Major wind-driven ocean currents are shifting toward the poles
Enhanced warming subtropical ocean is expanding the tropics

Date:
September 25, 2020
Source:
Alfred Wegener Institute, Helmholtz Centre for Polar and Marine
Research
Summary:
The severe droughts in the USA and Australia are the first sign
that the tropics, and their warm temperatures, are apparently
expanding in the wake of climate change.



FULL STORY ==========================================================================
The severe droughts in the USA and Australia are the first sign that
the tropics, and their warm temperatures, are apparently expanding in
the wake of climate change. But until now, scientists have been unable
to conclusively explain the reasons for this, because they were mostly
focusing on atmospheric processes. Now, experts at the AWI have solved the puzzle: the alarming expansion of the tropics is not caused by processes
in the atmosphere, but quite simply by warming subtropical ocean.


========================================================================== Forest fires in Australia and California, droughts and water shortages
in the Mediterranean -- in the last few years, events such as these
have become more frequent. Researchers attribute this to the fact
that the tropics, the warm region surrounding the Equator, appear to
be expanding. And that leads to the affected areas becoming hotter and
drier. According to the official definition, the tropics extend across
the Equator between the latitudes of 23 degrees North and 23 degrees
South. The central area is humid, with a great deal of precipitation,
while the marginal regions in the north and south are hot and dry. As
a result of climate change, however, for some time now the dry regions
have been expanding northwards in the Northern Hemisphere -- as far as
Southern California -- and southwards in the Southern Hemisphere.

But up to now, climate researchers have had a problem. They couldn't conclusively explain this obvious expansion of the tropics using their
climate models. The models simply didn't show the magnitude and the
regional characteristic of the observed expansion. A team working with
the physicists Hu Yang and Gerrit Lohmann at the Alfred Wegner Institute, Helmholtz Centre for Polar and Marine Research in Bremerhaven (AWI)
has now discovered the likely cause. As the AWI experts report in the
Journal of Geophysical Research Atmospheres, the reason for the expansion appears to be an altered warming of the ocean. To date, experts assumed
that processes in the atmosphere played a major role -- for instance a
change in the ozone concentration or the aerosols.

It was also thought possible that the natural climate fluctuations
that occur every few decades were responsible for the expansion of the
tropics. For many years researchers had been looking in the wrong place,
so to speak.

"Our simulations show that an enhanced warming over the subtropical ocean
in both the Northern and Southern Hemispheres are the main drivers," says
Hu Yang, the study's lead author. These subtropical warming patterns
are generated by the dynamic of subtropical ocean gyres, measuring
several hundreds of kilometres in diameter, which rotate slowly. These
currents are especially well-known in the Pacific, because the majority of floating marine litter is concentrated in them. "Because the currents in
the region bring together the surface warming water masses particularly intensely, it's easier for the subtropical ocean surface to accumulate
warmth than in other regions -- and the same applies to plastic,"
says Lohmann. As a result of this warming of the subtropical ocean, the tropical warm ocean regions are expanding. According to his calculations,
this phenomenon is the catalyst for the tropics expanding to the north
and south. "Previous researchers had been taking an overly complicated
approach to the problem, and assumed it was due to complex changes in
the atmosphere. In reality, it's due to a relatively simple mechanism
involving ocean currents." What led the experts to explore this avenue:
data on ocean gyres that they happened to come across five years ago --
data on ocean temperatures and satellite-based data, freely available on databases. Both sources indicated that the gyres were becoming warmer
and more powerful. "That's what led us to believe that they might be a
decisive factor in the expansion of the tropics," explains Hu Yang.

The AWI experts were right: their findings perfectly correspond to actual observations and the latest field data on tropical expansion. Just like
in reality, their climate model shows that the tropics are now stretching farther to the north and south alike. In the Southern Hemisphere, the
effect is even more pronounced, because the ocean takes up more of the
overall area there than in the Northern Hemisphere.

Yet when it comes to the question of whether the droughts in Australia, California and the Mediterranean are due to the expansion of the tropics, Gerrit Lohmann can't give a definitive answer. "When talking about climate change, it's always difficult to quantify the respective parameters with absolute certainty," he says. "However, we can safely assume that the
ocean currents and expansion of the tropics make droughts and hurricanes
more likely to occur."

========================================================================== Story Source: Materials provided by Alfred_Wegener_Institute,_Helmholtz_Centre_for_Polar_and
Marine_Research. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Hu Yang, Gerrit Lohmann, Jian Lu, Evan J. Gowan, Xiaoxu Shi,
Jiping Liu,
Qiang Wang. Tropical Expansion Driven by Poleward Advancing
Midlatitude Meridional Temperature Gradients. Journal of Geophysical
Research: Atmospheres, 2020; 125 (16) DOI: 10.1029/2020JD033158 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200925113344.htm

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