While sea ice in the Arctic has shrunk remarkably over the past few decades, sea ice around Antarctica has been dancing to the beat of a different drum. You might expect that as the world warms, sea ice would dwindle no matter which end of the planet it’s on, but the two regions are quite different.

While the North Pole sits in an ocean surrounded by land, the South Pole is in a continent surrounded by water. Antarctic sea ice grows outward from the coast, aided by the isolating winds that encircle the continent and carry frigid, inland air that pushes the ice around. So even as warmer water reaches under the floating ice shelves of Antarctica’s glaciers, persistently eating away at them, the growth of winter sea ice is more closely tied to wind patterns.

Climate models project a big decline in Arctic sea ice, with the end of summer becoming essentially sea-ice-free within a few decades at the current rate of warming. But in Antarctica, the models project smaller long-term declines.

In reality, Arctic sea ice extent has so far dropped faster than the model projections. Antarctic sea ice, however, has grown a bit since satellite monitoring started in 1979 (though not by enough to offset the Arctic loss). Between 2000 and 2014, that growth picked up speed—the same time period over which the growth in global average surface temperatures temporarily slowed due to a series of La Niña years in the Pacific.

As researchers work to understand what was controlling the behavior of the Antarctic sea ice, hypotheses have focused on changes in the winds that control ice extent. One idea, for example, was that ultraviolet radiation coming through the hole in the ozone layer above Antarctica was altering atmospheric circulation. That has since been ruled out as a large factor, but the spatial pattern of sea ice extent change in the 2000s hinted that the La Niña conditions in the Pacific might be responsible.

A group of researchers led by Gerald Meehl of the National Center for Atmospheric Research tested this hypothesis by examining spatial patterns of air pressure and surface winds. Looking back at weather data, they saw that the major low pressure center that inhabits the Amundsen Sea region deepened in the 2000s. That brings stronger winds from the continent over the Ross Sea, which is where most of the growth in sea ice extent took place. The big question is why this low pressure center, known as the Amundsen Sea Low, went even lower.

To explore this, the researchers first looked for correlations with recorded conditions elsewhere. They found that precipitation along the equator in the eastern Pacific was strongly linked to the Amundsen Sea Low—less precipitation in the Pacific means lower pressure over the Amundsen. That’s not a random correlation. The convection that causes precipitation moves heat upward into the atmosphere, and the influence of this on atmospheric circulation propagates all the way to Antarctica.

A La Niña event consists of colder surface water along the eastern equatorial Pacific, and colder water leads to less convection and precipitation. So the La Niñas of the 2000s should produce lower air pressure in the Amundsen Sea. Keeping track of the dominoes here, you can trace them from the La Niñas through enhanced southerly winds to growing sea ice in the Ross Sea.

There was also a weaker correlation with precipitation over the tropical Atlantic, with higher precipitation leading to lower air pressure in the Amundsen Sea this time. This connection had the biggest impact in March, April, and May, which happens to be the season that saw most of the Antarctic sea ice extent growth in the 1980s and '90s.

To evaluate the importance of these correlations, the researchers employed a climate model simulation run with the subdued eastern equatorial Pacific convection of the 2000s. The simulated Amundsen Sea low pressure center behaved much like the real one did over that time period. Another simulation run with stronger convection over the warm tropical Atlantic ocean also strengthened the Amundsen Sea Low in some seasons but couldn’t explain the observed behavior as well.

Running the same sorts of simulations for the 1980s and 1990s, however, showed that tropical Atlantic water would contribute to sea ice growth in the Ross Sea during the same March/April/May season that jumps out in the observed data. The eastern Pacific, on the other hand, had the opposite influence during this time period, counteracting some of that sea ice growth.

So, the researchers conclude, the variability of the tropical Atlantic was responsible for a significant portion of Antarctica’s slight sea ice growth in the 1980s and '90s. Between 2000 and 2014, the same phase of variability in the equatorial Pacific that impacted global average surface temperatures boosted Antarctic sea ice extent in a bigger way.

In fact, if you looked at climate model simulations run for the last IPCC report, you could pick out ones that happened to have the same phase of natural Pacific variability as the real world did in the 2000s. These runs also simulated increases in Antarctic sea ice extent over that time period. So while the recent growth of Antarctic sea ice is a puzzle that has taken some work to put together, it looks like we have the data we need to make sense out of it.

Nature Geoscience, 2016. DOI: 10.1038/ngeo2751 (About DOIs).