Deep Argo will help unlock mystery of deep ocean effects on climate

Gregory C. Johnson, a NOAA oceanographer at the Pacific Marine Environmental Lab, and John M. Lyman, an oceanographer at NOAA’s cooperative institute with the University of Hawaii, the Joint Institute for Marine and Atmospheric Research, have written an essay published online in Nature Climate Change about how a new international program can help fill a major gap in information about the heat in the deep ocean. We caught up with Johnson to learn more.

1. You write in your essay in Nature Climate Change that there is a deep ocean measurement gap. What do you mean?

Oceanographers from many countries have worked together on a program called Argo to deploy a global array of robotic floats that measure temperature and salinity from the surface to an ocean depth of almost 1-1/4 miles (or 2 kilometers) year-round and world-wide. These data have revolutionized scientific understanding of variations in temperature, salinity, and ocean circulation in the upper half of the ocean. However, there is still only sparse sampling of the deep ocean. It’s dependent upon ship-based research expeditions that revisit about two dozen coast to coast survey lines every 10 years. This sampling shows that the deep ocean has been changing decade to decade, but limited data don’t allow scientists to track how much the temperature and salinity are changing year to year. To get this information, we need to measure the deep ocean more often and in more places than research ships can afford to visit. This is why we are developing Deep Argo.

2. How will Deep Argo work?

Deep Argo is a planned global array of autonomous floats that sample from the surface of the ocean to about 3-3/4 miles (or 6 km) depth, allowing scientists to monitor continuously changing ocean temperature and salinity in the entire open ocean except for the deepest trenches. It is a technical challenge to design floats that can withstand the high pressures in the ocean abyss, and instruments that accurately measure temperature and salinity changes. This past June 2014, colleagues at the Scripps Institution of Oceanography and NOAA deployed prototype Deep Argo floats that are currently reporting back data from deep in the ocean. Research groups from several other countries are also working to establish regional arrays of Deep Argo floats. Scientists hope to combine the regional arrays to create a global Deep Argo array. Like Argo, Deep Argo is an international effort. In the US, NOAA and university scientists are working together on Deep Argo.

3. Why is it important to measure what’s happening below 1.2 miles (two kilometers) depth?

Ocean temperature, salinity, and currents are changing even in the very deepest regions of the ocean, because of changes in the atmosphere and ice around Antarctica and in the northern North Atlantic Ocean. Near the poles, ocean waters are cooled, become dense, and sink to the bottom of the ocean. From here they spread throughout the global deep ocean. The deep Greenland Sea has been warming at a rate of around 0.1 ºC per decade over the past several decades, the same rate as the average warming of the ocean surface over that time. And since the 1990s, the deepest waters around Antarctica have been warming at about a third that rate, and freshening as well. Measuring when, where, and just how much the entire ocean is warming helps climate scientists to evaluate how well their computer models of Earth’s climate are simulating changes in response to human activity, specifically greenhouse gas emissions. 

4. How does measuring change in the temperature and salinity, or salt concentration, help us understand how and where sea levels may rise?

Ocean water expands as it warms, and as it freshens, so measuring the temperature and salinity of the ocean tells scientists how much changes in sea level are due to this effect. It helps scientists sort out how much of the changes are due to other factors. Sea level also varies because of changes in the winds, exchanges of water between the land and ocean, from melting ice and snow, as well as changes in storage of water in the ground and reservoirs. Sorting these effects out is key to predicting long-term future patterns and rates of sea level rise.

5. Once we have a global array of Deep Argo floats bringing back information about the deep ocean, what can it tell us about the world we live in?

Since most of global warming is happening in the ocean, measuring where, how much, and how fast the ocean is warming will help climate scientists project where, how much, and how fast the atmosphere will warm, and sea levels will rise in the future. As the Earth has warmed over the past century or so, more energy has been entering the climate system than has escaped back into space. Scientists estimate that over the past 4 decades, 1 percent of that energy has gone into warming and adding moisture to the atmosphere. Another 3 percent has warmed the land, and 3 percent has melted ice. The ocean has taken up a massive 93 percent of that energy, and much of that warming has been transferred through ocean mixing and currents to the subsurface ocean. Deep Argo data will allow climate scientists to test and improve their models of long-term climate change. It will also allow climate scientists to make sure their predictive models start out with an accurate estimate of the current state of the oceans, including the deep oceans, which is important for improving the accuracy of models that can predict regional patterns of climate change over the next few decades. More accurate predictions that look out to longer time frames allow scientists to provide decision-makers with better information on which to base policies, as well as adaptation strategies.

For more information, please contact Monica Allen, director of public affairs for NOAA Research at 301-734-1123 or by email at monica.allen@noaa.gov