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Oceanography. The study of the oceans Essential to an understanding of weather and climate Oceans play 3 important roles in weather and climate: Source of atmospheric water vapor Exchanges energy with the atmosphere Transfer heat poleward. Structure of the Ocean. Surface Zone

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oceanography
Oceanography
  • The study of the oceans
  • Essential to an understanding of weather and climate
  • Oceans play 3 important roles in weather and climate:
    • Source of atmospheric water vapor
    • Exchanges energy with the atmosphere
    • Transfer heat poleward
structure of the ocean
Structure of the Ocean
  • Surface Zone
    • Top 100 meters with a constant temperature
    • Waves and currents mix this layer
    • Only about 2% of the ocean water
  • Thermocline
    • Transition zone between top and bottom layer
    • Temperature decreases rapidly with depth
    • Water between 100 meters and 1000 meters
  • Deep Zone
    • Below 1000 meters
    • Uniform temperature of -1˚ C to 3˚ C
differences in ocean structure
Differences in Ocean Structure
  • The temperature variation with depth is a function of latitude
  • Deep zone temperatures are similar for polar, mid-latitude, and tropical areas
  • Tropical waters have the warmest surface zone temperature, polar regions have the coldest (because of amount of insolation)
  • Tropical and midlatitude regions have the steepest thermocline
sea surface temperature distributions
Sea Surface Temperature Distributions
  • Ocean currents establish the following pattern:
    • Western coasts in mid-latitudes and subtropics are bordered by cool water
    • Western coasts in tropical latitudes are bordered by warm water
    • Eastern coasts in the middle latitudes are bordered by warm water
    • Eastern coasts in polar regions are bordered by cool water
ocean currents
Ocean Currents
  • A massive, ordered pattern of water flow
  • Global-scale wind patterns initiate this flow
  • In each hemisphere, a large gyre exists in each ocean
  • Gyre – swirling ocean currents
  • Gyres rotate clockwise in the Northern Hemisphere
  • Surface water will move 45˚ to the right of the wind direction due to friction and Coriolis
  • Ekman Spiral – flow of water gets weaker with depth, and continues to turn to the right
  • The net movement of water moves at right angles to the direction of the wind (toward the right)
upwelling
Upwelling
  • Winds blowing along a coastline will result in the net flow of water away from the coast
  • Upwelling – cold water from deeper in the ocean moves up to replace water moving away from the coast
  • Typically found off the western coasts of continents because of subtropical highs
  • Deep water contains many nutrients vital to marine organisms – so abundant life is found
el ni o
El Niño
  • Periodic warming of the equatorial Pacific Ocean off the coast of South America
  • Upwelling conditions cease, resulting in much warmer surface water
  • Lasts for several months
  • Disrupts marine food chain
  • Occur periodically every 2 to 7 years
  • Triggered when trade winds weaken or reverse direction
southern oscillation
Southern Oscillation
  • High pressure is found over cold water
  • Low pressure is found over warm water
  • Normally, warmer water is by Indonesia and cold water is by South America
  • Therefore, Darwin will have lower pressure and Tahiti will have higher pressure
  • In an El Niño, the situation flips
  • Lower pressure over Tahiti and higher pressure over Darwin
  • This see-saw relationship between Darwin and Tahiti is called the Southern Oscillation
affect of weather patterns
Affect of Weather Patterns
  • Western Pacific experiences drought
  • Low pressure follows the warmer surface water, which moves eastward towards South America
  • This alters the typical path of the subtropical jet stream
  • Some areas experience predictable weather changes in an El Niño
  • Chicago doesn’t experience any typical change (perhaps less snowfall)
la ni a
La Niña
  • Cooler than normal sea surface temperatures off South America
  • The counterpart to El Niño
  • Trade winds are stronger, and upwelling of cold water increases as a result
  • Normally, but not always, follows El Niño
  • Typically last for 9-12 months
  • As with El Niño, some areas experience predictable changes…but not Chicago
other oscillations
Other Oscillations
  • Pacific Decadal Oscillation
    • Occurs over periods of several decades
    • North Pacific oscillation
    • May interact with El Niño
    • Cause unknown
  • Arctic Oscillation
    • Changes from decade to decade
    • Relationship between pressure in the Arctic and central Atlantic Ocean
    • Affects strength and direction of jet stream
    • Cause also unkown
tropical cyclones
Tropical Cyclones
  • Occur in the tropical oceans in the late summer and fall
  • Large circular swirl of clouds
  • Couple of hundred miles in diameter
  • Intense area of low pressure
  • Different from mid-latitude cyclones in that they do not possess fronts
  • Name depends on location, but no physical difference
    • Hurricane: Atlantic or Eastern Pacific
    • Typhoon: Western Pacific
    • Cyclone: Indian Ocean, Australia
structure of a tropical cyclone
Structure of a Tropical Cyclone
  • Eye – central portion of storm, lowest pressure, weak winds, relative lack of clouds
  • Eye Wall – Narrow, circular wall of thunderstorms surrounding the eye (most intense part of the hurricane – million tons of air moves through every second!)
  • Spiral Rainbands – lines of thunderstorms spiraling into the eyewall like a pinwheel
ingredients for a hurricane
Ingredients for a Hurricane
  • Preexisting area of disturbed weather
    • ITCZ
    • Easterly tropical waves
    • MCC (complex of thunderstorms)
  • Sea surface temperatures greater than 80˚ Fahrenheit
  • Warm ocean water must be at least 200 feet deep (waves can’t bring up cold water)
  • Absence of wind shear
  • Not closer than 5˚ latitude from Equator
vertical structure of a tropical cyclone
Vertical Structure of a Tropical Cyclone
  • Intense low pressure at the surface
  • Subsidence in the eye causes lack of clouds
  • High pressure is found aloft
  • High clouds will spin clockwise away from the eye in the Northern Hemisphere
  • The high pressure aloft helps to vent the hurricane
stages of development
Stages of Development
  • Tropical Disturbance – area of disorganized disturbed weather (thunderstorms)
  • Tropical Depression – area of low pressure and cyclonic rotation is noticed; storm assigned a number for a name
  • Tropical Storm – Sustained winds of 39 mph or more; storm obtains a name from the list
  • Hurricane – Sustained winds of 74 mph or more
  • “Supertyphoon” – Sustained winds > 150 mph
typical movement in its life
Typical Movement in its Life
  • Tropical cyclones typically move around the edge of the subtropical highs
  • Storms usually form in the lower latitudes and move along with the trade winds
    • Earlier storms develop in the Carribbean Sea and Gulf of Mexico (smaller bodies of water that heat up faster)
    • Cape Verde Hurricanes: Storms that form off the coast of Africa and later develop into hurricanes (Aug/Sept)
  • Recurvature – turning of a tropical cyclone to the north and northeast when they are west of the subtropical high
  • Entering the prevailing westerlies will cause the hurricane to later move off towards the east
  • All hurricanes are different, and wobbles can occur
damage from hurricanes
Damage from Hurricanes
  • Wind – hours of strong winds due to intense pressure gradient
  • Storm Surge
    • Pileup of water that occurs as the cyclone nears the shore
    • #1 killer and destroyer of property near the coast
  • Wind and storm surge damage worst on the right flank of the hurricane facing the direction of motion
  • Saffir-Simpson scale – rating of 1 to 5 of the damage to be expected from a hurricane (wind & storm surge)
  • Flooding
    • Hours of torrential rains
    • #1 killer and destroyer of property away from the coast
killing a hurricane
Killing a Hurricane
  • Simply removing the ingredients that formed a hurricane will weaken it
  • Landfall, cold water (lack of latent heat fuel)
  • Wind shear (destroys circulation)
  • La Niña years –stronger jet stream, more shear, lower number of hurricanes in the Atlantic
  • Satellites, radar, and aircraft constantly monitor the storm and the environment in order to forecast its movement and behavior
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