In early December 2023, Xiao-Hai Yan was pleased to find out that one of his students at the University of Delaware had an article accepted for publication in the scientific journal Geophysical Research Letters. Being one of the top journals in the field, Yan was immensely proud of his student, Kelsea Edwing, as it was her first paper accepted for publication. Yan had barely processed the good news when he was informed that another one of his students, Lei Huang, also had a paper accepted for publication by the very same journal. 

Having two student papers accepted for publication does not happen very often, let alone hours apart, on the same day and by the same top journal. 

“Using remote sensing and big data to study the ocean's roles in global climate change is one of strong focus areas of our University and College,” said Yan, the Mary S. Lighthipe Professor of Marine Studies in SMSP and the Director of UD’s Center for Remote Sensing. “The fact that two of our students from the same research group were first authors on papers that were accepted for publication by Geophysical Research Letters within two hours — and are now published in the same issue — is an extremely rare and impressive accomplishment.” 

Both Edwing and Huang graduated from UD in 2023. Edwing earned her master’s degree while Huang earned his doctorate. Edwing now works as a contractor for the National Oceanic and Atmospheric Administration while Huang is a post-doctoral researcher at the University of Miami. They both spoke positively about their experience working in Yan’s lab. 

“I really enjoyed working with this lab group under the supervision of Dr. Yan,” said Huang. “He gave us as much freedom as possible to choose our research topics and that gave me the chance to choose the research topic that I wanted to study.” 

Edwing echoed these sentiments and added that Yan was very encouraging. 

“Dr. Yan made sure that he was available to us whenever we needed to talk through multiple forms of communication,” said Edwing. “He always tried to get us different types of opportunities, whether that be a conference or a collaboration with other faculty and students. He was great at all the different aspects of being an advisor.” 

Marine heatwaves

Edwing’s paper looked at extremely warm ocean temperature events known as marine heatwaves (MHWs) on the east coast of the United States and the role they play in the marine carbonate system. 

By absorbing excess carbon dioxide from the atmosphere, the world’s oceans act as one of the best tools in the fight against global warming. MHWs, however, have been shown to significantly impact marine environments by inhibiting their ability to absorb excess carbon dioxide from the atmosphere. While this phenomenon has been studied in deep, open ocean waters, less is known about the effects of MHWs in coastal waters.  

Edwing focused on MHWs from 1992 to 2020 in the Mid-Atlantic Bight (MAB), a coastal region running from Massachusetts to North Carolina, and the South Atlantic Bight (SAB), which extends from Cape Hatteras, North Carolina to the Upper Florida Keys. The study found that MHWs are having a substantial impact on the coastal oceans’ ability to absorb carbon dioxide, and are, in some cases, even causing the coastal ocean to send carbon back to the atmosphere. 

The paper was published in the Geophysical Research Letters Scientific Journal and led by Edwing, who graduated from UD’s College of Earth Ocean and Environment’s School of Marine Science and Policy (SMSP). The paper arose out of work she conducted for her master’s thesis in Yan’s lab. 

Co-authors on the paper include Yan; Wei-Jun Cai, associate dean for research and the Mary A.S. Lighthipe Chair of Earth, Ocean and Environment; Zelun Wu, from Yan’s lab; Xinyu Li, from Cai’s lab at UD; and Wenfang Lu, from the School of Marine Sciences at Sun Yat-Sen University, Zhuhai, China.  

MHWs are described in the paper as prolonged, anomalously warm seawater events that can persist for days or months and can get as big as a thousand kilometers or more. Over the past two centuries, they have increased in frequency, intensity, and duration. 

Edwing said she and her co-authors sifted through enormous amounts of data on the two regions and found that MHWs impacted each regions’ ability to absorb CO2 from the atmosphere. 

“Extreme temperatures from marine heatwaves can reduce these regions’ abilities to absorb carbon from the atmosphere so much that if the temperatures are warm enough, these regions will actually release CO2 back to the atmosphere,” said Edwing. “In the context of climate change, it’s important to understand this phenomenon because we rely on these regions to take CO2 out of the atmosphere. If they’re not being as efficient as they were in the past or if they’re releasing CO2 back to the atmosphere, that is something we need to understand.” 

Research has shown that the MAB and SAB regions normally serve as net sinks of atmospheric CO2, meaning they absorb more carbon from the atmosphere than they release. The MAB, however, showed a larger impact from MHWs when it came to the absorption of CO2 than the SAB. Edwing said this mostly had to do with non-temperatures factors, such as wind, that play a bigger role in amplifying or reducing the impact of the MHWs in the SAB. 

Edwing knows of only one other paper that has explored the relationship between MHWs and CO2 flux, and that paper focused on the Pacific open Ocean. Without any published research like this at this point on the east coast, the paper is foundational work, providing a substantial link between MHWs and coastal air-sea CO2 flux. 

“There are a lot of questions that arise from our conclusions, especially the non-temperature factors that need to be considered,” said Edwing. “There’s a lot of future work that can go into this area. I was trying to get the ball rolling and say, ‘Hey, there’s this very interesting thing that is happening, especially in this coastal region.’ It’s the cusp of a new direction to study.”

Southwest Indian Ocean

From the 1960s through the early 2000s, sea levels in the Southwest Indian Ocean fell. But in the last 20 years, the Indian Ocean has seen a rapid rise in sea level, faster than the global average, and has experienced the strongest warming among the global oceans. 

Specifically, the tropical Southwest portion of the Indian Ocean has experienced the strongest sea level rise of any other area of the Indian Ocean. Because of this, Huang was interested in seeing if he could determine what physical mechanisms were driving the warming and sea level rise in this portion of the Indian Ocean. 

While previous studies have shown the effect of thermal expansion in the upper 300 meters of water, Huang’s study showed that there have been larger contributions coming from the deeper ocean, characterized as anything between 300 meters to 2,000 meters, over the past two decades. 

Huang served as the lead author on the paper and co-authors included Yan; Wei Zhuang, from Xiamen University; Wenfang Lu, from Sun Yat-sen University; Deanna Edwing, from Yan’s lab; and Yang Zhang, from Xinfeng Liang’s lab.  

Using monthly satellite data from 1993 to 2021 from the Archiving, Validation and Interpretation of Satellite Oceanographic (AVISO) platform, as well as data from the Argo program — which has been providing temperature and salinity observations in the upper 2000 meters of the ocean since the early 2000s, and data from Gravity Recovery and Climate Experiment (GRACE) satellites, Huang and the research team had a trove of data to pull from to examine the physical cause of warming and sea level rise in the Tropical Southwest Indian Ocean. 

They discovered that there has been a significant contribution from the ocean volume increase and deeper ocean warming — specifically, anomalous water mass spreading from the Southern Ocean over the past two decades. 

With the deeper ocean contributing more to sea level rise and temperature changes in the Southwestern Indian Ocean than previously thought, this study points to the need for more observations for the abyssal ocean, beneath 2000 meters. 

“This study found the critical contribution in the layer that is 300-2,000 meters but we still don’t have enough data below these layers, below 2,000 meters,” said Huang. “We found a stronger warming in this deeper ocean, but we don’t know what changed below 2,000 meters. There is a need for observations and direct assessment for the deep ocean beneath 2,000 meters. Our knowledge for this part is less than the upper ocean because of the relatively sparse observations.”

Since joining UD in 1990, Yan has advised over 100 graduate and undergraduate students and postdoctoral researchers who have gone on to careers in academia, government, and industry. He has amassed an impressive list of accomplishments including the National Science Foundation’s Presidential Faculty Fellow Award, and the University of Delaware's Outstanding Doctoral Mentoring and Advising award.