Transcript El Nino and La Nina

What causes Earth’s climate and climate change?
The Ocean
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Recall that the ocean is a natural thermostat
 annual sea surface temperature variation
 2 C in tropics, 8 C in middle latitudes, 4 C in
polar regions
 global average 17 C
 releases and absorbs heat over decades to centuries,
whereas the atmosphere does the same but in days
to weeks
Water has a high specific heat
 requires high energy to raise its temperature: 1
calorie (4.18 J) of energy to raise water temperature
by 1 C
Why is seawater salty?
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Seawater is 96.5% water, the rest is sodium chloride
(NaCl) (about 3%) and other dissolved salts
Why is seawater salty?
 land sediments carried by rivers into oceans
 2.5 billion ton per year
 dissolved cations (Na+, Mg2+, Ca2+, etc.) leached
from rocks
2 anions such as chloride (Cl ) and sulfate (SO4 ) have
accumulated over centuries from gases escaping
from Earth’s interior through volcanic eruptions
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less important are dust blown in from deserts and
anthropogenic pollutants
Na+ resides longer in the sea than Ca2+ because
marine animals remove Ca2+ to make carbonate
skeletons
Na+ is removed by adsorption to clay minerals but
slow process
The composition of seawater
E.A. Mathez, 2009, Climate Change: The Science of Global
Warming and Our Energy Future, Columbia University Press.
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Addition of salt into water decreases freezing point (-1.9
C instead of 0 C) and increases density (1.026 g cm-3
instead of 1.0 g cm-3)
In oceans, increased temperature decreases density
Processes that alter salinity
 evaporation removes water, so increases salinity
 precipitation or influx of fresh river water decreases
salinity
 freezing removes water, so increases salinity
 melting of ice adds water, so decreases salinity
 salinity of oceans varies from place to place
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Salinity is low at equator and at poles because of high
precipitation (equator) and low evaporation (poles)
 also at mouths of large rivers
High salinity at semiclosed seas in arid regions
 Persian Gulf, Red Sea, and Mediterranean Sea
Ocean depths
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Density
 increases with higher salinity
 increases with lower temperature
So, deep water is typically denser, colder, and more
saline than shallow water
Ocean is stratified by density into 3 major layers
 0-20 m: thin warm surface layer
 called a mixed layer because it affected by waves
and temperature changes => rapid mixing/changes
 20-500/900 m: thermocline
 zone where temperature and salinity change
rapidly with depth
 depth/thickness varies from location to location
and season to season
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below the thermocline
 called the deep zone
 slight variation in temperature and salinity
 65% of ocean water is in this layer
 but in winter at high latitudes, thermocline can
extend all the way to the ocean bottom (like in
Norwegian-Greenland Sea)
Vertical profiles of density,
temperature, and salinity
through the upper several
hundred meters of the ocean
E.A. Mathez, 2009, Climate Change: The Science of Global
Warming and Our Energy Future, Columbia University Press.
The average annual salinity of ocean surface water, 2005
E.A. Mathez, 2009, Climate Change: The Science of Global
Warming and Our Energy Future, Columbia University Press.
A conductivity, temperature,
and depth measurement
device
E.A. Mathez, 2009, Climate Change: The Science of
Global Warming and Our Energy Future, Columbia
University Press. Photograph by E.A. Mathez
North Atlantic
Deep Water
(NADW)
Antarctic Bottom Water (AABW)
The global ocean conveyor system
E.A. Mathez, 2009, Climate Change: The Science of Global Warming and
Our Energy Future, Columbia University Press. Source: IPCC, 2001
The global ocean conveyor system
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The global ocean conveyor system is also known as
Thermohaline Circulation (THC)
This circulation is driven by differences in density of sea
water, which is controlled by temperature and salinity
 generally less than 0.1 ms-1
 overturns entire ocean depth every 100-1000 years
Begins with the downwelling of water in the North
Atlantic and Southern Ocean
 water flows to and wells up in the Pacific Ocean and
flows as shallow water to replace the downwelling
water
This system exerts/moderates a stabilising influence on
global climate for hundreds to thousands of years
 but can change abruptly as well
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Warm, near surface water forms in the Atlantic Ocean at
about 35 N and flow northward at a depth of about 800
m
In the north, the water sinks because it loses heat to the
atmosphere and being now cold and more dense, it
sinks, and starts to flow southward, all the way to the
Southern Ocean
 North Atlantic Deep Water (NADW)
Water in the Antarctica rises because the seas here are
less dense, but sinks again as the NADW are cooled
again
Water wells up at less salty, warmer, and shallower
Indian Ocean and Pacific Ocean
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Deep water forms in North Atlantic, rather than North
Pacific because North Atlantic is saltier (by 5%) than
North Pacific
 more precipitation in North Pacific than North Atlantic
(which has higher evaporation)
 the global conveyor system acts to redistribute the
salt to correct this salt imbalance between these two
oceans
Northward water flow into the North Atlantic brings
enormous amount of heat, equivalent to 30% of annual
solar energy, to Europe
 even though Europe is at the high latitudes, its
weather is mild because of the conveyor system that
brings heat here from the equator
Effect of global warming
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More ice from the Arctic melting, adding freshwater into
the North Atlantic
Retreating ice cover exposes more of the ocean surface,
allowing more moisture to evaporate into the atmosphere
and leading to more precipitation (rain and snow)
So, the increased freshwater into the North Atlantic
increases the buoyancy of the ocean and makes it
harder more the warm water from the equator to sink to
the bottom
 hence, NADW might slow down or stop!
 ironically, causing global cooling (ice age, perhaps?)
 average Europe temperature might fall 5-10 C
(colder)
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Mediterranean Sea has also high salinity and its water
flows into Atlantic Ocean, making it saltier by 6%
 increased use of freshwater means less freshwater
flowing from rivers into the Mediterranean Sea,
causing higher salinity in both the Mediterranean Sea
and Atlantic Ocean
The water balance of the continents and oceans
Region
Evapotranspiration
Precipitation
Runoff
(in millimeters/year)
Europe/Asia
795
1353
558
Africa
582
696
114
North America
403
645
242
South America
946
1,564
618
All land
480
746
266
Atlantic Ocean
1,133
761
–372
Indian Ocean
1,294
1,043
–251
Pacific Ocean
1,202
1,292
90
All oceans
1,176
1,066
–110
E.A. Mathez, 2009, Climate Change: The Science of Global Warming and
Our Energy Future, Columbia University Press. Source: Hartmann, 1994
Ocean surface currents
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Ocean currents driven by winds
 their interaction with the atmosphere have important
consequences for both climate and weather
 confined mostly to the upper kilometre or two of the
ocean
 typical speeds
-1
 horizontal flow or currents are 0.01-1.0 ms
 vertical speeds within the stratified ocean are
0.001 ms-1
Ocean gyres correspond nearly to the wind gyres
 rotate clockwise in the Northern Hemisphere
 rotate counter clockwise in the Southern Hemisphere
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Continents positions affect wind gyres, deflecting them
into boundary currents flowing poleward, parallel to
coastlines
 Gulf Stream in North Atlantic
 Kuroshio Current in North Pacific
 Brazil Current, along coats of South America
 others such as East Australian and Mozambique
Currents
http://www.crd.bc.ca/watersheds/protection/geology-processes/globaloceancurrents.htm
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Boundary currents are important because
 they carry heat from the equator to the north, making
the weather in the north milder
 Gulf Stream can carry heat from the equator to the
mid latitudes in a month (mean flow: 100 mil. m3 of
water per second)
 they carry water vapour
 they help to remove CO2 from the atmosphere
 warm waters has less CO2 than colder waters
Upwelling and downwelling: Ekman transport
In the Northern Hemisphere:
Deeper waters are richer in nitrates and phosphates supports
growth of plankton and, in turn, fish, such as occurring in Peru
and Ecuador
The food chain:
Phytoplankton → Zooplankton → Predatory zooplankton →
Filter feeders → Predatory fish
http://www.crd.bc.ca/watersheds/protection/geology-processes/globaloceancurrents.htm
Upwelling zones
http://www.absoluteastronomy.com/topics/Upwelling
El Nino and La Nina
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Every several years, coastal areas in Peru and Ecuador
experience rapid warming, increasing from -2  to 4 C in
a month
 usually occurs around Christmas and lasts until
May/June
 anchovies and sardines disappear
 birds, fur seals and other animals that depend on fish
die
 unusual heavy rains in coastal areas, and droughts in
Andes in south Peru and northeast Brazil
 caused by El Nino (boy child or Christ child in
Spanish)
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El Nino is not a local phenomenon
 global: affects weather around the world
 severe droughts in Australia, Indonesia, southern
Africa, and Egypt
 milder weather (but stormier winter) in North America
El Nino cycle/occurrence is pseudo-periodic; cycle not
consistent, but usually every 2 to 7 years
El Nino is caused by a change in atmospheric and ocean
circulation across the entire equatorial Pacific Ocean
NOAA/PMEL/TAO
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In normal years, easterly winds (from east to west) in the
Northern and Southern Hemisphere converge along the
equator, blowing westwards (toward west)
 this causes the usual upwelling in coastal areas such
as Peru and Ecuador
 the westward flowing surface water piles up in the
western Pacific, causing sea level to rise about 60-70
cm higher than in the eastern Pacific
 the upwelling brings the thermocline nearer in the
east, while in the west, the thermocline is much
deeper
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In the mean time, the warm, moist air in the western
Pacific rise, causing rain, and warm air’s place is
replaced by easterly winds
 a convection circulation that starts with rising air in the
west, flowing toward the east Pacific, then sinking in
the east Pacific, then flows back to the west Pacific
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In an El Nino year, this coupled air-ocean circulation
cycle is reversed
 easterly winds weaken
 warm Pacific water flows toward east, sea level
flattens out (less difference between sea level at the
west and east Pacific)
 upwelling in coastal areas at South America stops
 thermocline along the equator deepens to tens of
meters in the east
 equatorial undercurrent stops
Warm air in the east Pacific rises, causing heavy rains in
coastal areas in east Pacific
 but no rain in west Pacific, causing drought
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Changes in circulation patterns are caused by changes
in atmospheric air pressure
 Normal years: high pressure at east Pacific, low
pressure at west Pacific
 El Nino: low pressure at east Pacific, high pressure at
west Pacific
 this swing in air pressure is known as the Southern
Oscillation
 El Nino is an ocean phenomenom, but Southern
Oscillation an atmospheric phenomenom
 but the two (ocean and atmosphere) are coupled
to cause El Nino, they are known together as
ENSO (El Nino-Southern Oscillation)
Atmospheric pressure:
Normal years: low at Darwin, high at Tahiti,
causing easterly (east-to-west) trade winds
El Nino year: high at Darwin, low at Tahiti,
weakening easterly winds
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The Southern Oscillation Index (SOI) measures the
change in air pressure measured between the eastern
(Tahiti) and western (Darwin) Pacific
 +ve SOI means
 Darwin < Tahiti
 air pressure is lower in the west, higher in the east
 normal years (high index)
 La Nina (girl child in Spanish) (a very high index)
 -ve SOI means
 Darwin > Tahiti
 air pressure is higher in the west, lower in the east
 El Nino year (low index)
http://www.pmel.noaa.gov/tao/elnino/faq.html
2nd strongest period
strongest period
Climate variability and the global harvest : impacts of El Niño and other oscillations on agroecosystems by Cynthia
Rosenzweig and Daniel Hillel, Oxford University Press, 2008
Climate variability and the global harvest : impacts of El Niño and other oscillations on agroecosystems by Cynthia
Rosenzweig and Daniel Hillel, Oxford University Press, 2008
El Nino has shown to give low grain yield in South Asia and Australia, but high grain
yield in the North American prairies
Climate variability and the global harvest : impacts of El Niño and other oscillations on agroecosystems by Cynthia
Rosenzweig and Daniel Hillel, Oxford University Press, 2008
El Nino of 1982-83 on Brazil
Climate variability and the global harvest : impacts of El Niño and other oscillations on agroecosystems by Cynthia
Rosenzweig and Daniel Hillel, Oxford University Press, 2008
Long term effects
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El Nino and La Nina, however, has no effect on global
warming in the long run
 effects only on the short term
 droughts or heavy rains will give poor yields in a
short period
 or may give good yields in some cases such as
more rains in usually dry areas
El Nino and La Nina cancel each other out to give zero
net change over the long run