Teleconnections - El Nino
GEOG1002
Environmental
Systems and Processes
.
Introduction
El Nino Southern Oscillation (ENSO)
• INTRODUCTION - normal weather
• MECHANISMS
• El Nino - Teleconnections
• La Nina
• Impacts: Indonesia
• Further material
INTRODUCTION
normal weather
Normal weather conditions in
Equatorial Pacific
• Surface
wind
patterns
Normal weather conditions
in Equatorial Pacific
• Warm waters in
E Pacific
• cooler waters in
W Pacific
• lower surface
pressure in E
than W
MECHANISMS
• E-W pressure difference
drives winds from
Galapagos to Indonesia
• Trade winds blow W
across Tropical Pacific at
equator
• Pile up warm surface
water in W. Pacific
• Sea surface 0.5 m higher
at Indonesia than
Ecuador
Mechanisms
• Atmospheric pressure differences
– Trade winds and S-E winds Peru/ Ecuador
coasts drag surface water
• Earth rotation deflects currents
– N in N hemisphere
– S in S hemisphere
• upwelling where
surface water moves
away
– colder, nutrient-rich
water replaces moved
water
– upwelling along coast
and equator limited to
narrow bands
(100
miles wide)
• SST 8o higher in W
than E
Sea surface temperature
May 1988 (equatorial Pacific cold)
Note "tongue" of recently upwelled water
extending W along equator
Thermocline
• dividing layer
between warm
surface water and
deep cold water
– thermocline flat if
no wind
– normal surface
winds drag surface
water to West
• raises close to
surface in E
• depress in W
– E winds (red arrow) drag surface water W
along the equator
– Earth's rotation deflects the W current
toward the right in the N and toward the left
in the S, driving surface water from equator
and bringing up the water from below
(upward arrows).
•Winds
•warm surface water
accumulates in W
Pacific.
•low density warm water
•sea level is two feet
higher on the W side
when the winds
blowing full
•Thermocline
•boundary - warm
surface water - cold
deep water (darker
blue) tilted.
•almost to sea surface
in E eq. Pacific.
Ecosystems
• Cold water beneath thermocline rich in nutrients
• where thermocline shallow
• mixing with surface water
• phytoplankton flourish (nutrients and sunlight)
• primary productivity
•zooplankton graze on phytoplankton
feeding higher up food web
• diverse marine ecosystem
• major fisheries
Winds
• winds control upwelling
• controls phytoplankton
• controls higher forms
• winds responsible for surface temperature pattern
El NINO
El Nino
• Trade winds relax in C. and E. Pacific
• Thermocline flattens
e.g. 1982-1983 110o W
17o thermocline
(black) dropped
to 150 m
Effect of El Nino
• reduced upwelling to surface
• cut off supply of nutrient-rich thermocline
water to eutrophic zone
– drastic reduction in primary productivity
• effect on higher levels of food chain
– including commercial fishing
• rise in SST in E. Pacific
– (cold tongue weakens / disappears)
SST:
• 1988
• 1992 (El Nino)
Southern Oscillation Index
(SOI)
• Early evidence / links
– 1920s - Sir Gilbert Walker
– investigating timing of monsson in India
– noticed pressure links
between E and W sides of Pacific
– rises in E, falls in W & vice-versa
• Southern Oscillation Index
– pressure difference E-W
Upper image: high index phase (Nov. 1988) pressure is higher (darker red) near & to E.
of Tahiti than Darwin
Lower Image: low index phase (Nov. 1982) reduction or reversal of pressure difference
Southern Oscillation
• monsoon seasons with low index
– drought in Australia, Indonesia, India &
parts of Africa
– unusually mild winters in Canada
• high index
– E-W pressure difference causes
surface air to flow W
• low index
– Strength of Easterly surface winds reduced
or reversed direction
• low index phase usually
accompanied by El Niño conditions
• Late 1960s - Jacob Bjerknes first clearly
understandable description of life cycle of
storms in temperate latitudes
– key to link between pressure differences of
SO and warm waters of El Niño
• part of same phenomenom - ENSO.
SST Effects on Atmosphere
• Heavy rainfall confined to areas of
warmest water
– normal years W. Pacific - Indonesia
– E. Pacific cool water - upwelling
– El Nino years
• C. Pacific warms
• moist air above ocean warms - forms deep
clouds
• rain migrates E.
• no / late Monsoon rains in normal areas
• heavy rainfall in C. Pacific
• leads to atmospheric pressure variations:
– pressure increase in W.
– pressure decrease in E.
• Southern Oscillation Pressure
change
– weakens Trade winds
– positive feedback
TELECONNECTIONS
• ENSO - Large changes in
global atmospheric
circulation
– shifts in tropical rainfall
affect wind patterns over
much of world
• jet streams (5-15 miles alt)
change course in El Nino
winters
• high pressure over N. American W. Coast
(Washington, Oregon)
– higher than normal temperature
– storms diverted from here to coast of N.
Alaska
• similar effects lead to storms in Gulf of
Mexico heavy rain in S. USA
• similar Brazil,
Chile, Argentina
Extreme global conditions 1982-84
• changes in weather in regions far
removed from tropical Pacific
Extreme temperature events
Extreme precipitation
Global El Nino precipitaion
anomolies
Global El Nino precipitaion
anomolies
Timing of El Nino
• irregular intervals 2 - 10 years
• no two events alike
– 1950s-1980s event preceeded by
stronger easterlies on equator
– 1982-83 no such 'warning'
• came later in season
examine historical evidence
• SST records
– merchant ships and coastal stations
crossing equator - 1930s+
• atmospheric pressure and rainfall
– e.g. Darwin - over 100 years
• fisheries records from S. America
• writings of Spanish colonists
Peru/Ecuador - C15th
• proxy evidence
– coral samples
– tree rings
– centuries++ timescale
– debate re. history of el nino
Cold Event - La Nina (El Viejo)
• unusually cold ocean temperature in
Equatorial Pacific
• La Niña (cold) conditions December 1988
• Normal Conditions December 1990
• El Niño (warm) Conditions December
1997
• Effects essentially
opposite to El Nino
– warm winters in SE
USA
• Typical impacts
IMPACTS: INDONESIA
Indonesia 1997/1998
• Vast population - over 200M
• Developing (TIGER) economy
• econ. crash in 1997
– civil unrest
– change of government 1998 (Suharto Habibie)
• forest clearing (Kalimantan, C. Sumatra)
– fire; Haze
• timber
• subsistence farmers
• oil palm (major culprit) + rubber
plantations • important for economy
• large corporations blamed for around
80% of fires
• 1997 El Nino - delay/weakening of rains
– no rains May to September
– start of rainy season 3-4 months late
– reduced rain in monsoon season (Nov. to
March)
• some fire burned on into April 1998
– fires out of control
– 1.5 to 2M acres of forest burned
• rainforest not normally burn
– logging
– drought due to El Nino
• El Nino severity enhanced by released
greenhouse gasses in burning?
• peat deposits underground burned
(months/years)
• environmental concerns low priority for
government in economic crisis
fires caused smoke ... smog
• El Nino high pressure system
favours smog development
El Nino indirect links to fires:
• environmantal damage
• public health
• economy
estimated cost US$4.5bn
• direct losses by timber/agriculture
• impact of air polution on public health
and tourism
• transport / industry accidents
• economic impact - damages
• claim S. California rains helped air
quality there