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Oceanography - PowerPoint PPT Presentation

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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 l.jpg

  • 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

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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

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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

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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

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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)

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  • 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

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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

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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

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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)

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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