University of Delaware oceanographer Xiao-Hai Yan has been working to untangle questions surrounding climate change dynamics in the ocean throughout his career.

Yan, the Mary A.S. Lighthipe Professor in Marine Studies and director of UD’s Center for Remote Sensing, is internationally known for using satellites in tracking the notorious weathermaker El Niño and in developing new techniques for monitoring global climate change and coastal responses.

But understanding how ocean currents behave and what lies beneath the ocean’s surface is not a problem that can be tackled in one sitting. Rather, the work is incremental and requires looking at the problem in new ways, with new tools and techniques.

Recently, Yan and colleagues published several papers that explained how three important regions of the ocean influence our global climate system: the Western Pacific Warm Pool (WP), the subpolar North Atlantic (SPNA) and the deep ocean.

Switch and engine of global climate system

In their work, the researchers found that the Western Pacific Warm Pool (WP)— a huge water mass in the Indo-Pacific, a region that covers part of the Indian Ocean and parts of the Pacific near Indonesia, is warmer and less dense than the surrounding seas. This pool has migrated westward over the past thirty years (in agreement with other studies), and that the water’s volume has increased 14 percent since 1982.

Scientists think of the warm pool as an “engine” that is driving observable climate changes because of its capacity to collect heat due to its large size (it’s larger than the continental United States) and its ablity to move around the Pacific during an El Niño.

The WP is characterized by a permanent surface temperature greater than 28 degrees Celsius (82 degrees Fahrenheit). Because it is so warm, increases or decreases in the WP’s water temperature can have a significant effect on the global ocean, particularly on wind patterns, ocean circulation patterns and the overall size of the water mass. At the same time, additional heat from the upper layer of the ocean may sink into the deep ocean and lead to overall warming approximately 1,000 to 6,500 feet below the water’s surface.

Yan explained that during the recent global surface warming “hiatus,” observed from 1998 to 2013 and caused by heat redistribution and storage in the deep ocean, almost all of the ocean basins became warmer.

“It is difficult to say which ocean basin has the largest contribution to the global deep ocean warming, but our data shows that the Indian Ocean is important, as it has accounted for about 30 percent of global subsurface and deep ocean heat taken up during the hiatus period,” Yan said.

Temperature, Yan continued, can be considered an index of heat variability, while eddy kinetic energy (also known as turbulence energy) is an indicator of ocean currents that result from water temperature changes that cause movement, a process known as convection.

Looking at data from 1998 to 2013, the research team found the warming to be highly irregular during the “hiatus” in the sub-polar North Atlantic (SPNA). To understand this variability, Yan and colleagues studied sea surface temperature and turbulence energy related to ocean current patterns observed in the Labrador Sea. The Labrador is an arm of the North Atlantic Ocean situated between Greenland and the Labrador Peninsula that is flanked by continental shelves on all but the southeastern side.

The researchers found that during the hiatus period, turbulence energy near the west Greenland current (WGC) varied from year to year, which the researchers suspect may be driven by the North Atlantic Oscillation and Subpolar Gyre circulation change. The more eddy’s there were, the more turbulence was created in the central Labrador Sea, which increased the amount of heat sent to deep ocean, providing a cooling effect on the surface. They recently reported these findings in a paper in the Journal of Geophysical Research (Ocean).

In a separate study, Yan and colleagues used a machine learning technique known as a “random forest algorithm” to aggregate large quantities of satellite data and estimate subsurface temperature anomalies using sea surface height, temperature, salinity, and wind, along with data from Argo, the main system for monitoring ocean heat content.Remote sensing and satellite technology approaches are not effective in this case, Yan said, because electronic magnetic waves cannot penetrate the ocean’s surface.

By breaking down the temperature variations into natural rising and falling (heaving) and forced warming due to winds and ocean mixing, the researcher’s determined that the temperature changes in the western SPNA are mainly caused by surface heat being transferred from the water’s surface to the deep ocean.

“This is why we call it a ‘switch,’ because it switches the heat away from the surface into the deep ocean,” said Enhui Liao, a doctoral student in Yan’s research group.

Contributing to global understanding of climate change

As scientists continue to grapple with understanding what’s in store under a changing climate, Yan views his research team’s ongoing work as important background that may have interdisciplinary applications. A paper published in Earth’s Future, for example, was recently cited among the American Geophysical Union’s top 10 most downloaded papers in 2016-2017.

“Our work focuses on climate change and the effect of temperature changes on physical processes in the ocean, but it also could be applied to understanding biogeochemical processes such as ocean acidification and carbon cycling,” Yan said.

Since joining UD in 1990, Yan has advised over 50 graduate and undergraduate students and postdoctoral researchers who have gone on to careers in academia, government and industry, including notable posts at Princeton University; Applied Research Laboratories in University of Texas at Austin; Xiamen University and Fuzhou University in China. Additionally, Yan has been instrumental in forging academic and research ties with Xiamen University, UD's partner in the Joint Institute for Ocean and Coastal Resource Management, a dual degree program in oceanography. His career accolades include the National Science Foundation’s Presidential Faculty Fellow Award.