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Subject: G8) Why do hurricanes hit the East coast of the U.S., but never
the West coast?
Contributed by Chris Landsea
Hurricanes form both in the Atlantic basin (i.e. the Atlantic
Ocean, Gulf of Mexico and Caribbean Sea) to the east of the
continental U.S. and in the Northeast Pacific basin to the
west of the U.S. However, the ones in the Northeast Pacific
almost never hit the U.S., while the ones in the Atlantic basin
strike the U.S. mainland just less than twice a year on average.
There are two main reasons. The first is that
hurricanes tend to move toward the west-northwest after they form
in the tropical and subtropical latitudes. In the Atlantic,
such a motion often brings the hurricane into the vicinity of the
U.S. east coast. In the Northeast Pacific, a west-northwest track
takes those hurricanes farther off-shore, well away from the U.S.
west coast. In addition to the general track, a second factor is
the difference in water temperatures along the U.S. east and west
coasts. Along the U.S. east coast, the Gulf Stream provides a source
of warm (> 80°F or 26.5°C) waters to help maintain the hurricane.
However, along the U.S. west coast, the ocean temperatures rarely get
above the lower 70s, even in the midst of summer. Such relatively cool
temperatures are not energetic enough to sustain a hurricane's strength.
So for the occasional Northeast Pacific hurricane that does track
back toward the U.S. west coast, the cooler waters can quickly
reduce the strength of the storm.
Recently Chenoweth and Landsea (2004),
re-discovered that a hurricane struck San Diego, California on October
2, 1858. Unprecedented damage was done in the city and was described as
the severest gale ever felt to that date nor has it been matched or
exceeded in severity since. The hurricane force winds at San Diego are t
he first and only documented instance of winds of this strength
from a tropical cyclone in the recorded history of the state. While
climate records are incomplete, 1858 may have been an El Niño year,
which would have allowed the hurricane to maintain intensity as it moved
north along warmer than usual waters. Today if a Category 1 hurricane
made a direct landfall in either San Diego or Los Angeles, damage from
such a storm would likely be on the order of a few to several hundred
million dollars. The re-discovery of this storm is relevant to climate
change issues and the insurance/emergency management communities
risk assessment of rare and extreme events in the region.
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