Where Do the Hurricanes Come From?

Introduction

• A tropical cyclone is a rapidly-rotating 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

– Different names: hurricanes, typhoons, cyclones– Genesis region generally in the tropics, especially between 5N-15N and 5S-

15S.– Western Pacific has the highest average number of tropical cyclones per

year

164.4

8.95.4

2.5

3.4

4.3

1. SST > 27 oC (Poleward of about 20o SST too cold for formation. Highest frequency in late summer to early autumn when water is warmest.)

2. Warm ocean mixed layer is thick enough to supply energy (this is why they weaken quickly upon landfall)

3. Unstable atmosphere with a moist lower/middle troposphere (central and western ocean basins)

4. Low vertical windshear (Otherwise upward transfer of latent heat disrupted)

5. Coriolis force (do not form between 5N-5S where Coriolis force is too weak)

6. Pre-existing low-level rotating circulations (tropical waves and other disturbances)

Necessary environmental conditions for tropical cyclone formation

Video: Hurricane Katrina, The Storm that Drowned a City

• https://www.youtube.com/watch?v=974O47UmrII

Four stages of tropical cyclone formation

Organized convection >200 km, >24 hours

Vortex w/ max wind <38 mph

Vortex w/ max wind 39-73 mph

Vortex w/ max wind >74 mph

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

• 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 hurricanes.

Tropical depression

Hurricane

Tropical storm

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.

Saffir-Simpson scale classifies hurricanes into five categories based on:

- central pressures (ended in 1990s)- maximum sustained wind speeds (used

now)

Hurricane Intensity Scale

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 troubleFrequency 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