Review Evaluates Evidence for Intensifying Indian Ocean Water Cycle

A review paper by Janet Sprintall, an oceanographer at Scripps Institution of Oceanography at UC San Diego, and colleagues considers whether an intensification of the Indian Ocean’s water cycle is natural or a consequence of human-caused climate destabilization. 

The answer has “big implications for climate risk assessments and for the densely populated regions around the Indian Ocean that are vulnerable to the effects of climate change,” said Caroline Ummenhofer of Woods Hole Oceanographic Institution (WHOI) and lead author of the review published July 20 in the journal Nature Reviews Earth and Environment.  

The Indian Ocean has been warming much more than other ocean basins over the past half-century. While basin-wide temperature changes can be unequivocally attributed to human-induced climate change, researchers say it is difficult to assess whether recent heat and freshwater changes in the Indian Ocean represent an anthropogenically forced transformation of the hydrological cycle. That cycle governs, among other things, the annual South Asian monsoon that results in the rainfall that is intricately linked and vital to the economies of several countries in the region. 

What complicates the assessment is factoring in natural variations, regional-scale trends, a short observational record, climate model uncertainties, and the ocean basin’s complex circulation.

The team concluded that while some changes in the Indian Ocean appear to be a consistent response to anthropogenic global warming, “in general our ocean observational records are still far too short to distinguish the naturally driven variability from the man-made changes,” said Sprintall, a co-author of the study that included input from seven research institutions in the United States and Australia. “This tells us that we need to continue measuring our oceans—particularly below the surface—so that we can better understand these long-term changes and their causes, and in turn can improve our prediction and response to them.”

The paper brought together various scientific expertise, tools, and data sources to address key questions regarding climate change in the Indian Ocean, said Ummenhofer, associate scientist in the Physical Oceanography Department at WHOI. 

“The different scientific communities need to come together and have very open discussions about what we can tell from our data, how we can compare apples and oranges, and how we can bring all of this information together to have a better understanding of the entire Indian Ocean system,” she said.

Quantifying the changes in the Indian Ocean heat and freshwater balances warrants a multi-pronged approach that integrates in situ observations, remote sensing by satellites to measure rainfall and sea surface salinity, improved numerical modelling simulations, and paleoclimate research to understand how nature behaved in the distant past, the authors note.

Corals are an important paleoclimate archive because their calcium carbonate skeletons incorporate the chemical properties of past ocean environments and thus past conditions. 

“Including the long-term perspective provided by corals alongside that of observations and remote sensing data enriches our understanding of complex climate and ocean systems and improves our ability to anticipate future changes in a warming world,” said co-author Sujata Murty, a WHOI adjunct scientist and assistant professor in the Department of Atmospheric and Environmental Sciences at the University at Albany, State University of New York. 

The Indian Ocean, the paper notes, “is particularly vulnerable to anthropogenic climate change,” in part because the ocean is bounded to the north by the Asian continent. This means that heat from the Pacific Ocean that enters the Indian Ocean through the Indonesian Seas cannot easily exit the basin.

The basin “could be a kind of canary in a coal mine,” said Ummenhofer, because those changes now being observed in the Indian Ocean could also happen in other ocean basins. “We can all benefit from having better observations and a better understanding of the ocean so that we can know whether the changes are a climate change signal or part of a natural cycle.”

This research was supported by the WHOI Postdoctoral Scholar Program, the U.S. National Science Foundation, the Australian Research Council, the Andrew W. Mellon Foundation Award for Innovative Research, and the James E. and Barbara V. Moltz Fellowship for Climate- Related Research.

– Adapted from Woods Hole Oceanographic Institution