Changing ocean currents are pushing more and more heat into colder Southern Hemisphere waters

The oceans absorb more than 90% of all the extra heat trapped by the emissions we have produced by burning fossil fuels. This heat is enormous. It’s like detonating an atomic bomb underwater, every second of every day.

Authors


  • Moninya Roughan

    Professor of Oceanography, UNSW Sydney


  • Junde Li

    Postdoctoral research associate

The ocean is not warming everywhere at the same rate. We know that the heat is concentrated in the fast, narrow currents that flow along the eastern coasts of the world’s continents and carry warm water from the tropics to the poles.

In the southern hemisphere, these currents – known as western boundary currents – are warming faster than the global average at their southern limits, creating hotspots of warming oceans.

Until now, we didn’t know exactly why. These western boundary currents are particularly important in the southern hemisphere, which is made up of more than 80% ocean against only 60% for the northern hemisphere.

Our new research has found a critical part of the puzzle: strong easterly winds at mid-latitudes shift south, driving western boundary currents farther south and leading to faster ocean warming in those regions.

What are these currents and why are they important?

These warm water currents are like fast-flowing rivers in the oceans. They flow rapidly in a narrow band along the western side of the world’s major ocean basins, passing through the densely populated coasts of South Africa, Australia and Brazil where hundreds of millions of people live.

These currents often play a role in regulating local climates. Consider the best known of these currents, the northern hemisphere’s Gulf Stream, which has ensured for millennia that Europe is much warmer than it would otherwise be given its latitude.

In the southern hemisphere we have three main subtropical western boundary currents, the Agulhas Current in the Indian Ocean, the East Australian Current in the Pacific Ocean, and the Brazil Current in the Atlantic Ocean.

In recent decades, these currents have become hotspots for warming oceans, carrying increasing amounts of heat southward. Since 1993, the East Australian Current has been moving south at about 33 kilometers per decade, while the Brazil Current is moving south at about 46 kilometers per decade. Currents push heat and moisture into the atmosphere as they flow. In their southernmost part, the heat they carry moves the colder ocean and warms it rapidly. These areas of the ocean are warming two to three times faster than the global average.

As the currents carry more heat energy, they also generate more ocean eddies – large rotating spirals of water that break away from the main current. If you have looked closely at the way a fast flowing stream flows, you will see small eddies forming and dissolving all the time.

Why are these vortices important? Because they are how heat ends up in cold seas. As the eddies become faster and more heat-laden, they act as pathbreakers, carrying the heat farther south and eventually into the deep ocean. That’s why NASA will soon launch a new satellite to track these vortices, responsible for up to half of all heat transfer to the deep.

Our team has planned a research cruise next September aboard the RV Investigator, Australia’s research vessel, to explore the eddies beneath the trajectory of this new satellite. This will shed new light on vortex processes in the warming ocean.

How do the winds integrate?

Western boundary currents are driven by large-scale winds blowing across ocean basins.

You may have heard of the trade winds. These are the winds that traders and sailors have used for centuries to travel from east to west, taking advantage of the winds blowing consistently from the southeast across the tropics and subtropics.

Further south, the strongest winds are the prevailing westerlies, better known to sailors as the Roaring Forties. These westerly winds carry cold fronts and rain, and often drift north to dump rain over Australia.

These westerly winds can change course over time, moving north and south, in a pattern known as the southern annular mode.

At present, this belt of strong westerly winds has strengthened and moved south in what is called the positive phase of the mode. Since 1940, this climatic regime has increasingly favored this positive phase, which tends to bring drier conditions to Australia.

When we analyzed changes in the tropical trade winds over the past three decades, we found that they, too, had moved poleward by 18 km per decade since 1993.

What does that mean? Trade winds have been pushed further south as the annual southern mode increases. As they move south, they drive the western boundary currents further south.

Although these currents are carrying increasingly warm waters south from the tropics, they have not really strengthened. On the contrary, they became less stable in their southern regions as they lengthened. When the currents are pushed south, they transfer thermal energy into the cold seas through chaotic eddies mixing the warmer water with the cold. These whirlpools are not small, they are between 20 and 200 kilometers wide.

What does this mean for people and nature?

Western boundary currents have long played a key role in stabilizing our climate, carrying heat south and moderating coastal climates. As these currents become distorted and less predictable, they will alter the distribution of heat, the dissolution of gases in seawater, and the distribution of nutrients in the oceans. In turn, this will mean major changes in local weather patterns and marine ecosystems.

More intense eddies are also likely to warm our coastal oceans by bringing warm waters closer to shore.

For many people, these currents are out of sight, out of mind. They won’t stay that way. As these vital currents change, they will alter the lives and livelihoods of hundreds of millions of people who live along the coasts of South Africa, Australia and Brazil.

Moninya Roughan receives funding from the Australian Research Council

Junde Li receives funding from the Australian Research Council

/ Courtesy of The Conversation. This material from the original organization/authors may be ad hoc in nature, edited for clarity, style and length. The views and opinions expressed are those of the authors.

Comments are closed.