Geography 311
 
El Nino Southern Oscillation (ENSO)
 
1. The Eastern Equatorial Pacific
 (1). Dry Zone
  A. From the coast of South America (Ecuador and northern Peru)
  westward to about the 165 oE meridian (approximately 7000
  miles long).
  B. Approximately from the 3 oN to 10 oS (north-south width).
  C. A cold water tongue (zone)
  (A) The Peru cold ocean current: The ocean surface
  water flows from the Antarctica to the equator.
  (B) Upwelling
  a. The nutrient-rich cold water rises to the ocean
  surface from below.
  b. Blooms of phytoplankton (a tiny plant) produce a
  thin layer of chlorophyll that can be identified
  from the satellite image.
  c. Coast upwelling off the South America.
  (a). 100 miles wide.
  (b). The ocean surface water flows to the left of
  the prevailing southerly flow, away from the
  coast (the Ekman’s spiral).
  d. The equatorial upwelling
  The surface water divergence associated with the
  subtropical Highs in the both hemisphere.
  (a) Northern Hemisphere: The equatorial water
  flows away from the equator (to the right of
  the easterly wind).
  (b) Southern Hemisphere: The equatorial water
  flows away from the equator (to the left of the
  easterly wind).
  D. The ITCZ is to the north of the dry zone.
 (2) large rainfall variability
 A. Strong easterlies: strong divergence and upwelling, less
  rainfall.
 B. Weak easterlies: weak divergence and upwelling, more rainfall.
 C. Westerlies: more disturbances and more rainfall.
 D. The desert islands of the central equatorial Pacific:
  (A). Normal year: little or no rainfall.
  (B). El Nino years (low SOI index year): torrential rains day
  after day, month after month.
2. The Western Equatorial Pacific
 (1) Indonesia and northern Australia (marine continents).
 (2) Warm ocean water and humid climate.
 (3) A warm water pool between 5 oN and 10 oS supplies the thermal
  energy to drive the Hadley circulation in both hemispheres.
 (4) Strong convective activity (thunderstorm clouds) normally.
3. The Walker’s Circulation (named by Bjerknes in 1969)
 (1) A longitudinal (east-west) standing (Up and down) atmospheric
  circulation over the equatorial Pacific.
 A. The surface easterlies and upper westerlies.
 B. The western equatorial Pacific (Indonesia and North
   Australia): rising airs (convections).
  C. The eastern equatorial Pacific (Peru and Ecuador coasts,
 Galapagos Islands, Tahiti, Christmas Island and Easter
  Island): subsidences (sinking airs).
 (2) Caused by a longitudinal temperature gradient.
 (3) Part of the Walker”s Southern Oscillation: The east-west
  standing circulations over the entire tropical latitudes of the
  earth (the Pacific, Atlantic, and Indian oceans).
 (4) The Southern Oscillation Index (SOI)
  A. In South Pacific, when air pressure rises in the east, it
  usually falls in the west (the seesaw in southern Pacific
  pressures).
  B. SOI calculations
  (A) Santiago (Chile, 33 oS) air pressure - Darwin
  (Australia, 12 oS) air pressure.
  (B) Tahiti air pressure - Darwin air pressure.
  C. Low index period : El Nino
  The weakening of the equatorial easterlies.
4. El Nino (The Christ Child or the Christ Boy)
 (1) Definition:
  A warm and heavy rainy period along the coasts of Ecuador and
  Peru near the Christmas-time and lasts for several months ( to
  June).
 (2) Ten El Ninos in the past 40 years:
  A. Weak El Ninos: 
  Water temperatures rose about 1 to 2 oF over the eastern
  equatorial cold and dry zone.
  B. Strong El Nino (1982-83)
  (A) The strongest and unusual one in the century 
  a. Set in late and last 10 months into 1983.
  b. Not preceded by a period of stronger than normal
  easterlies on the equator (caught scientists by
  surprise).
  (B) West of the dateline to Indonesia:
  The easterly surface winds began to weaken in May 1982
  and eventually shifted to the westerly winds in a few
  weeks.
  (C) Sea level at Christmas Island in the middle Pacific
  rose several inches.
  (D) By October, sea level rose up to a foot spreading
  6,000 miles eastward to Ecuador.
  (E) The western Pacific: Sea level dropped exposing
  and destroying the upper Layers of the coral reefs
  surrounding many islands.
  (F) Sea-surface temperatures at the Galapagos Islands
  and along the coast of Ecuador rose from typical levels
  in the low 70s well up into the 80s(oF).
  (G) Losses of sea birds and fish species:
  a. Deaths and disappearances of sea birds at 
 Christmas Island. 
  b. 25% of the fur seal and sea lion adults and all of
 pups had died when the condition returned to
 normal in mid-1983.
  c. Tropical and subtropical fish species migrated
 poleward: sardines in Chilean water.
  d. Unexpected harvest of warm-water scallops on the
 normally cold coast of Ecuador.
  e. Ecuador and Peru: anchovy harvest failed. 
  (H) Effects on land
  a. Ecuador and northern Peru: 
 100 inches of rain fell during a six month period:
 Transforming the coast desert into grassland
 dotted with lakes (explosions of grasshoppers,
 toads, birds, mosquitoes).
  b. Tahiti and Hawaii: unusual severe hurricane
 attacks (steered by abnormal wind patterns). 
  c. The central Pacific: heavy monsoon rainfall.
  d. The western equatorial Pacific (Indonesia and
 Australia): drought. 
  e. Southern California and the southern USA: floods.
  f. Northern USA and Canada: mild weather and lack of
 snow.
  g. Africa and India: drought. 
  (I) world wide economic loss: $8 billion.
 (3) Causes (Mechanisms)
  A. Strengthening of trade winds (easterlies):
  (A) western equatorial Pacific (Indonesia, northern
  Australia): rising sea level (warm water piles up and
  is sustained by the strong easterlies).
  (B) Eastern equatorial Pacific (Ecuador and Peru):
  lowering sea level, rising thermocline almost to the
  surface, strong upwelling.
  B. Weakening of trade winds in both hemispheres (Wyrtiki’s
  theory): 
  The equatorial Pacific is characterized by the easterlies
  (trade winds) during the normal year.
  (A) The upwelling in the dry zone also weakens or even
  disappears: Thermocline flattens out.
  (B) The Kelvin wave (eastward flow of the warm water
  which was piled up on western Pacific during period of
  strong easterlies) progresses eastward across the
  equatorial Pacific, raising sea level and bringing warm
  El Nino to South America.
  (C) Sea level flattens out, dropping in the west and
  rising in the east.
  (D) Shift of the convective zone from western Pacific
  to central Pacific and eastern Pacific.
  C. Abnormal warming of the ocean water which supplies energy
  to the overlying atmosphere by means of sensible heat
  transfer and evaporation.
   D. Hadley circulation is intensified and it is in turn
  accelerated the Ferrel westerlies by transferring the
  angular momentum from the tropics to the subtropical jet
  stream that may cross over southern USA including southern
  California.
  E. Over South America, the ITCZ is displaced southward more
  than usual.
 (4) Thermolcline
  A. The dividing layer between the warm surface water and the
  deep cold water: A zone of strong vertical water temperature
   gradient.
  B. Rich nutrients in the cold water below the thermocline.
  C. Absence of winds: flat thermocline.
  D. Strong easterlies (trade winds): 
  The thermocline rises in the eastern equatorial Pacific and
  plunged in the western equatorial Pacific (shallow in the
  east and deep in the west).
  E. Weak easterlies (El Nino, low SOI)
  Thermocline flattened out (rise on the west and drop on the 
  east). 
5. La Nina
 (1). Christ girl.
 (2). A extreme cold and dry period in the eastern equatorial Pacific
   including northern Peru and Ecuador. 
 (3). A reversal of El Nino.
 (4). A period of extreme cold water tongue in the eastern equatorial
   dry zone.
 (5). A period of strong easterlies (high SOI).
 (6). The Subtropical Highs in both hemispheres intensify.
 (7). The jet stream stays in northern United States, bringing heavy
   rains to northern California, Oregon and Washington states.
6. Pineapple connection (express)
 (1). A subtropical jet that extends from near Hawaii Islands to
  southern California, bringing winter storms during the El Nino
   time.
 (2). Convective zone (thunderstorm zone) moves from the
 maritime continents to the central Pacific near the
  international date line. 
 
Please click on the following links for details (optional):
 

http://www.pmel.noaa.gov/tao/elnino/nino-home.html

http://www.elnino.noaa.gov/lanina.html

http://www.noaanews.noaa.gov/stories2006/s2748.htm

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml#current