Climate Change and the Atmosphere

Sectors Creating Greenhouse Gas Emissions

• Transport: 26%

• Business: 18%

• Residential: 16%

• Energy: 20%

• Agriculture: 11%

• Waste: 4%

• Other: 4%

• The total does not add up to 100% due to rounding.

Global Average Temperature Change

• The past eight years (until 2023) have been confirmed to be the warmest on record and now the past ten years are.

• 2023 shattered climate records, with major impacts, according to the World Meteorological Organization (WMO).

• 2024 is on track to be the hottest year on record, with warming temporarily hitting 1.5°C.

• WMO confirms 2024 as the warmest year on record at about 1.55°C above pre-industrial levels. The past ten years have all been in the Top Ten warmest years.

Atmosphere

• Troposphere: Approximately 16 km thick, containing about 75% of the gases.

• Stratosphere: Extends from 16 km to 50 km, containing about 24% of the gases; the ozone layer is located here.

Properties of Troposphere and Stratosphere

• Troposphere

  • Ground level to 8-20 km.

  • Temperatures typically decrease with altitude, reaching as low as -60°C.

  • Significant vertical mixing and turbulence.

  • Substances are washed back to Earth by gravitation and precipitation.

  • This is where weather occurs, and where we live.

  • Almost all water vapor and clouds are in this layer.

• Stratosphere

  • Extends from the tropopause (top of the troposphere) to approximately 50 km.

  • Temperatures increase with altitude, reaching up to 0°C.

  • Little vertical mixing; very slow exchange of gases with the troposphere.

  • Substances entering this layer remain for a long time, unless broken down by UV radiation.

  • Isolated from the troposphere by the “tropopause”.

  • Contains the ozone layer.

Weather and Climate

• Weather and climate primarily occur in the troposphere.

• Weather is the day-to-day variation in temperature, air pressure, humidity, cloudiness, precipitation, etc.

• Climate is the result of long-term weather trends.

• Meteorology is the study of the atmosphere, climate, and weather.

Climate Defined

• The average trend in temperature and rainfall that produces and supports a unique assemblage of plants and animals.

• Determined by general atmospheric circulation and precipitation patterns.

  • These patterns are influenced by the Earth’s rotation and tilt on its axis.

• Climate tends to be stable over time.

  • Abrupt changes in climate are a threat to ecosystems.

  • The rate at which organisms can adapt is a critical factor.

The Sun's Role

• The sun drives the Earth's weather and climate.

• Uneven heating of the Earth’s surface leads to global wind patterns.

• Convection occurs at both global and local scales.

Convection Cells

• Air flows from high to low-pressure areas.

• Rising air creates low pressure.

• Falling air creates high pressure.

• The system is driven by energy from the sun.

Jet Streams

• Strong west-to-east winds (200 – 400 KPH) occurring in the upper tropospherebetween major convection cells.

• Have a large impact on local weather patterns by moving ground-level air masses.

Fronts

• Fronts are boundaries between air masses of different temperatures (densities).

• Responsible for most of the weather we experience.

Thermohaline Circulation

• The thermohaline circulation also called the "conveyor belt" involves cold, deep ocean currents and shallow, warm water circulation patterns.

• Heat from the tropics is redistributed to northern Europe via the Oceanic Conveyor System.

• North Atlantic Deep Water (NADW) and the Gulf Stream are key components.

• Ocean currents move huge amounts of excess heat from the equator toward Europe.

• Deepwater formation occurs near Greenland.

• The top 3 meters of the ocean has as much heat capacity as the entire atmosphere.

• Atlantic Meridional Overturning Circulation (AMOC)

• Meltwater from Greenland and the Arctic is weakening ocean circulation, speeding up warming down south.

Atlantic Ocean Circulation

• Atlantic Ocean circulation is nearing a 'devastating' tipping point; collapse in the system of currents would occur at such speed that adaptation would be impossible.

Ocean-Atmosphere Oscillations

• Decades-long patterns of oscillations in pressure, winds, and currents include:

  • North Atlantic Oscillation (NAO)

  • Interdecadal Pacific Oscillation (IPO)

  • El Nino/La Nina Southern Oscillation (ENSO) which has a global impact on weather patterns.

IPCC

• The IPCC (Intergovernmental Panel on Climate Change) was established in 1988 by the United Nations Environment Program (UNEP) and the World Meteorological Society to evaluate climate science.

• It has three working groups:

  • WG1: The Physical Science Basis of Climate Change

  • WG2: Climate Change Impacts, Adaptation, and Vulnerability

  • WG3: Mitigation of Climate Change

Climate Change

• The International Panel on Climate Change (IPCC) Sixth Assessment Report (2021-2023) states that "It is unequivocal that human influence has warmed the atmosphere, ocean, and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere, and biosphere have occurred."

• The oceans are warmer.

• There is a greater frequency of extreme weather events.

• Glaciers and ice caps are melting.

• The sea level is rising.

• Humans are driving these changes.

Past Climate Change

• To understand climate change, it is helpful to look at how climate has changed in the past.

Proxy Measurements

• Proxy measurements for temperature (paleoclimatology) include

  • Historical accounts

    • Diaries, letters, and official records.

    • Warming period in Europe from 1100 to 1300.

    • “Little Ice Age” from 1400 to 1850.

  • Tree rings.

  • Ice core samples.

  • Pollen deposits.

  • Lake and marine sediments.

  • Coral growth.

Tree Rings

• Tree rings represent annual growth patterns influenced by climate conditions.

CO2 and Methane Levels

• Currently at the highest levels in 800,000 years.

Milankovitch Cycles

• There are cyclic changes to the Earth’s climate.

• $T = 41,000$ years (Tilt)

• $P = 23,000$ years (Precession)

• $E = 100,000$ years (Eccentricity)

Changes in Eccentricity (Orbit Shape)

• Occur in 100,000-year cycles.

• Earth's orbit shape varies between 0.0034 (almost a perfect circle) to 0.058 (slightly elliptical).

Changes in Obliquity (Tilt)

• Occur in 41,000-year cycles.

• Relative to orbital plane 22.1-24.5°

Axial Precession (Wobble)

• Occurs in 26,000-year cycles.

Rapid Climate Change in the Past

• There are indications of rapid climate change in the past.

• The transition from the last glacial period (which ended ~16,000 years ago) to the present interglacial period was punctuated by a brief and intense return to cold conditions around 11,700 years before present, called the Younger Dryas event.

• This episode lasted about 1300 years.

• At the end of this event, temperatures rapidly rose, perhaps by as much as $10°C$ in 10 years!

Younger Dryas Trigger

• A deluge of fresh water from glacial Lake Agassiz, flowing into the North Atlantic via the Great Lakes and the St. Lawrence Seaway, may have triggered the Younger Dryas event.

• The lake normally emptied down the Mississippi to the Gulf of Mexico.

• It’s possible that a natural dam broke and released the water of Lake Agassiz through the Great Lakes, down the Saint Lawrence to the North Atlantic.

Global Climate Change: The Greenhouse Effect

• The Earth acts as a greenhouse.

Energy from the Sun

• Energy is packaged in electromagnetic waves and travels through space at the speed of light.

• Longwave radiation vs. shortwave radiation.

Types of Radiation

• Long-wave and short-wave radiation differ in their ability to pass through the atmosphere:

• The SUN emits primarily shortwave radiation:

  • Gamma rays & x-rays are blocked

  • Most of the UV is blocked

  • Most visible and near IR light passes through

• The EARTH emits longwave radiation:

  • Much of this is absorbed and re-radiated by the atmosphere

• The atmosphere is essentially transparent to visible light (shortwave radiation).

Energy Balance

• For a system in balance: Energy in = Energy out

• If Energy in ≠ Energy out there will be heating or cooling of the system as a whole until a new equilibrium is established.

Greenhouse Effect

• Incoming sunlight = 100%:

  • Reflected by clouds: 20%

  • Reflected by atmosphere: 6%

  • Reflected by Earth's surface: 4%

  • Absorbed by atmosphere: 19%

  • Absorbed by land and oceans: 51%

• Radiation from atmosphere:

  • Radiated to space from atmosphere: 64%

  • Radiation absorbed by atmosphere: 15%

• Radiation from Earth:

  • Radiated to space from Earth: 6%

  • Conduction and rising air: 7%

  • Latent heat in water vapor: 23%

• Greenhouse gases bounce energy in all directions, warming the atmosphere.

Earth's Energy Budget

• Incoming solar radiation: 340.4 $W/m^2$

• Reflected by clouds & atmosphere: 77.0 $W/m^2$

• Reflected by surface: 22.9 $W/m^2$

• Total reflected solar radiation: 99.9 $W/m^2$

• Net absorbed: 0.6 $W/m^2$

• Total outgoing infrared radiation: 239.9 $W/m^2$

• Emitted by atmosphere: 169.9 $W/m^2$

• Emitted by clouds: 29.9 $W/m^2$

• Atmospheric window: 40.1 $W/m^2$

• Absorbed by surface: 163.3 $W/m^2$

• Emitted by surface: 398.2 $W/m^2$

• Back radiation: 340.3 $W/m^2$

• Thermals (conduction/convection): 18.4 $W/m^2$

• Latent heat (change of state) evapotranspiration: 86.4 $W/m^2$

• Absorbed by atmosphere: 358.2 $W/m^2$

Radiation and Wavelengths

• Short-wave radiation vs. Long- wave radiation differ in their ability to pass through the atmosphere.

Temperatures

• If the Earth had no atmosphere, the globally averaged surface temperature would be about $-19°C$. But, since the Earth has an atmosphere, the average surface temperature is about $+14°C$.

Avoiding Extremes

• Venus has a dense atmosphere composed of 96.5% carbon dioxide and 3.5% nitrogen, leading to a runaway greenhouse effect with surface temperatures over $460°C$.

• Mars has a very thin atmosphere with surface temperatures ranging from about $-87°C$ to up to $-5°C$.

Radiative Forcing

• A key concept of Climate Change: Radiative “forcing”

• The influence a particular factor has on the energy balance of the “Atmosphere – Ocean – Land System”

  • Positive forcing leads to warming

  • Negative forcing leads to cooling

  • Usually expressed as Watts per meter squared ($W/m^2$)

• Radiative forcing is the change in energy balance at the top of Earth’s atmosphere caused by some factor (like greenhouse gases, aerosols, or solar changes).

Increased Greenhouse Gases

• The Major contributor to a warming world.

Major Greenhouse Gases

• Water ($H_2O$) is the most abundant GHG

  • Short residency time in the atmosphere

  • Warmer air holds more water à more clouds

• Carbon Dioxide ($CO_2$) is the primary anthropogenic GHG

  • Increased from 280 ppm to ~420 ppm from 1750 to 2023 (50% increase)

  • The main anthropogenic source is burning fossil fuels

  • Stays in atmosphere ~ 120 years

  • $CO_2$ acts as the “Control Knob” on the Earth’s climate

Fossil Fuel Use

• Fossil fuel use is the primary driver of increased atmospheric CO2.

• Swedish scientist Svante Arrhenius (1859-1927) hypothesized that CO2 would lead to warming of the earth (1896).

• About 3 kg of CO2 is produced from each kg of fossil fuel burned.

• About 8 billion metric tons of fossil fuels are burned each year.

Atmospheric CO2

• CO2 levels are increasing by about 1.8 to 2.0 ppm / year

• CO2 levels are highest during N. Hemisphere Winters

• Current levels are higher than anytime in the past 800,000 years

Global Carbon Cycle

• Currently the oceans and forests + soils are acting as sinks.

Global Greenhouse Gas Emissions by Sector

• Total global greenhouse gas emissions in 2016 were 49.4 billion tonnes $CO_2$eq.

Carbon Sources and Sinks

• Global carbon cycle: numbers are GtC (billion metric tons of carbon)
  Industrial Sector

• Iron and steel: 7.2%

• Non-ferrous metals: 0.7%

• Chemical & petrochemical: 3.6%

• Food & tobacco: 1%

• Paper & pulp: 0.6%

• Machinery: 0.5%

• Energy use in Industry: 24.2%
  Agriculture, Forestry & Land Use: 18.4%

• Livestock & manure: 5.8%

• Agricultural soils: 4.1%

• Rice cultivation: 1.3%

• Crop burning: 3.5%

• Deforestation: 2.2%

• Grassland: 0.1%

• Cropland: 1.4%
  Waste: 3.2%

• Landfills: 1.9%

• Wastewater: 1.3%
  Energy: 73.2%

• Fugitive emissions from energy production: 5.8%

• Energy use in buildings: 17.5%

  • Commercial: 6.6%

  • Residential buildings: 10.9%

• Energy in Agriculture & Fishing: 1.7%

• Unallocated fuel combustion: 7.8%

• Other industry: 5.2%

• Cement: 3%

Transport: 16.2%

• Road Transport: 11.9%

• Aviation: 1.9%

• Shipping: 1.7%

• Rail: 0.4%

• Pipeline: 0.3%

• The climate cost of the first 60 days of Israel’s military response was equivalent to burning at least 150,000 tonnes of coal.

Top CO2 Emitting Countries (2022)

• China: 12,667,428,430 tons (-0.39% change)

• United States: 4,853,780,240 tons (1.78% change)

• India: 2,693,034,100 tons (6.52% change)

• Russia: 1,909,039,310 tons (-1.22% change)

• Japan: 1,082,645,430 tons (0.65% change)

Major Greenhouse Gases: Methane (CH4)

• Much more potent than CO2, but shorter time in atmosphere (~ 12 years)

Global Methane Budget (2008-2017)

• Total Emissions: 550-594 Tg CH4/yr (Bottom-up view) 557-635 Tg CH4/yr (Top-down view)

• Change in Atmospheric Abundance: 100 (0-49) Tg CH4/yr

• Total Sinks: 500-798 Tg CH4/yr (Bottom-up view) 501-574 Tg CH4/yr (Top-down view)

• Emissions and Sinks (Tg CH4 yr):

  • Wetlands: 113-154 (Bottom-up) / 81-131 (Top-down)

  • Agriculture and waste: 191-223 / 207-240

  • Fossil fuel production and use: 84 (26-40) / 30 (22-36)

  • Biomass and biofuel burning: 37 (11-49) / 23 (14-30)

  • Other natural emissions: 59 (48-74) / 37 (11-49)

  • Sink from chemical reactions in the atmosphere: 489-749 / 474-532

  • Sink in soils: 41 (21-50) / 32 (23-45)

Methane by Source (2023)

• Anthropogenic:

  • Energy

    • Oil: 10

    • Natural Gas: 20

    • Coal: 50
      *Bioenergy: 5

  • Waste: 60

  • Agriculture: 130

• Natural

  • Wetlands 180

  • Other: 15
    Biomass burning 16.

Major Greenhouse Gases:

• Ozone (tropospheric) (O3)

  • Product of photochemical reactions in the atmosphere

  • Concentrated around cities in warmer months

  • Short residency time

• Nitrous Oxide (N2O)

  • Increased fertilizer use, transportation, industrial processes

  • Long residency time ~114 years

• CFCs (chlorofluorocarbons)

  • No natural sources / long lasting ~100 years

  • Also deplete stratospheric ozone

Greenhouse Gas Overview

• CO2: 79.7%

• CH4: 11.1%

• N2O: 6.1%
  *HFCs, PFCs, SF6 and NF3: 3.1%

Selected Greenhouse Gases

Changes to Albedo

• Albedo = the ability to reflect light energy (0 = absorbs 100%; 1 = reflects 100%)

Surface Albedo

Forest Conversion

• Conversion of Forests to Agricultural Use has an overall cooling effect (a negative radiative forcing)

  • Generally higher albedo

  • Less transfer of latent heat (evaporation) to atmosphere.

Sulphate Aerosols

• Scatter incoming solar radiation back to space Reflection of incoming solar radiation back to space Affect cloud formation and reflectivity: “Clean” cloud without sulphate aerosols vs “Polluted” cloud with sulphate aerosols (cloud is more reflective)

Clouds

• Clouds cool the Earth by reflecting incoming sunlight

• Clouds warm the Earth by absorbing infrared radiation emitted from the surface and reradiating it back down.

• Clouds and aerosols interact

• Currently net cooling, but could become net warming

Climate Sensitivity

• Climate sensitivity is a measure of how responsive the temperature of the climate system is to a change in radiative forcing.

• It is usually expressed as the temperature change associated with a doubling of the concentration of carbon dioxide in Earth's atmosphere.

• The IPCC Sixth Assessment Report stated: Equilibrium climate sensitivity is very likely in the range 2.0°C to 4.0°C (high confidence).

Thermal Inertia

• Thermal inertia by the world ocean Increases in atmospheric CO2 do not lead to an immediate increase in temperatures. It takes time for a new equilibrium to be reached - there is a climate lag Much of this lag can be attributed to the thermal inertia of the world oceans

• This means that even if GHG emissions were reduced immediately, the earth would likely continue to warm for another century or more.

Ocean Temperatures

• Record ocean temperatures put Earth in ‘uncharted territory’, say scientists.

Global Warming

• Ocean: 93.4%

• Continents: 2.1%

• Atmosphere: 2.3%

• Glaciers and ice caps: 0.9%

• Arctic sea ice: 0.8%

• Greenland Ice Sheet: 0.2%

• Antarctic Ice Sheet: 0.2%

Climate Feedback Systems.

• A little push can go a long way.

Climate Feedbacks include

• Vanishing artic ice --> Temperatures rise --> As reflective ice disappears, darker ocean waters absorb more heat --> Arctic sea ice melts --> (repeats).

• Melting permafrost --> Temperatures rise --> Permafrost bogs thaw --> Bogs release methane, a greenhouse gas 25 times more powerful than CO2 --> (repeats).

• Clogging the ocean sink --> Temperature rise --> Warming waters shut down exchange of CO2 from the surface to the deep ocean --> Ocean waters dissolve less CO2 --> (repeats).

• Amazon rainforest die-off --> Temperature rise --> Rainfall decreases --> Drought and wildfires increase in the Amazon --> (Fewer rainforest trees mean less water is pumped into the atmosphere) More trees die and decompose, or burn, releasing CO2 --> (repeats).

Climate feedback systems

• Closer look at some of the previous.

Positive and negative feedback mechanisms

• Negative feedback

  • More CO2 à more plant growth à more carbon storage

  • Retreating Ice Shelves à more open water à increased phytoplankton à more Carbon storage

  • Increased surface temperature à increased LW radiation to space.

• Positive (amplifying feedback)

  • Warmer temperatures à melting permafrost à more CH4 released

  • Melting snow à lower albedo à more melting

  • Higher temperature à more evaporation à more H2O in atmosphere

  • Warm oceans absorb less CO2 , which leaves more in Atmosphere à warming

Stefan–Boltzmann law

• Describes the energy radiated from a black body in terms of its temperature.

• … specifically, the total energy radiated per unit surface area per unit time, is directly proportional to the fourth power of the black body's temperature T.

• $j* = σT^4$

Global Warming Mechanics

• Includes Melting polar ice caps, decreases in oceanic salinity, melting of Greenland/ Antarctica, Loss of solar reflectivity, Sea level rise, Temperature change which can then affect thermohaline currents which has a state change in global climate. All of which is related to Thaw of northern permafrost and increase atmospheric methane thanks to humans burning fossil fuels which also increases atmospheric carbon (greenhouse gases).

Climate Change Evidence

• Temperature anomaly

Computer Models and Data Comparison

• Models including only natural factors do not match observations.

• Models including anthropogenic factors match observations more closely.

IPCC Summary

• It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred. The scale of recent changes across the climate system as a whole – and the present state of many aspects of the climate system – are unprecedented over many centuries to many thousands of years.

IPCC Findings

• Human-induced climate change is already affecting many weather and climate extremes in every region across the globe.

• Evidence of observed changes in extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has strengthened since AR5.

Indisputable Evidence

• The atmosphere and ocean have warmed.

• The amounts of snow and ice have diminished.

• Sea level has risen.

• The concentrations of greenhouse gases have increased.

ppm vs Temperature

• Increasing temperature and increasing CO2 levels track together.

Declining Glaciers and Ice Caps

• Glaciers, Ice Caps, Ice Shelves, and snowpacks are declining across the globe.

• The graph shows cumulative average mass balance of reference glaciers around the world as measured by the World Glacier Monitoring Service.

The Melting Snows of Kilimanjaro

• Data ranging from 1912 to 2002 show significant glacial ice loss.

Retreating Glaciers

• Glaciers in the Andes, Alps, Himalayas, and the Rockies are rapidly retreating…

• Argentina’s Upsala Glacier retreating ~ 200 m / year

• Pasterze Glacier, Austria has almost disappeared.

Arctic Impacts

• The impacts of Global Warming are greatest in the Arctic

  • The average temperature rise in the Arctic is about twice the global average.

  • Rapid ecosystem changes

  • Lost of summer ice in Arctic

  • Polar bears extinct by 2050?

  • Tree line moving north

  • Summer shipping lanes are becoming available

• The Antarctic and Greenland ice sheets hold enough water to raise sea level ~ 70 meters

Arctic Ice Loss

• Loss of artic ice - new record set

• Arctic Ice free within 20 years??

• 2020 Arctic Sea Ice Minimum at Second Lowest on Record (Lowest was 2012)

Global Warming Variation

• The amount of global warming varies…

Potential Impacts of Global Warming and Climate Change

• Droughts and Floods

• Climate Change will impact world food security

• Increase in Percentage Change in Yields from Present and 2050

• Increased storm intensity and destructiveness

• Global Warming Will Bring Violent Storms And Tornadoes, NASA Predicts ScienceDaily (Aug. 31, 2007)

• Greater spread of infectious diseases

  • As temperatures rise, diseases once limited to tropical regions are showing up further north and south of the equator.

    • Hantavirus

    • Malaria

    • Dengue Fever

• Sea Level Rise

  • Melting glaciers, ice caps, ice sheets, etc.

  • Thermal expansion of water

  • Estimates range from 10 to 140 cm or more by the end of the century

    • Some island nations are unlikely to survive. – Tuvalu: mean elevation ~ 4.5 m asl.

  • Threatens hundreds of millions globally

• Extinctions: many plants and animals cannot move or adapt fast enough

  • There are serious threats to numerous species

  • Entire ecosystems are at risk …we predict, on the basis of mid-range climate-warming scenarios for 2050, that 15–37% of species in our sample of regions and taxa will be 'committed to extinction'. Thomas et al. Nature 07 Jan 2004

• All sorts of other bad things are happening that are likely related to global warming…

  • Increasing desertification

  • Acidification of the world ocean

  • Saltwater intrusion in coastal areas

  • Large declines in forests and more wildfires

  • Thawing of permafrost & drying up of peat bogs (which releases more methane)

  • Increased unrest, food riots, wars, etc.
    *More feedback leading to global warming

Temperature Change

• Temperature change (°C) above preindustrial

Mitigation

• Reduce our “carbon footprint”

  • Develop alternatives to fossil fuels

  • More efficient use of energy

    • Better industrial processes

    • Improved transportation

    • Live closer to work – better cities

    • Cap and Trade schemes

  • Consume Less Meat; major change to lifestyle.

  • Save forests, plant more trees
    *Adapt to a new global reality…

  • Less Water, Less Food, Higher Prices

  • More Floods, More Droughts

  • More Severe Storms

  • More Disease

  • Loss of ecosystem services

  • Displaced people and climate refugees There is the potential for catastrophe by the middle of the century.
    *Geoengineering…

  • Can we use technology to save the planet?

    • Fertilize areas of the sea with iron to increase CO2 uptake by phytoplankton which would then sink to the ocean floor.

    • Injecting sulfate aerosols into the stratosphere

    • Create “artificial trees” which would absorb CO2 directly from the atmosphere

    • Use satellites to release reflecting particles around the earth

Climate Change and the Ozone Hole

• Ozone-depleting gases are also potent greenhouse gases

• As heat is trapped in the troposphere, the stratosphere becomes colder, creating conditions more conducive to ozone depletion

• Changes in Stratospheric temperatures are impacting weather patterns in the southern hemisphere

Solar Constant

• Kiehl and Trenberth, 1997: Earth’s Annual Global Mean Energy Budget, Bull. Am. Met. Soc. 78, 197-208. (see also: Trenberth 2009 for updated model) solar constant = 1368 $W/m^2$ at the top of the atmosphere

El Niño & La Niña

• Changes in ocean water temperatures have global impacts

El Niño /La Niña Southern Oscillation (ENSO)

• An El Niño is a major warming in the Eastern Tropical Pacific Ocean

• Has dramatic effects on the world’s climate and weather systems

• Occurs about every 2 to 7 years and is characterized by a weakening or reversal of normal circulation patterns (Walker Circulation)

• Characterized by a negative “Southern Oscillation Index” SOI SOI = (Tahiti Sea Level Pressure) - (Darwin Sea Level Pressure)

• Note: There are other demonstrated and hypothesized climate oscillations: North Atlantic Oscillation (NAO), Interdecadal Pacific Oscillation (IPO), etc.

Southern Oscillation Index

• A measure of the difference in sea level air pressure between Tahiti and Darwin, Australia If SOI is negative (TSLP < DSLP) it is an indication that an El Nino is forming.

Normal Conditions in the Pacific Ocean

• Atmosphere The Southern Oscillation Index Impacts the distribution of warm surface waters in the Pacific Ocean .

Walker Circulation

• Surface: westward trade winds Upper troposphere: eastward winds Western Pacific – Warm / Moist – Low pressure – Stormy Eastern Pacific – Cool / Dry – High pressure Tranquil Cold water upwelling along the coast of South America

El Niño Conditions

• Low pressure moves to the tropical Central and Eastern Pacific High pressure forms over the tropical Western Pacific The trade winds weaken or reverse over the Pacific The West Pacific becomes drier (droughts common) The Central and East Pacific become very stormy -- Tends to promote warming

ENSO's Cool Phase (La Niña)

• Generally follows a strong El Niño event A major cooling of the east tropical Pacific Ocean An overly-strong Walker Circulation / strongly positive SOI Correlated with a greater number of tropical storms