March 13, 2014 | The pause in global warming since 2001 can be traced to a natural shift in weather patterns that causes stronger trade winds along the equator in the Pacific Ocean, new research indicates. When the patterns change, global temperatures will quickly warm, according to the researchers.

The unusually strong winds increase the ocean’s vertical circulation, sending more heat into deep water (about 125 meters, or 400 feet, below the surface), and bringing more cool water up to the surface. The cooler surface water cools the air and temporarily offsets the warming from increasing greenhouse gases, according to the new study in Nature Climate Change.

The research was led by Matthew England, deputy director of the Climate Change Research Center at the University of New South Wales, Australia. NCAR scientist Jerry Meehl was a co-author.

Global average temperatures have warmed significantly since the beginning of the twentieth century, but the warming has been uneven, with periods when average temperatures leveled off or even cooled. Earth has been in such a warming hiatus since 2001, despite measurements at the top of the atmosphere that show the planet has been retaining more heat. Scientists had previously found evidence that the so-called missing heat was going into the deep ocean. The new research goes a step further by showing the processes behind the heat’s movement.

The research team used observations and climate models to study the effects of the increased trade winds on heat transfer to the deep ocean. They found that stronger Pacific trade winds can transfer a significant amount of heat to the water. The current phase of increased winds has a cooling effect on global average temperatures of 0.11 degrees Celsius (0.2°F), they found. This offsets about 50 percent of the greenhouse gas-induced warming that would have occurred during the current warming hiatus.

“This is another piece in the puzzle that helps us understand how natural changes to our climate system temporarily counteract the warming that otherwise would be happening due to increasing greenhouse gases,” Meehl said.

When the current pattern ends, the atmosphere’s excess heat will no longer be driven into the deep ocean, Meehl said. Instead a warming trend will resume.

Matthew H. England, Shayne McGregor, Paul Spence, Gerald A. Meehl, Axel Timmermann, Wenju Cai, Alex Sen Gupta, Michael J. McPhaden, Ariaan Purich and Agus Santoso (2014), Recently intensified Pacific Ocean wind-driven circulation and the ongoing warming hiatus, Nature Climate Change, doi:10.1038/nclimate2106.

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