The oceans aren't level. Over the span of decades, atmospheric weather patterns push water the water around, causing sea levels in connected basins to rise and fall somewhat predictably. However, since 2000 the two huge basins in the southern hemisphere have broken their trend, rising jointly over 2 millimeters per year.

A new paper in Geophysical Research Letters proposes that this new development, found using satellite-derived sea height data, could be due to changes in a large climate system that wraps around the entire southern hemisphere. The map above shows the different oceanic basins (which aren't always separated by continental bodies). The Indian ocean and southern Atlantic form the largest such connected system, and historically a rise in one would result in the fall of another.

In the bottom graph, the south Atlantic and Indian Ocean break their pattern of rising and falling. Philip R. Thompson andMark A. Merrifield

The graph to the left shows the relationships between sea level height for four pairs of connected ocean basins. In the top three systems, the thin lines show how historically, when one basin rises, the other falls, and vice versa. However, in the bottommost graph, showing the south Atlantic and Indian oceans, these trends break in the late 1990s, and both oceans rise in tandem.

The thick lines at the bottom of each graph show how a different weather pattern dominates how the water is redistributed between connected basins. Changes in the east to west wind pattern seem to be driving the difference in Indian/southern Atlantic system.

The scientists have several hypotheses for what's driving the shift in wind patterns, all of which are linked to climate change.