncertainty about the magnitude of the trend by a factor of two.

NOAA’s Argo partners first launched Deep Argo floats capable of measuring ocean temperature, salinity and other data to a depth of 3.7 miles (6,000 meters) in 2014 in the Southwest Pacific off New Zealand.

Since then, Deep Argo float arrays designed to measure changes in the ocean depths have been expanded in the Southwest Pacific and added to the South Atlantic off Brazil and Argentina, the southern Indian Ocean between Australia and Antarctica, and the North Atlantic between Florida and North Africa.

“Right now, Deep Argo consists of several pilot groups in key regions,” Johnson said. “If we can build a global group, we’ll be able to quantify the rate of warming over shorter time periods to see how that rate is changing.”

“We have some evidence that this rate is changing, but we need to be able to analyze it better. Measuring how temperature and salinity patterns are changing in the deep ocean will also help predict climate change decades in advance.”

Deep Argo is part of the larger Argo program, supported by NOAA’s Global Ocean Monitoring and Observation Program. Since its inception in 1999, the Argo program has revolutionized our ability to track changes in the ocean through a global suite of autonomous profiling floats, providing nearly four times more information about the ocean than all other observing tools combined.