Oceanography

1
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

2
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

3
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

4
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

5
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)

6
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

7
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

8
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

9
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 doesnt experience any typical change
  (perhaps less snowfall)

10
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 changesbut not Chicago

11
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

12
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

13
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

14
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 cant bring up cold water)
• Absence of wind shear
• Not closer than 5 latitude from Equator

15
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

16
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

17
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

18
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

19
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