If you’re climate-curious, you’ve probably seen some of the research revealing why globally averaged surface air temperatures have warmed less quickly over the last decade or so than they did in the 1990s. The oceans are the dominant heat reservoir in the climate system, and they have been in a greedy phase lately, giving up a little less warmth to the atmosphere.

This has largely been the product of a string of La Niñas in the Pacific driven by stronger easterly trade winds. In those conditions, a pool of colder deep water takes the place of warmer surface water in the eastern Pacific. The warmer water that would normally be there is instead moved westward and mixed downward.

But here’s the puzzling thing: while records show a buildup of heat below the surface, the heat's generally not in the Pacific. If that’s where so much downward mixing is taking place, where is the warm water going?

Using one dataset (a “reanalysis” that uses a model to generate global coverage from all the available measurements), over 70 percent of the heat accumulated in the upper 700 meters of the global ocean between 2003 and 2012 ended up in the Indian Ocean. Heat content in the Pacific, on the other hand, actually dropped a bit over that time period.

To find out why, a team led by University of Miami and NOAA researcher Sang-Ki Lee experimented with a global climate model. One configuration of the model was driven by the pattern of ocean surface heat exchange (determined by things like sunlight, water temperature, and air temperature) calculated by a historical reanalysis covering the last century. The model simulated the way the rest of the climate system responded to that two-way transfer of heat energy. For comparison, a second configuration of the model was driven by constantly average surface heat exchange.

The simulations meant to mirror historical conditions did a good job of matching changes in heat content of the upper 700 meters of ocean water—including the recent trends in the Indian and Pacific Oceans. Almost none of the Indian Ocean warming came from above, though. Instead, it was warm Pacific water flowing through Indonesia—a transfer that strengthened between 2003 and 2012. The Pacific, meanwhile, absorbed a lot of additional heat energy through its surface, but gave it up to the Indian Ocean.

The stronger easterlies and La Niña conditions in the Pacific are also responsible for transferring more water into the Indian Ocean, connecting the dots here. The Pacific easterlies push warm surface water westward, mixing it down beneath the surface and bringing up cool water in the east. At the same time, winds and slight sea level differences pushed warmer water from the western Pacific into the Indian Ocean.

If this were to keep up, warmer Indian Ocean water would bleed into the Atlantic around the southern tip of Africa, adding an additional warming boost to that ocean basin, as well.

Like stolen gear bought from a pawn shop, the Indian Ocean has ended up with some of the atmosphere’s heat, slowing the surface warming trend a bit. When we start to get solid El Niño conditions again (as appears to be happening this year), the oceans will turn generous with their heat, and the Pacific will give back to the atmosphere it so recently robbed.

Nature Geoscience, 2015. DOI: 10.1038/ngeo2438 (About DOIs).