Lec 9: WBC, Equatorial currents, El Nino

• Western Boundary Currents


  Currents that flow along the western side of ocean basins

  
• Equatorial Currents


  Currents that flow along the equator
• El Niño


  A periodic warming of the central and eastern Pacific Ocean

  
• The sea surface maximum is offset to the west in a more realistic schematic

  
• Western Intensification

  1. Water moving eastward (in the westerly winds band) is deflected more than water moving westward (in the trade winds zone)

  2. Westerly zone → water is transported towards the gyre center over the entire ocean width

  3. Trade wind zone → little deflection; flow pushes water to west side of the ocean

  4. Gyre center is offset to the west

  5. Pressure gradient is larger on the west side (steeper slope) → stronger geostrophic currents

  
• The pycnocline is also pushed down, meaning the boundary current is also deeper (besides being faster)
• Front

  Encounter of two water masses/currents with distinct characteristics

  
• Meander


  Snakelike bends in the current
• Eddy


  Isolated ring formed when a meander is tight enough
• Eddies are important for transporting heat, nutrients and marine organisms around
• Warm core rings

  Spin clockwise
• Cold core rings


  Spin counter-clockwise
• Both warm and cold core rings can reach sizes of 100-300 km in diameter and can reach from the surface to the seafloor
• The rotating currents at the rims of warm and cold core rings can reach speeds of ~90 cm/s
• Cold core rings last longer because they are not limited in area (such as for warm core rings, which are limited to move between the Gulf Stream and land)
• Equatorial Surface Currents

  Currents that travel westward along the equator, creating a pile up of water in the western side of the ocean

  
• Equatorial Countercurrent

  Current that travels eastward along the equator as a result of surface divergence

  
• The Equatorial Countercurrent is better defined in the Pacific Ocean
• Pacific Ocean Circulation

  • Dominated by 2 large subtropical gyres
  • Equatorial countercurrent is much better developed in Pacific, largely because the Pacific ocean basin is bigger and more unobstructed
• Normal Conditions in the Pacific

  1. Warm water pool to the west, from flowing equatorial currents

  2. Low pressure to the west, associated with rising air, clouds and precipitation

  3. High pressure coastal South America, associated with sinking air and clear, fair, dry weather

  4. Sloped thermocline

  5. Peru coast → upwelling → richest fishing grounds
• El Niño Conditions

  1. High pressure coastal South America weakens

  2. Reduces the pressure difference → reduce trade winds

  3. Warm water pool spreads along equator (September; reaches South America by December/January)

  4. Precipitation center also moves

  5. Thermocline flattens out

  6. Peru: weak or no upwelling

  
• La Niña Conditions

  1. Larger pressure difference → stronger trade winds

  2. Band of cooler-than-normal water across the equatorial Pacific

  3. Thermocline slope accentuated

  4. Peru: stronger upwelling

  
• El Niño is characterized by unusually warm ocean temperatures in the eastern equatorial Pacific. The warmer water associated with El Niño displaces colder water in the upper layer of the ocean causing an increase in sea surface height because of thermal expansion.

  
• El Niño and La Niña events occur every 2 to 20 years and last from 12 to 18 months
• ENSO Index

  Positive = El Niño, Negative = La Niña, Near zero = "normal"
• Very strong El Niño events can result in flooding, erosion, droughts, fires, tropical storms and effects on marine life worldwide