Chapter 8

 

  • Oceanography is the study of the oceans and is vital in understanding weather and climate as well.
  • Oceans play 3 important roles in determining weather and climate:
    • They are the source of atmospheric water vapor
    • They exchange energy with the atmosphere
    • They transfer heat poleward through ocean currents
  • The oceans provide the majority of water needed to form precipitation.
  • On average, the ocean gains energy during the summer and loses it in winter, and the maximum exchanges of energy occur in the Northern Hemisphere winter to the east of North America and Asia where warm ocean currents flow poleward.
  • In maintaining the Earth’s radiation balance, around 30° latitude, the ocean and atmosphere each transport about the same amount of heat, whereas Equatorward of 30°, the ocean transfers the majority of heat required to maintain balance.
  • The oceans and atmosphere work together to maintain balance, through winds, currents, and atmospheric conditions.
  • The ocean can be classified into three different vertical zones based on temperature:
    • Surface Zone-uniform or mixed layer, constant temperature due to wind mixing, around 2% of world’s ocean waters are within this zone.
    • Deep Zone- bottom layer, below 1000 meters, cold water at temperatures between 1° and 3° C, temperature is uniform in this layer.
    • Thermocline- transition zone between Surface and Deep layer, temperature decreases rapidly with depth down to 1000 meters.
  • The zones can change depending on latitude, although the deep zone is the same in all regions (Polar, Mid-latitude, and Tropical).
  • Interactions between the atmosphere and ocean occur at the surface and result in the transfer of heat and moisture, so Sea Surface Temperature (SST) distribution is important.
  • An Ocean Current is a massive, ordered pattern of water flow.  They closely resemble surface wind patterns.
  • In general, warm currents tend to flow poleward or westward, while cold currents tend to flow toward the Equator.
  • Gyre- an ocean circulation that forms a closed loop that stretches across and entire ocean basin.
  • Surface currents generally flow at an angle of around 45° to the wind to the right of the wind in the Northern Hemisphere and to the left of the wind in the Southern Hemisphere, due to Coriolis and Friction forces.
  • Ekman Spiral- the progressive turning of ocean currents from the surface down to 100 meters due to the combined effect of wind on the sea surface and the Coriolis Force.  The top layer is driven by surface wind and each layer below it is driven by frictional drag of the layer above.
  • Ekman Transport- the effect of the Ekman Spiral to move water masses at a right angle (90°) to the direction of the surface wind.  In the NH, it is to the right, in the SH to the left.
  • Upwelling- a wind driven ocean circulation pattern in which cold, nutrient-rich ocean waters are forced up to the surface as a consequence of Ekman Transport.
  • Upwelling is very important for marine organisms at the surface and if it ceases, it can have catastrophic effects on coastal regions.

 

El Nino and La Nina

 

  • El Nino is a natural, periodic warming of the equatorial Pacific Ocean between South America and the Date Line.  It is an excellent example of ocean/atmosphere interaction and its effect on climate.
  • Typically, under normal conditions, SSTs off of Peru are cold due to coastal upwelling, and waters in the western Equatorial Pacific are warm.  The westerly (east to west) trade winds also push the water westward raising the level of the ocean one half meter higher than around South America’s western coast.  Due to the water’s warmth over the western Pacific, more precipitation also occurs.
  • An El Nino event is triggered when the trade winds weaken or reverse direction and blow from west to east, bringing with it warmer waters towards South America, along with increased precipitation.
  • During El Nino, two distinct changes occur in the equatorial Pacific Ocean:
    • Cold coastal waters are replaced by warm waters in the eastern Pacific
    • The height of the ocean surface drops over Indonesia (western Pacific) and rises in the eastern Pacific (South America), forcing the thermocline to lower near South America and prevent upwelling.
  • These changes can affect the global weather patterns by changing the Subtropical Jet, which can then change the weather of the mid-latitudes region, not just the tropics.
  • El Nino warming occurs periodically every 2 to 7 years.

 

  • The Southern Oscillation is the seesaw in atmospheric pressure between the western and eastern Pacific that is commonly associated with El Nino and La Nina.  It is measured through the Southern Oscillation Index (SOI).
  • El Nino and the Southern Oscillation can often occur together, but also can happen separately.  When they occur together it is called ENSO.
  • La Nina- cooler-than-normal sea surface temperatures in the eastern tropical Pacific Ocean.  It is the counterpart of El Nino.
  • During a La Nina, intense trade winds move warm waters towards Western Pacific, and there is increased cold water upwelling in the eastern equatorial Pacific. 
  • La Nina conditions typically last 9 to 12 month.
  • In many cases, La Nina produces the opposite climate variations from El Nino.
  • Other oscillations similar to the El Nino-Southern Oscillation have been uncovered in other areas of the world.
  • The Pacific Decadal Oscillation (PDO) is the seesaw of atmospheric pressure and SSTs that occurs over the North Pacific over periods of several decades.
  • The North Atlantic Oscillation (NAO) is an oscillation where atmospheric pressure seesaws between the polar low near the North Pole and the subtropical high over the Atlantic Ocean.

 

 

Tropical Cyclones: Hurricanes and Typhoons

 

  • Tropical Cyclones are circular low pressure storms with winds of at least 35 knots (39 mph) which are driven by atmosphere-ocean interactions and originate in the tropical oceans.
  • Hurricanes are tropical cyclones in the tropical regions of North and Central America.
  • Typhoons are tropical cyclones in the region of the western Pacific.
  • A cyclone is a center of low pressure.
  • The Eye is the innermost part of a strong tropical cyclone’s center, and is almost completely clear of clouds.  Surrounding the eye is the Eyewall, a narrow, circular region of intense thunderstorms.
  • A tropical cyclone must be powered by an energy source.  This energy source is the large latent heat of water.
  • A tropical cyclone also adds fuel to itself by drawing surface air towards its low pressure center, where the strong pressure gradient means the means the winds grow stronger closer to the eye.  As wind blows faster, more evaporation occurs, and this means more water vapor in the air to fuel thunderstorms.
  • Evaporation and Condensation are the keys to understanding the power of tropical cyclones.
  • Tropical cyclones cannot develop in regions with SSTs lower than 26.5°C, so they are constrained to the tropics region where warm SSTs produce more evaporation.  They are generally only seen in the summer and fall seasons as well.
  • Tropical cyclones do not form near the Equator (within 5 degrees latitude) b/c the Coriolis force is 0 and air tends to flow straight into low pressure centers, so it feels up before the pressure can drop enough.
  • The Eye of the storm is nearly calm and serves to break up the front and back of the storm as it passes over.  It is calm because in this region air sinks instead of rising, making it very stable.
  • When the eye becomes very small and distinct, this is a sign of the storm strengthening, when it becomes wider, it implies some weakening.
  • The stronger the cyclone, the stronger the upper level High Pressure and stronger surface Low Pressure.
  • Wind shear is one way to break up a cyclone because the winds are changing fast enough in the vertical to disrupt the vertical development of the storm.
  • The heaviest rainfall is seen in Rainbands, which spiral into and blend it with the eyewall.
  • The strongest winds are seen on the north and east sides of the cyclone b/c the storm’s forward motion adds or subtracts from the winds.
  • Tropical cyclones usually begin small as Tropical Disturbances, or Easterly Waves in the Atlantic Ocean.
  • A Tropical Depression is the next step in development, when cyclonic rotation is observed and a lower pressure center is located.
  • After that, the storm becomes a Tropical Storm when winds reach a consistent speed of 35 knots.
  • A Hurricane, or Typhoon, is classified when winds reach 65 knots, and this is when the eye normally forms.  There are also SuperTyphoons (winds greater than 150 mph) in the Western Pacific.
  • Most storms will lose their strength within a week, but some become Extratropical Cyclones, a low pressure system that forms outside of the tropics, and is usually associated with cold air and fronts near its center.
  • The Atlantic Hurricane Season starts on June 1st  (typically in the Gulf of Mexico or Western Caribbean) and lasts until November 30th, however they can continue into December if SSTs remain high enough and there is no wind shear present.
  • The peak of hurricane activity in the tropical waters south and southeast of the US is in mid-September.
  • Approximately 80% of intense hurricanes in the Atlantic begin as Easterly Waves off the coast of Western Africa and then drift westward, also referred to as Cape Verde Hurricanes.

 

How do they cause destruction?

 

  • The most obvious threat of a tropical cyclone is the powerful wind.
  • Wind damage is classified using the Saffir-Simpson Scale, which rates hurricanes on a scale of 1 to 5 based on the damage their winds would cause upon landfall.
  • Storm Surge is another threat where winds in a tropical cyclone push ocean water in front of them, and the stronger the wind, the more water is piled up by the winds, causing more flooding.
  • Storm Surge is the deadliest weapon in the tropical cyclone’s arsenal, typically causing 90% of all hurricane related deaths because water is heavier than air.
  • Flooding caused by heavy rains is another threat.