We May Finally Know Why Oceans in The Southern Hemisphere Are Getting So Warm

The oceans absorb more heat than any other body of water.

The heat is huge. It's like we exploded an atom bomb in the water.

The ocean is not warming at the same rate as elsewhere. The heat is concentrated in the fast, narrow currents that flow along the east coast of the world's continents.

The western boundary currents in the Southern Hemisphere are warming more quickly than the global average.

We haven't figured out why yet. In the Southern Hemisphere, the western boundary currents are more important than the northern ones.

Strong easterly winds in the mid-latitudes are moving south, driving the western boundary currents further south and speeding up ocean warming in these areas.

What are these currents and why do they matter?

The streams of warm water are similar to rivers in the ocean. They flow quickly in a narrow band along the western side of the world's major ocean basins, passing densely populated coastlines in South Africa, Australia, and Brazil.

Local climates are regulated by these currents. Europe is much warmer than it would be if it weren't for the Gulf Stream, a current that has existed for thousands of years.

There are three major sub-tropical western boundary currents in the Southern Hemisphere.

The currents have become a hot spot for ocean warming. The East Australian Current has moved at a rate of 33 kilometers per decade, while the Brazil Current has moved at a rate of 46 kilometers per decade.

The currents send heat and humidity into the air. They warm the ocean quickly in their southernmost reaches. The areas of the ocean are warmer than the global average.

The ocean eddies are generated when the main current carries more heat energy. You can see small eddies dissolving all the time if you look closely at the way a fast-flowing stream flows.

Why are these eddies important? They are the way heat ends up in the cold sea.

As the eddies get faster and more loaded with heat, they act as path-breakers and carry heat to the deep ocean.

Up to half of all heat transfer to the deep can be tracked by a new satellite that will be launched by NASA.

The RV Investigator, Australia's research vessel, will be the site of a research cruise in September next year.

This will show us more about the eddy processes in the ocean.

How do the winds fit in?

Large-scale winds blowing across ocean basins drive the western boundary current.

Have you heard of the trade winds? The winds traders and mariners used for centuries to go from east to west took advantage of winds blowing from the southeast across the tropics and subtropics.

The strongest winds are known as the westerlies. The westerly winds carry cold fronts and rain and often stray north to dump it over Australia.

Depending on the pattern of the Southern Annular Mode, these westerlies can change track over time.

In what's known as the mode's positive phase, this belt of strong westerly winds has strengthened. This positive phase tends to bring dry conditions to Australia.

Since 1993, the tropical trade winds have moved 18 km per decade.

This means what? The trade winds have been pushed further south. The western boundary currents are driven further south as they move south.

The currents are carrying warm water from the tropics but have not become stronger. They've become less stable in the south.

When the currents push south, they transfer heat into the cold seas through chaotic eddies. The eddies are between 20 and 200 kilometers in width.

What does this mean for people and nature?

Western boundary currents carry heat to the south and moderate coastal climates to the north.

As the currents warp and become less predictable, they will change how heat is distributed, how gases are dissolved in the water, and how the ocean's resources are spread.

Major changes will be made to local weather patterns.

Warming our coastal oceans is likely to be accomplished by moving warm waters closer to shore.

Many people don't pay attention to these currents. They're not going to stay that way.

Hundreds of millions of people living along the coast of South Africa, Australia, and Brazil will be affected by the changing currents.

Junde Li and Moninya Roughan are professors at the University of New South Wales.

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