Lec 7: Ocean forces & Atmosphere

• Three dominant forces in the ocean

  • Friction
  • Gravity
  • Coriolis
• Friction

  Due to relative motion between two fluid parcels; wind stress, lateral and bottom friction, friction between layers

  
• Gravity


  Gives rise to pressure gradients, buoyancy, and tides
• Coriolis

  Results from motion in a rotating coordinate system
• Pressure

  • Depends on the weight of overlying air/water
  • The thicker the column of air/water, the higher the pressure
  • Pressure decreases with height
• High pressure centers


  Associated with cold, dense air
• Low pressure centers

  Associated with warm, less dense air
• Pressure Gradient


  Given a difference in pressure between locations, the air flows from HIGH to LOW pressure

  
• Coriolis Effect


  • Changes the intended path of a moving body in a rotating system
  • Causes objects moving on the Earth to follow curved paths due to Earth's rotation to the East
  • Objects actually travel in straight lines but appear to follow curved paths as the Earth rotates underneath them

  
• Coriolis Effect


  • Increases towards the pole (zero at the Equator)
  • Increases with velocity
• Coriolis Effect

  1. If a parcel moves in the meridional (north-south) direction, it is changing its distance to the axis of rotation

  2. Angular momentum would change BUT angular momentum has to be conserved
• Air travelling northwards in the northern hemisphere

  Experiences an eastward acceleration
• Air travelling southwards in the northern hemisphere

  Experiences a westward acceleration
• Air travelling northwards in the southern hemisphere

  Experiences a westward acceleration
• Air travelling southwards in the southern hemisphere

  Experiences an eastward acceleration
• Eastward motion in the northern hemisphere


  Leads to a southward acceleration
• Westward motion in the northern hemisphere


  Leads to a northward acceleration
• Eastward motion in the southern hemisphere


  Leads to a northward acceleration
• Westward motion in the southern hemisphere


  Leads to a southward acceleration
• A parcel in the NH moving northward

  Decreasing angular momentum, to conserve angular momentum the angular velocity must increase, to increase the angular velocity an eastward acceleration must occur
• Coriolis Effect with Latitude

  • As the Earth rotates on its axis the velocity decreases with latitude
  • This change in velocity with latitude is the true cause of the Coriolis effect
  • 1600 km/h at the equator
  • 0 km/h at the poles
• Projectile 1: Launched from North Pole towards New Orleans, Louisiana


  • Flies in straight line, but lands west of target due to Earth's rotation
  • From North Pole perspective, path appears to curve to the right
• Projectile 2: Launched towards New Orleans from the Galapagos Island (directly south, at the equator)


  • Lands 200 km east of New Orleans (offshore of Alabama) due to difference in eastward velocity between Galapagos and New Orleans
  • From Galapagos perspective, projectile appears to curve to the right
• Coriolis effect is caused by the difference in velocity of different latitudes on Earth
• Maximum Coriolis effect at the poles
• There is no Coriolis effect at the equator
• If an object moves eastward, the same as Earth's rotation


  It is travelling faster than the earth's surface, so it must move to larger radius of rotation to conserve angular momentum (i.e. move toward equator)
• If an object moves westward, opposing Earth's rotation

  It is travelling slower than the Earth surface, so it must move to smaller radius of rotation to conserve angular momentum (i.e. move toward poles)
• The atmosphere and ocean function as one linked system
• Solar radiation heats the surface of the Earth, creating atmospheric winds that drive most the surface currents and waves in the ocean
• The atmosphere and ocean use the high heat capacity of water to constantly exchange energy, shaping Earth's global weather patterns
• Dry air consists almost entirely of nitrogen (N2) and oxygen (O2) with other gases like argon, carbon dioxide, and water vapour (H20(v)) in trace amounts
• As you go higher in the atmosphere

  It is colder
• Troposphere

  Lowest layer of atmosphere, extends from surface to about 12 km (7 miles) up, where all weather occurs, temperature decreases with altitude
• At high latitudes air temperature is below freezing
• Density of air

  Temperature has a big effect, warm air is less dense so it rises, cold air is more dense so it sinks
• Radiator uses convection to heat a room: Heater warms nearby air causing it to expand and rise, cold window cools nearby air causing it to contract and sink, forming a convection cell
• Pressure Gradient


  Given a difference in pressure between locations, pressure exerts a force from HIGH to LOW pressure, air always moves down the pressure gradient

  
• Atmospheric water vapour


  There is a maximum amount that can be held, increasing with temperature, warm air is typically moist, cool air is typically dry, the maximum amount of water that can be held as vapour is the saturation vapour pressure
• As water evaporates, water molecules which are lighter than air molecules are added to the atmosphere, making moist air less dense than dry air