How more powerful Pacific cyclones may be fueling global warming

Increasingly powerful tropical cyclones in the North
Pacific Ocean may be fueling a
powerful north-flowing ocean current
, helping to boost the amount of heat it
ferries to northern latitudes. By enhancing the speed of some ocean whirlpools
called eddies, and suppressing the spin of others, the passing storms may be
accelerating the heat-carrying Kuroshio Current — and that could warm the
planet further, researchers report in the May 29 Science

Researchers have long predicted that climate change would
increase the intensity of tropical cyclones around the planet. Some
observational data, including a recent study of tropical
cyclone intensity
over the last four decades, suggest that this
supercharging of storms is already happening.

Yet tropical cyclones’ own influence on the climate isn’t
generally included in most climate simulations. The interaction of these relatively
short-lived storms with a calm, quiescent ocean has been considered
insignificant in the long-term climate picture, says Yu Zhang, a physical
oceanographer at the Ocean University of China in Qingdao.

But, in reality, the ocean is anything but quiescent, she says:
It is full of energetic eddies, large swirls of water
that spin off of large, fast-moving currents (SN: 6/9/03). These swirls,
known as mesoscale eddies, tend to persist for perhaps a few months, span 10 to
100 kilometers across and can extend more than 1,000 meters deep. That makes the
ephemeral eddies key players in mixing up and
redistributing
the ocean’s heat, salt and nutrient content (SN: 9/27/08).

“The collision of these two giant monsters — tropical
cyclones and mesoscale eddies — will probably lead to dramatic climatic impacts
that are far beyond our imagination,” Zhang says.

A third powerful player in the North Pacific is the
Kuroshio Current, a swift, broad ocean current that originates off the east
coast of the Philippines and transports warm, tropical waters northward toward
Japan, warming the climate, nourishing rich fishing grounds and allowing the
world’s northernmost coral reefs to thrive. The Kuroshio is analogous to the North
Atlantic’s Gulf Stream
, which brings warm waters and a balmy climate to
northwestern Europe (SN: 1/31/19).

The Kuroshio’s speed is primarily linked to winds.
Puzzlingly, the push of those winds has decreased in strength by more than 30
percent, on average, in the last 20 years, but the Kuroshio has not slowed as
much as would be expected, Zhang says.

She and her colleagues suspected that changes in the spin
of ocean eddies in the region — linked to the intensification of North Pacific
tropical cyclones — were helping to keep the Kuroshio speeding along. To
investigate, the researchers used a combination of satellite data and
ocean-based Argo floats, which track temperature and salinity. The team then examined
how eddies’ energy and spinning motion, called “potential vorticity,” were
changed when the swirls interacted with a passing tropical cyclone.

The storms, the team found, increased the strength of
eddies spinning counterclockwise while decreasing the strength of eddies
spinning in the opposite direction. When the eddies spin back into the current
— as they might, for example, off the coast of Taiwan — the strengthened counterclockwise
eddies, on balance, help accelerate the northward flow, the researchers say.  

phytoplankton swirl
A swirl of phytoplankton (milky blue) outlines an ocean eddy off the eastern coast of Japan, spun off by the collision of the warm Kuroshio Current with the frigid Oyashio Current. Tropical cyclones can increase or decrease the spin of some eddies, new research shows. On balance, those changes appear to be accelerating the Kuroshio’s delivery of heat to the north.N. Kuring/MODIS Ocean Color Team

Speeding up the Kuroshio could create a positive feedback
loop to global warming by delivering more heat to higher latitudes, helping to
warm them even more, the researchers say. That warming then could drive
stronger cyclones that could in turn speed up the current more in a continual
cycle.

A similar scenario, in which more powerful tropical
cyclones accelerate the Gulf Stream, is likely playing out in the North
Atlantic Ocean, Zhang adds. However, researchers are less clear about what role
climate change is playing in the intensification of North
Atlantic Ocean hurricanes
. In that region, another culprit that is supercharging
the storms is a large, natural ocean-atmospheric climate pattern known as the
Atlantic Multidecadal Oscillation. 

The authors “make a compelling case for a feedback of
tropical cyclones on global warming,” says Kerry Emanuel, an atmospheric
scientist at MIT. By strengthening the Kuroshio, this mechanism could, in
principle, “accelerate warming outside the tropics,” he says.

Takeyoshi Nagai, a physical oceanographer at Tokyo
University of Marine Science and Technology, notes that the study focuses on
how these eddies might be strengthening the Kuroshio in its more southerly portions.
But as the current winds to the north, nearer to Japan, the pattern of ocean
eddies becomes even more complex, and it’s less clear what impact storm-altered
eddies might have on this part of the current, he says. Better understanding of
that interplay would be especially useful, as that’s also the region where extra
heat could most affect climate and the fishing industry, by shifting, for
example, the mix of fish species.

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