The global ocean has been warming up as a consequence of anthropogenic, or human-made, climate change, with 90% of the anthropogenic warming stored in the world’s ocean, leading to an increase in ocean heat and sea level rise.

From 2003 to 2012, however, the surface temperature of the Earth experienced what is known as a global surface warming slowdown. Instead of heating up rapidly, as would be expected as a consequence of global warming, the global surface temperatures instead stayed flat.

Rather than this being proof that global warming isn’t happening, University of Delaware Professor Xiao-Hai Yan showed in previous research that the heat wasn’t simply disappearing and the ocean wasn’t magically cooling. Instead, the heat was being redistributed, specifically, to deeper layers of the ocean and especially the oceans in the Southern Hemisphere.

Unlike other areas of the ocean, during the global surface warming slowdown, the Southern Hemisphere oceans, especially the Southern Ocean, experienced rapid warming as most of the global ocean heat storage occurred in the Southern Hemisphere.

A new paper from UD doctoral student Lina Wang and her collaborators from around the world shows that now that the slowdown period has ended, the rapid warming that occurred in the Southern Hemisphere oceans from 2003 to 2012 has slowed down and even changed to cooling in certain regions — specifically the Southeast Indian Ocean and South Pacific Ocean.

The research was recently published in the Geophysical Research Letters scientific journal.

This shift can be attributed to changing winds which caused the downwelling of warmer waters to the deep ocean during the slowdown period. Now that the winds have changed their course, the cooler subsurface water is once again being pumped up.

“Lina and her collaborators found that the deep part of the Southern Ocean is no longer warming. It shows a cooling,” said Yan, the Mary A.S. Lighthipe Professor in Marine Studies and Wang’s advisor. “This might indicate that no more heat is going to the Southern Ocean as it did in the slowdown period. It indicates the acceleration of global warming is happening and may last for quite a long time. It’s coming back to how it was before the slowdown period.”

The data used in the study was gathered from floats of the international Argo program, which since 2006, has been providing continuous real-time ocean observations in the upper 2,000 meters of the ocean from a near-global array of autonomous profiling floats.

This data helped the researchers determine that the Southern Hemisphere oceans experienced warming during the slowdown period because of the strengthening of westerly winds, which helped transport warm water into the deeper layers of the ocean.

“Because of the changing wind, less warm water is going into the deep ocean compared to the previous period so it now shows a cooling,” said Wang, a doctoral student who is completing her dual degree program with UD’s College of Earth, Ocean and Environment and Xiamen University in China. “If you think about it, if less heat is added into the deeper layer of the ocean, the water temperature becomes cooler. When the ocean doesn't receive more warm water, we can say that compared to before, it becomes cooler.”