GEOL 2503 Introduction to Oceanography
Dr. David M. Bush
Department of Geosciences
University of West Georgia
Topic 13: Hurricanes
POWERPOINT SLIDE SHOW NOTES
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Topic 13: Hurricanes
More and more people are moving to the coast, putting more and more people and
property in the hazardous coastal zone.
Garden City, South Carolina after Hurricane Hugo (1989). Hugo was noteworthy because
it hit both Puerto Rico and South Carolina at about the same strength. Two different
geologic settings. Note sand pushed by storm waves (called overwash sand) into the
community, homes that have been floated off their foundations and smashed into other
homes, and downed utility lines. Garden City had a lot of older homes, built at ground
level. Newer homes must be built elevated on pilings to make the living space higher
than flood waters.
North coast of Puerto Rico after Hurricane Hugo (1989). Overwash sand just like in South
Carolina, but palm trees are more well-adapted to high wind, and most homes are built
out of reinforced concrete, so there was a lot less damage in PR compared to SC.
Deadliest and strongest U. S. hurricanes. The 1900 killed 6,000 on Galveston Island,
Texas, probably 2,000 more in Texas, and maybe 4,000 more in Mexico.
How the deadliest U. S. hurricanes compare.
There are cycles in Atlantic hurricane activity and intensity. Major hurricanes are those
of categories 3, 4, and 5 on the Saffir-Simpson hurricane scale. This is for all storms in
the western Atlantic Ocean, including U. S., the Caribbean, Mexico and Central America.
Maximum hurricane activity for the U. S. This is just for hurricanes that strike the U. S.
Hurricanes
What is a cyclone? Note the directions of rotation in the two hemispheres. Remember
the Coriolis effect?
There are two classes of cyclones. Hurricanes and tropical storms are tropical cyclones.
Northeasters, also known as winter storms, are extratropical cyclones.
Hurricanes are simply rising air, causing a low-pressure area at the surface. Air moves in
toward the rising column of air, and the Coriolis force causes the air to be deflected to
the right in the northern hemisphere. The result is a spiraling and rising of air to create
essentially a chimney effect. The faster the air rises, the lower the surface air pressure,
and the stronger the storm.
Hurricane formation. Visit www.srh.noaa.gov/jetstream and see topic 10, tropical
weather. Take some time to explore some of the other topics, and the NOAA web site.
Recall the Intertropical convergence zone. It is the birthplace of hurricanes.
You need very warm ocean surface temperatures in order to have enough evaporation
and rising air to drive the convection that creates hurricanes. The Atlantic hurricane
season runs from June 1 through November 30, but the peak months of activity are
August, September, and October. It takes the solar input of the summer to provide
enough energy for hurricanes to form.
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Remember the El Niño story? Under normal conditions, so much cold water upwells
along the western sides of continents to keep the sea surface too cool to generate
hurricanes.
The hierarchy of tropical cyclones. The wind speeds refer to the rotating winds.
The growth of a hurricane.
The main parts of a tropical cyclone. The most intense part of a hurricane, the eye wall,
surrounds the eye.
The eye is a column of sinking air, free of clouds.
Different names in different ocean basins, but they are all the same.
The subtropical ridge is an important steering mechanism. It’s part of the system of
sinking air (high pressure) centered around 30° N.
Tropical disturbance
Tropical depression
Tropical storm
Hurricanes
The Saffir-Simpson Hurricane Scale. Surface air pressure is given in millibars (mb), a
metric unit of measurement. Standard air pressure of 14.7 psi is equal to 1013 mb. The
lower the pressure, the stronger the hurricane.
There is strong evidence that hurricane activity is cyclical.
Relatively more active and less active years in the Atlantic/Caribbean basins.
Storm surge. You hear a lot about storm surge during a hurricane. It is the deadliest part
of a hurricane, accounting for 90% of hurricane-associated deaths worldwide. In the U.
S., storm surge-related deaths are rare because of excellent evacuation and sheltering
plans and facilities. Storm “tide” is an old term for storm surge, no longer used to avoid
confusion with astronomical tides.
This diagram is a little complicated. Storm surge is water pushed onshore by the
hurricane. But there is another important factor—astronomical tides. The low and high
tides common along most coasts. The measured “high water level” (which used to be
called the “storm tide”) is equal to the storm surge plus the astronomical tides. This
diagram shows the high water level for a hurricane making landfall at the same time as a
high astronomical tide. It is important to consider that a hurricane could make landfall at
a time of low tide, in which case the high water level would be lower than that shown
here.
The water level recorded at a tide gauge station in Charleston, SC, as Hurricane Hugo
passed inland in 1980. The dashed line is what is called the “predicted” water level, in
other words, the astronomical tides. The solid line is the “observed” water level, what
really happened. Subtract the predicted from the observed and you are left with the
storm surge. The highest storm surge occurs when a hurricane makes landfall. That is
when the forces pushing water toward the land are at a maximum. Note that Hugo
made landfall near high tide. If it had made landfall six hours earlier or six hours later it
would have been low tide and the high water level would have been much lower.
The top diagram makes the point that the storm surge we have been talking about is
really a water surface without waves. In order to understand the full power of a
landfalling hurricane you must add storm waves on top of the storm surge. So when you
hear on the evening news that a hurricane created a storm surge of 10 feet, there were
storm waves on top of that 10 feet of water. That’s the real power of storm surge.
Essentially moving the surf zone inland so that waves are breaking on top of houses and
buildings and utilities, etc.
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The bottom diagram compares the wind driven surge height to the so-called pressure
surge. The pressure surge is the rising up of the sea surface simply because the
hurricane is an area of low pressure. Low pressure means less weight of atmosphere
pushing down on the sea surface compared to outside the hurricane. So in the center of
the hurricane, the sea surface mounds up a little bit. It’s not much, as indicated by the
relative sizes of the mounds of water shown. A rule of thumb is that for every 1 mb drop
in air pressure there is about a 1 cm rise in the water level. Even the strongest
hurricanes have a minimum air pressure of about 900 mb. That’s 113 mb below
standard air pressure and would account for a 1.13 meter rise in sea level. Consider that
the storm surge from such a strong hurricane might be 5-6 meters or more.
As this hurricane makes landfall, imagine the wind histories at cities A, B, and C. A sees
only winds blowing onto the land. C sees only winds blowing off the land. B sees winds
blowing along shore from A to C as the storm approaches. After the storm moves inland,
B sees winds blowing alongshore from C to A.
The right, front part (relative to looking from the eye in the direction of travel) of a
hurricane is the most destructive portion. Here you have the forward motion of the
hurricane adding to the rotating wind speed. This was the situation when Hurricane
Andrew made landfall near Miami in 1992. Andrew was one of the strongest storms to
hit the U. S.
Hurricanes not only vary in strength but also in size. Andrew was an extremely powerful
storm with wind speeds right near 155 mph, but it also was very small. Note the size of
Floyd. Floyd was not nearly as strong as Andrew, but impacted a much larger area.
Hurricane Katrina about to make landfall in 2005. Katrina was devastating in terms of
loss of life. It flooded much of New Orleans. A lot of New Orleans is actually below sea
level, so once the protective levees are broken or overtopped, there is no way for the
water to get out except by pumping. Katrina pushed water up into Lake Pontchartrain
which spilled over into New Orleans.
A satellite image of Lake Pontchartrain with the city of New Orleans just to the south. Notice
the Mississippi River winding its way through the city.
Lake Pontchartrain at normal level is still higher than some of New Orleans. At flood
level it spilled over the levee and into the city. The Mississippi River is at a higher
elevation that the city of New Orleans. New Orleans is in a very hazardous setting.
Scientists have been warning about the danger for years.
The highest elevation areas (the reds) in New Orleans are the levees which are meant to
contain the Mississippi River. When levees are broken or overtopped, disaster occurs.
The blue areas are below sea level and were flooded during Katrina.
The media makes a spectacle reporting from disasters.
Bicycling in a hurricane is not a good idea.
As I said, bicycling in a hurricane is not a good idea.
Which one is not an idiot?
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Tropical cyclone components.
GPS dropsondes
Dropsondes
Deploying dropsondes from hurricane hunter aircraft.
Hurricane hunter aircraft
Georgia has not been hit by a major hurricane since the 1890’s.
The 1898 Georgia Hurricane.
Risks to coastal Georgia
Inland flooding is a major hazard associated with hurricanes.
Tornadoes are another inland hazard from hurricanes.
Hurricane risks to Georgia
Adaptation strategies
Hurricane Tracking and Forecasting. There are many different forecast products. The
following slides show a few. Visit the National Hurricane Center web page for a lot more:
http://www.nhc.noaa.gov/
Track forecast cone
Track forecast cone
Forecasting wind speed probabilities
Measured wind field of Hurricane Katrina
Predicted storm surge