Washington (AFP) - An apparent slowdown in the Earth's surface warming in the last 15 years could be due to that heat being trapped in the deep Atlantic and Southern Ocean, researchers said Thursday.

The findings in the journal Science suggest that such cycles tend to last 20-35 years, and that global warming will likely pick up again once that heat returns to surface waters.

"Every week there's a new explanation of the hiatus," said co-author Ka-Kit Tung, a University of Washington professor of applied mathematics and adjunct faculty member in atmospheric sciences.

"We looked at observations in the ocean to try to find the underlying cause."

Tung and Xianyao Chen of the Ocean University of China studied deep-sea temperatures from floats that sample the water as deep as 6,500 feet (2,000 meters) depth. They found that more heat began to sink around 1999, just when the rapid warming of the 20th century began to flatline.

The movement of more heat into the water explains how surface temperatures could stay close to the same, even as mounting greenhouse gases trap more solar heat at the Earth's surface, researchers said.

They also found that contrary to earlier studies, the Pacific Ocean was not the hiding place for the heat.

"The finding is a surprise," Tung said.

"But the data are quite convincing and they show otherwise."

The change also coincided with an increase in saltier, denser water at the surface of the northern part of the Atlantic, near Iceland.

This dynamic caused changes in the speed of the huge current in the Atlantic Ocean that circulates heat throughout the planet, the study said.

"When it's heavy water on top of light water, it just plunges very fast and takes heat with it," Tung said.

"There are recurrent cycles that are salinity-driven that can store heat deep in the Atlantic and Southern oceans," Tung added.

"After 30 years of rapid warming in the warm phase, now it's time for the cool phase."

Researchers said the current slowdown may last another decade, then the rapid warming is likely to return.

The study was funded by the US National Science Foundation and the National Natural Science Foundation of China.