The place where the ocean's cold deep water blends with the warmer water of the upper ocean is on the move due to climate change, new research reveals.

Tropical corals in the western Pacific Ocean revealed that the depth where warm surface water and colder, deeper water meet, known as the thermocline, is getting shallower. The new study is the first physical evidence supporting what climate modelers have been predicting as the effects of global climate change on the ocean circulation below surface waters.

"Over several decades, specifically since the mid- to late-1970s, the records show that the mean depth of the thermocline has been getting shallower," said study team member Branwen Williams, who conducted the research while a Ph.D. student at the Ohio State University in Columbus. Williams is now a postdoctoral researcher at the University of Toronto in Ontario.

The thermocline's upward shift may be due in part to a shift in a long-lived pattern of climate variability similar to the El Niño phenomenon, called the Pacific Decadal Oscillation (PDO). The thermocline shift in the 1970s coincides with a shift in the PDO from a negative phase to a positive phase, said Andrea Grottoli, a study team member also from Ohio State.

During a positive, or warm, phase, the surface waters of the west Pacific become cool and part of the eastern ocean warms.

"We think the thermocline rose when the PDO shifted," Grottoli said, "that it was a cumulative effect of both the natural variability of the PDO plus the warming global temperatures."

What happens deeper in the waters is often unknown because satellite data and physical measurements are mainly restricted to the ocean's surface.

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