The ferocious winds of tropical cyclones churn up ocean water as they spin over the surface, significantly affecting the transport of heat in the ocean, a new study finds.
And as global warming heats up the ocean surface, this process could be intensified, preventing the potential shutdown of the oceanic conveyor belt. The intensification could also have strange effects on storms.
Tropical cyclones include tropical storms, typhoons and hurricanes — all spinning storms fueled by warm waters in tropical oceans.
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The main ocean circulation, called the global conveyor belt, moves warm water from the tropical oceans to the poles along the ocean's surface and cold water from the poles to the tropics along the ocean's bottom.
As the warm water travels northward, it becomes colder and denser until it sinks when it reaches the poles.
This sinking provides the "push" in what scientists call the "push-pull" mechanism of ocean circulation.
Though no one had studied it much, scientists knew that some force must be causing the "pull" that brought cold water back up to the surface in the tropics.
Like mixing cream and coffee
In tropical areas, water sits in rigid layers according to density, with the densest water at the bottom and the least dense on the top, like oil floating on water. This structure is called "stably stratified" because it is not easily mixed.
"Something is required to break through this strong stratification," said the study's lead author, Michael Huber of Purdue University.
That something turns out to be a tropical cyclone.
"When a tropical cyclone moves over the ocean, inside of the ocean, it's generating waves," Huber said.
Just as waves break on the shore, violently mixing together, these "internal waves" can break and mix together water layers of different densities.
Imagine pouring cream on top of coffee and shaking the cup — the two liquids, which have different densities, would mix together.
This internal mixing "bring[s] cold water up, and put[s] hot water down," Huber told LiveScience, thus providing the "pull" in the "push-pull" mechanism and accounting for about 15 percent of heat transport in the ocean.
His results are detailed in the May 31 issue of the journal Nature.
An intensified effect?
The result could invigorate ocean circulation and cyclones.
Here is how: By churning up cold water, tropical cyclones tend to leave a "cold wake" behind themselves that can depress ocean temperatures by as much as 8 degrees Celsius (about 14 degrees Fahrenheit) — a phenomenon that has been well documented by meteorologists, Huber said.
This colder water eventually warms to match normal temperatures for the area, but until it does so, it can stifle the intensification of secondary storms trailing behind the original storm.
Many scientists have assumed that as global warming raises sea-surface temperatures, tropical storms will become more intense, more frequent and/or longer-lived.
But Huber and his colleagues predict that any of the three scenarios above will just churn up more cold water, lessening the overall oceanic-surface temperature increase.
The cyclones will in effect be boosting oceanic circulation, the study found — a conclusion directly contrary to well-known fears that global warming could shut down the global conveyor belt.
However, as the "pull" part of the ocean circulation intensifies with stronger cyclones, it could send more warm water north and south, exacerbating global warming's effect in the polar regions.
Huber says that while the tropics may not initially see as much warming as predicted, the rise in temperature in the Arctic and Antarctic will slow down the entire conveyer belt in the long run as the deep cold water flowing back toward the equator gets hotter.
"As global warming continues to happen, after it happens for a while, eventually, the water that is being upwelled, mixed up, will actually not be so cold anymore", he said.
With less cold water to dampen the effect, tropical cyclones will have all the fuel they need from the tropical ocean.
Though this won't happen for several hundred years, Huber said, "there's no question that it will happen."
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