Extreme
precipitation in the tropics comes in many forms: thunderstorm
complexes, flood-inducing monsoons and wide-sweeping cyclones like the
recent Hurricane Isaac.

Global
warming is expected to intensify extreme precipitation, but the rate at
which it does so in the tropics has remained unclear. Now a Massachusetts Institute of Technology study
has given an estimate based on model simulations and observations: With
every 1 C rise in temperature, the study finds, tropical regions will
see 10% heavier rainfall extremes, with possible impacts for flooding in
populous regions.

“The
study includes some populous countries that are vulnerable to climate
change,” says Paul O’Gorman, the Victor P. Starr Career Development
Assistant Professor of Atmospheric Science at MIT, “and impacts of
changes in rainfall could be important there.”

O’Gorman
found that, compared to other regions of the world, extreme rainfall in
the tropics responds differently to climate change. “It seems rainfall
extremes in tropical regions are more sensitive to global warming,”
O’Gorman says. “We have yet to understand the mechanism for this higher
sensitivity.”

Results from the study are published online this week in the journal Nature Geoscience.

A warm rain will fall

Global
warming’s effect on rainfall in general is relatively well-understood:
As carbon dioxide and other greenhouse gases enter the atmosphere, they
increase the temperature, which in turn leads to increases in the amount
of water vapor in the atmosphere. When storm systems develop, the
increased humidity prompts heavier rain events that become more extreme
as the climate warms.

Scientists
have been developing models and simulations of Earth’s climate that can
be used to help understand the impact of global warming on extreme
rainfall around the world. For the most part, O’Gorman says, existing
models do a decent job of simulating rainfall outside the tropics—for
instance, in mid-latitude regions such as the United States and Europe.
In those regions, the models agree on the rate at which heavy rains
intensify with global warming.

However,
when it comes to precipitation in the tropics, these models, O’Gorman
says, are not in agreement with one another. The reason may come down to
resolution: Climate models simulate weather systems by dividing the
globe into a grid, with each square on the grid representing a wide
swath of ocean or land. Large weather systems that span multiple
squares, such as those that occur in the United States and Europe in
winter, are relatively easy to simulate. In contrast, smaller, more
isolated storms that occur in the tropics may be trickier to track.

An intensity of extremes

To
better understand global warming’s effect on tropical precipitation,
O’Gorman studied satellite observations of extreme rainfall between the
latitudes of 30 degrees north and 30 degrees south—just above and below
the Equator. The observations spanned the last 20 years, the extent of
the satellite record. He then compared the observations to results from
18 different climate models over a similar 20-year period.

“That’s
not long enough to get a trend in extreme rainfall, but there are
variations from year to year,” O’Gorman says. “Some years are warmer
than others, and it’s known to rain more overall in those years.”

This
year-to-year variability is mostly due to El Niño—a tropical weather
phenomenon that warms the surface of the Eastern Pacific Ocean. El Niño
causes localized warming and changes in rainfall patterns and occurs
independent of global warming.

Looking
through the climate models, which can simulate the effects of both El
Niño and global warming, O’Gorman found a pattern. Models that showed a
strong response in rainfall to El Niño also responded strongly to global
warming, and vice versa. The results, he says, suggest a link between
the response of tropical extreme rainfall to year-to-year temperature
changes and longer-term climate change.

O’Gorman
then looked at satellite observations to see what rainfall actually
occurred as a result of El Niño in the past 20 years, and found that the
observations were consistent with the models in that the most extreme
rainfall events occurred in warmer periods. Using the observations to
constrain the model results, he determined that with every 1 C rise
under global warming, the most extreme tropical rainfall would become
10% more intense—a more sensitive response than is expected for
nontropical parts of the world.

“Unfortunately,
the results of the study suggest a relatively high sensitivity of
tropical extreme rainfall to global warming,” O’Gorman says. “But they
also provide an estimate of what that sensitivity is, which should be of
practical value for planning.”

The
results of the study are in line with scientists’ current understanding
of how global warming affects rainfall, says Richard Allan, an
associate professor of climate science at the University of Reading in
England. A warming climate, he says, adds more water vapor to the
atmosphere, fueling more intense storm systems.

“However,
it is important to note that computer projections indicate that
although the rainfall increases in the wettest regions—or similarly, the
wet season—the drier parts of the tropics … will become drier still,”
Allan says. “So policymakers may have to plan for more damaging
flooding, but also less reliable rains from year to year.”

Sensitivity of tropical precipitation extremes to climate change

                           

Source: Massachusetts Institute of Technology