Transcript Where do the hurricanes come from?

Where Do the Hurricanes Come From?
Introduction
• A tropical cyclone is a rapidlyrotating storm system
characterized by a low-pressure
center, strong winds, and a spiral
arrangement of thunderstorms
that produce heavy rain.
• Total diameter: about 600km
• Different names in different ocean
basins: hurricanes (Atlantic),
typhoons (Western Pacific),
cyclone (Indian Ocean)
Observation of tropical cyclones:
Aircrafts (The hurricane hunters)
Dropsondes
Observation of tropical cyclones:
Satellite (1960s-Now)
Genesis and tracks of tropical cyclones
2.5
16
4.4
8.9
5.4
3.4
4.3
– Different names: hurricanes, typhoons, cyclones
– Genesis region generally in the tropics, especially between 5N-15N and 5S15S.
– Western Pacific has the highest average number of tropical cyclones per
Necessary environmental conditions for
tropical cyclone formation
1.
2.
3.
4.
5.
6.
SST > 27 oC (Poleward of about 20o SST too cold for formation. Highest
frequency in late summer to early autumn when water is warmest.)
Warm ocean mixed layer is thick enough to supply energy (this is why they
weaken quickly upon landfall)
Unstable atmosphere with a moist lower/middle troposphere (central and
western ocean basins)
Low vertical windshear (Otherwise upward transfer of latent heat disrupted)
Coriolis force (do not form between 5N-5S where Coriolis force is too weak)
Pre-existing low-level rotating circulations (tropical waves and other
disturbances)
Video: Hurricane Katrina, The Storm
that Drowned a City
• https://www.youtube.com/watch?v=974O47UmrII
Four stages of tropical cyclone formation
Vortex w/ max
wind >74 mph
Vortex w/ max
wind 39-73 mph
Vortex w/ max
wind <38 mph
Organized convection
>200 km, >24 hours
Tropical Disturbances
• Tropical disturbances are disorganized clusters of
thunderstorms with weak pressure gradients and no rotation.
• Easterly waves, or ripples in the trade wind pattern, spawn
hurricanes in the Atlantic. They are depicted by plotting
streamlines, or lines of wind direction.
• The disturbance is located upstream of the wave axis.
Tropical depressions, tropical storms
and hurricanes
Tropical
storm
• When at least one closed isobar is
present, the disturbance is
classified as a tropical
depression.
• Further intensification, to wind
speeds of 60km/hr (37mph),
categorize the storm in the as a
tropical storm.
• Hurricane status is gained when
winds reach or exceed 120km/hr
(74mph).
• A high percentage of depressions
become tropical storms and an
even higher percentage become
Tropical
depression
Hurricane
Hurricane Paths
• Depends on the stage of development (storm intensity).
• Tropical disturbances and depressions are guided by the
trade winds and move west.
• For tropical storms and hurricanes, upper-level winds and
ocean temperatures have more influence on movement.
Hurricane Paths
• Hurricane movement may be highly erratic.
Hurricane Intensity Scale
Saffir-Simpson scale classifies hurricanes into five categories
based on:
- central pressures (ended in 1990s)
- maximum sustained wind speeds (used now)
Categories 3, 4, 5 are collectively called major hurricanes.
They account for 21% of hurricane landfall in U.S., but
cause 83% of the damage.
Storm surge height
Hurricane names
• Named alphabetically
• 6 lists in rotation
• When there is a hurricane that is too devastating, the name
will retire and be replaced by a new name
Hurricane
names
retired in the
2000s
Hurricane names that spell trouble
Frequency of retired names by letters of the alphabet
Trends in Hurricane Activity
• Destructive hurricane seasons result in public awareness
and general concern that hurricane activity is related to
global warming.
• There is evidence indicating that the increase in category
4 and 5 Atlantic hurricanes is due to global warming.
AMO and Hurricane Activity
• The Atlantic Multidecadal Oscillation (AMO) is a 60yr
oscillation in water temperatures and is a major factor in
the increase in Atlantic hurricane activity.
Summary
• Tropical cyclone genesis: Western Pacific has the
highest averaged number per year
• 6 necessary conditions for tropical cyclone
genesis
• 4 stages of tropical cyclone genesis
• Tropical cyclone tracks
• Tropical cyclone intensity scale. Category 1:
74mph, category 5: 155mph
• Hurricane names: alphabetically, 6 lists in rotation
• Trends and variability in tropical cyclone activity
Works cited
• http://www.theawl.com/2012/06/hurricanes-names