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Oceanography 10, T. James Noyes, El Camino College
8B-1
Hurricanes
Hurricanes are especially huge and powerful tropical storms. They have wind speeds of 75 mph
or more and cover well over 100 miles. Fueled by warm ocean water, they are enormously
destructive, flooding the nearby land and drowning people. Different parts of the world have
different names for hurricanes. In East Asia they are called “typhoons.” In the Indian Ocean,
they are called “cyclones.” (To some Australians, a hurricane is a “willy willy.”) Our word
“hurricane” comes from Hurican, a meso-american god of the wind.
How the Ocean Fuels Hurricanes
Warm water fuels hurricanes by warming the air above in two ways. The air is warmed by
contact with the surface of the ocean and – more importantly – by evaporation of warm ocean
water. The warmed air rises quickly, and lots of cooler air rushes in from the sides to replace it.
As the air comes in from the sides, it too warms 1 and rises as part of the giant convection cells
that make up hurricanes. The warmer the air is, the faster the air rises, and the faster the air rises,
the faster air from the sides moves in to replace the rising air, resulting in the strong winds of the
hurricane. (The winds are the air that moves in from the sides to replace the rising air.)
As the warm, moist air rises upward, it cools
and its water vapor condenses into clouds
and rain. Since lots of warm water
evaporates from the surface, the rising air
contains an enormous amount of water; it
eventually falls back into the ocean as rain.
LOTS of air rises,
LOTS of air comes
in from the side
= Strong Winds
Records show an indisputable increase in
hurricane activity since the 1990s (the IPCC
recognizes a correlation going back to at least the
1970s) that is probably attributable to global
warming. For example, waves created by
hurricane winds are larger than in the past,
consistent the idea that storm winds have gotten
stronger.
Research has shown that small changes in ocean
temperature can have a big impact on a storm:
a 1oC increase can raise a hurricane’s strength
by 1 category (e.g., 3 to 4) and will increase the
storm’s rainfall by about 20%.
Winds
Ocean
Hurricanes and Global Warming
While it is fair to say that global warming will
increase storms’ rainfall, it is not clear that global
warming will produce more storms. Hurricanes
are fueled by sucking heat out of the ocean, and
one big storm is more effective at removing heat
than 2 small ones. Thus, global warming may
lead to larger hurricanes each year, not more each
year. If we observe fewer (and larger) storms,
then would support the claim that global warming
is responsible, not undermine it.
1
Winds carrying lots of cool, dry air into the hurricane enhance evaporation, because there is lots of air with plenty
of “room” available in the air for more water molecules. So think of a hurricane this way: the dry winds help the
water heat the air, which makes the winds stronger, which helps the water heat the air even more, which makes the
winds even stronger, which…you get the idea. This is a positive feedback loop which helps a hurricane become
more and more powerful so long as it has a source of warm water.
Oceanography 10, T. James Noyes, El Camino College
8B-1
Oceanography 10, T. James Noyes, El Camino College
8B-2
Structure of a Hurricane
The center of a hurricane is primarily a place where air rises (except for the small “eye” at the
very center). Air outside the hurricane tries to move towards the center to replace the rising air,
but it turns away from the center because of the Coriolis effect; it cannot go directly in. Air
keeps trying to move towards the center and bending away, resulting in a slow, inward spiral 2.
As the air spirals in, it may warm, rise, cool, and sink several times. Thus, a hurricane is made
up of bands of clouds where the air is rising and gaps between them where air is sinking.
Costliest Hurricane:
Katrina
(Gulf Coast, 2005)
$100 billion+
Hurricanes and El Niño
Upper and lower levels winds change
in the Pacific Ocean during an El
Niño, and the effects ripple outward,
altering the jet stream and increasing
the wind shear (difference in the
direction and speed of upper and lower
level winds) in the subtropical
Atlantic. This wind shear tends to
break up hurricanes’ convection cells
as they are trying to form, so during El
Niño years (e.g., 2006) fewer and
weaker hurricanes hit the eastern and
southern coasts of the United States.
Hurricane Katrina (above) and hurricane structure (below).
National Oceanic and Atmospheric Administration.
2
Without the Coriolis effect, the hurricane cannot “wrap up” into its familiar circular, self-contained shape and
move as an independent unit.
Oceanography 10, T. James Noyes, El Camino College
8B-2
Oceanography 10, T. James Noyes, El Camino College
8B-3
Death and Destruction
Caused by Hurricanes
Hurricanes strike the east and
southern coasts of the United States
each summer and fall.
Hurricanes cause damage in a variety
of ways. Obviously, the strong
winds 3 can fling objects around and
tear down buildings. In addition,
hurricanes produce an enormous
amount of rain. However, the most
devastating aspect of a hurricane by
far is the hurricane’s storm surge 4.
Sea level rises beneath hurricanes,
more than 3 feet in the open ocean.
This may not seem like much, but the
strong winds of the hurricane push
the mound of water against the land,
causing more and more water to
pile up: sea level can go up 20 feet or
more! So when hurricanes hit the
coast, they flood the shoreline. (This
is how Hurricane Katrina overtopped
the levees, pouring water into New
Orleans.) In addition, huge waves
created by the strong winds can reach
much farther up the shoreline thanks
to the storm surge. These waves cause
much of the damage.
Deadliest Hurricane:
Bhola
(Ganges Delta, 1970)
300,000+ dead
(comparable to the
Indian Ocean
Tsunami of 2004)
Storm Surge and its effects. NOAA (top and bottom) and FEMA (middle).
3
Hurricanes are low pressure zones, and the strong winds blowing past buildings further reduces the air pressure via
a phenomenon called the “Bernoulli effect.” (Have you ever felt “pulled” towards a fast-moving 18-wheeler going
by as you’re walking along the side of the road? That’s the Bernoulli effect.) The air pressure inside a sealed house
can become so much greater than air pressure outside that it literally helps push the roof off the building. This is
why homeowners need to leave their windows open a little bit (just a crack) in spite of all the rain.
4
About 90% of hurricane deaths are caused by drowning.
Oceanography 10, T. James Noyes, El Camino College
8B-3
Oceanography 10, T. James Noyes, El Camino College
Why does the ocean rise beneath a
hurricane? The ocean is pushed
down strongly beneath the higher
air pressure outside the hurricane,
so the ocean rises beneath the
hurricane where the downward
push is weaker. (The water is that
is pushed down has to go
somewhere. This is somewhat like
a “see saw” or “teeter-totter”: if
one end goes down, the other must
come up.) Hurricanes are
associated with low atmospheric
pressure, because the air in a storm
system is warm, rising air. Warm air expands
(get “bigger”), so there is less air above each
location (it has spread off to the side), and
therefore less weight above (less pressure) as
well.
8B-4
Effects of Storm Surge.
National Oceanic
and Atmospheric
Administration.
Air
Molecules
Hurricanes cause much more property damage
by flooding the shoreline than with their
winds, which is why insurance companies will
not insure against flooding. You have to buy
flood protection from the national flood
insurance program run by the federal
government. (Similarly, the state government
has an earthquake insurance program in
California, because no private insurer wants to
insure against earthquake damage.) However,
Land
even though many people who buy the flood
insurance consider it too expensive, the
“I lost my property in
program typically pays out more each
Carla in ’61, in ’83 –
year to hurricane victims than it takes in
what was that, Alicia? –
and then again this time
from policies, so other people’s taxes
[Hurricane Ike, 2008].”
have to make up the difference. Many
James Clark, a resident
people don’t buy the more expensive
of Galveston, Texas
flood insurance 5, and thus can lose
everything if they are unlucky enough to be hit by a hurricane. We have similar problems with earthquakes and
earthquake insurance here in California. As we all know,
living in “paradise” is expensive, one way or the other.
Ocean
5
This division between the insurance companies paying for “wind damage” and the NFIP paying for “flood
damage” can make it very difficult to file a claim, because it is not always clear what caused the damage. The
judgments of homeowners, the NFIP, and the insurance company could all be different (and biased depending upon
what they stand to gain or lose). Moreover, the ambiguity can greatly delay payouts for claims, which may have to
be settled in court.
Oceanography 10, T. James Noyes, El Camino College
8B-4