Atmospheric Processes, Hazards and Management Structure and Composition of the Atmosphere

Large scale sea surface temperature changes across the tropical Pacific. The El Nino event is when a warm countercurrent flows southward along the coasts of Ecuador and Peru in contrast to the normal cold Peruvian current. A strong change occurs every 3 to 7 years, but milder ones can occur at the end of every year.

1. 
   The El Nino Event
   
   1. 
      The El Nino/Southern Oscillation (ENSO)
      

• 
  Sir Gilbert Walker identified an irregular oscillation between atmospheric pressures in Tahiti in central equatorial Pacific and Darwin
  
• 
  Tahiti normally has higher atmospheric pressure than Darwin, but during El Nino events, barometric pressure drops over large parts of southeastern Pacific, while in the western side pressure rises. When the event ends, pressure reverses
  
• 
  The seesaw pattern is the ENSO
  

1. 
   The Walker Circulation
   

• 
  The difference in air pressure causes air to move from subtropical high pressure cell over the southeastern Pacific to a low pressure area over the western Pacific
  
• 
  The ENSO changes the strength of the Pacific trade winds and the circulation of the oceanic currents
  

1. 
   The Walker Circulation During Normal Period
   

• 
  The northeast and southeast tradewinds blow towards the west, creating a frictional drag on ocean currents, resulting in a westward surface current
  
• 
  Pooling of warm surface water occurs around the west equatorial Pacific, about 100m deep. Piling causes water level to rise 40cm higher than in the eastern Pacific
  
• 
  Pooling causes the thermocline (boundary between warm surface water and cold subsurface water) to be much deeper in the western Pacific than the eastern Pacific
  
• 
  Warm water leads to higher temperatures and lower surface air pressure, intensifying convective rainfall in western Pacific.
  
• 
  On the eastern Pacific, the warm water migrating westward is replaced by cold water from below. The southeasterly trade winds cause the cold Peruvian current rich in plankton and fish. This combination results in a shallow thermocline (40m) and a shallow inversion, providing positive feedback, strengthening the trade winds
  
• 
  Western counterflow at 200mb results in subsidence over South America
  

1. 
   The Walker Circulation during an El Nino Event
   

• 
  Barometric pressure rises in Indonesia and subsides in South America, causing the pressure gradient to weaken or reverse
  
• 
  The weakening of the trade winds causes the warm pool to move gradually eastward, also due to the higher water level, causing Kelvin waves
  
• 
  Warm water influx covers the nutrient rich cold water, preventing upwelling, cutting off nutrients to plankton, causing a biological starvation chain
  
• 
  A south flowing warm current replaces the north flowing Peruvian current. This countercurrent is termed the El Nino, and increasing temperatures of about 1-4^oC further weakens the trade winds, increasing warm water movement and decreasing cold water upwelling. Positive feedback.
  
• 
  Thermocline is depressed, generating a positive feedback loop as cold water is prevented from upwelling
  
• 
  El Nino takes 3-6 months to reach local peak intensity. Stronger Hadley Cell activity when pressure situations return to normal eventually terminates the El Nino
  

2. 
   Past Occurrences of El Nino and Future Prediction
   

• 
  Development and decay of El Nino takes place over 12-18 months. El Nino itself occurs roughly every 3-4 years
  
• 
  Identifying development of ENSO is normally done by monitoring surface temperatures across the tropical Pacific. Unusually warm waters over the eastern Pacific indicate a strong El Nino, while colder than normal waters signify a La Nina.
  
• 
  Computer forecasting models based on sea temperature have been created to attempt to predict ENSO and its effects. One more successful one is the Lamont-Doherty Geological Observatory at Columbia University in New York. The ENSO of 1982-83, 1986-87, 1989-90 and 1991 were predicted 8 months ahead
  
• 
  However, ENSOs are largely unpredictable, especially with regards to magnitude, since the 1990 ENSO peaked in early 1992, weakened suddenly and strengthened again
  
• 
  Consequences are even harder to predict. Regardless, efforts are important as we need to take steps to manage climate change. Global warming could intensify ENSO events.
  

3. 
   Impacts of El Nino
   
   1. 
      Economic Impact on Peru and Ecuador
      

• 
  The cold upwelling Peruvian current encourages nutrients which are food source for plankton and millions of fish, especially anchovies. These fish support birds whose guano are mined for fertiliser. The cessation of cold water during an El Nino harms the local fishing industry due to the loss of fish, as well as the fertiliser industry.
  
• 
  Some inland areas, normally arid, receive unusual amounts of rain, and pastures and cotton flourish, yielding far more than normal
  

1. 
   Worldwide Weather Anomalies
   
   1. 
      Effects of the 1992-1993 El Nino
      

• 
  Heavy rains and flooding occurred in normally dry parts of Ecuador and Peru, up to 350cm from 10-13cm of rain per year
  
• 
  A warm winter and a very wet spring occurred for the United States. California was struck by storms, causing landslides, floods and coastal erosion. Heavy snowing in Sierra Nevada and mountains caused mudflows and flooding in Utah, Nevada and Colorado River
  
• 
  This ENSO caused up to 2000 deaths and $13 billion worth of damage
  

1. 
   Effects of the 1997 El Nino
   

• 
  Strongest since 1983. Severe droughts in Indonesia, worst in 50 years. Associated with widespread forest burning in Sumatra and Kalimantan, causing large amounts of smoke pollution. KL and Kuching airports closed, schools closed. 50000 people ill due to smog. 2 ships collided in Straits of Malacca due to poor visibility, 29 died. Garuda Airlines plane crashed near Medan in Sumatra, killing 234.
  
• 
  Hurricane Pauline on 9 October in Acapulco (south Mexico). Severity of storm attributed to El Nino peak causing instability and high sea temp
  
• 
  Southern California: winter storms and mudslides
  

1. 
   Teleconnections
   

• 
  Weather anomalies can be induced worldwide due to teleconnections: linkages over great distances of atmospheric and oceanic variables
  
• 
  Linkage between eastern and western Pacific: typical teleconnection
  
• 
  ENSO related with lower monsoon rain over South Asia. Parts of India experience more droughts during El Nino e.g. 1888: 1.5 million people died due to famines caused by droughts
  
• 
  Relations between ENSO and extreme weather regimes around the globe
  
• 
  Widespread and variable – ENSO has effects on global weather
  

When an area experiences a prolonged period of below average rainfall, drought conditions can prevail.

1. 
   Types of Droughts
   

• 
  Generally refers to an extended period of rainfall deficit when agricultural biomass is severely curtailed
  
• 
  Meteorological Drought – shortage of rainfall. Britain – used to be 14 consecutive days without rain. No direct ecological/economic impact, no effective human response
  
• 
  Climatic Drought – longer period of little or no rainfall when rainfall is expected
  
• 
  Hydrological Drought – water levels in rivers and ground are well below expected levels - a period of low flows due to meteorological drought and lack of groundwater recharge. Responses come from authorities, involve managing of water
  
• 
  Agricultural Drought – impact of reduced precipitation results in crop failure. Direct responses at a national level, involving compensation and other measures
  
• 
  Famine Drought – most severe, deaths from starvation, response at international level
  

2. 
   Causes of Droughts
   

• 
  Some climatic regimes have less predictable rainfall, high variability. Agricultrual, industrial and domestic water supply planning becomes more difficult. Increasing demand may cause supply problems during below average/average conditions
  
• 
  El Nino can have major effects in rainfall in Australia, Indonesia, parts of west USA
  

3. 
   Managing Droughts
   
   1. 
      Drought Prediction and Forecasting
      

• 
  Many strategies depend on preparedness. Climate studies to predict possibility of droughts relies on data from precipitation, temperature, evapotranspiration, soil moisture, stream flow, groundwater, reservoir and lake levels.
  
• 
  Assessment of probability and recurrence of droughts, intensity of droughts, duration, spatial extent, risks and potential effects on agriculture
  
• 
  Provide information enabling and persuading people and organisation to maximise successful crop production, minimise damage to crops and other assets
  
• 
  Monitoring El Nino and Sea Surface Temperature anomalies – 1988 British predicted rain in Sahel due to alteration in ocean currents
  

1. 
   Drought Mitigations
   

• 
  Short and long term actions, programs or policies implemented during and in advance of droughts, reducing degree of risk to lives, property and production
  
• 
  Mitigated by increasing water supply – drought reserve dams, large house and garden storage for stock – ideally provide for stock and domestic use.
  
• 
  Changing practices that can worsen drought effect – efficient use of water
  
• 
  Conservation of soil and water and managing livestock – herd reduction
  
• 
  Mitigating agricultural impact – low tech: stone bunds to reduce overland flows, small earth dams to store water. High tech: drip and sprinkle irrigation, fertiliser use, mechanisation of farming
  

1. 
   Response to Drought
   

• 
  Actions taken once drought has occurred – address impacts, expedite recovery. Resettlement, food, hygiene, medical care
  

1. 
   Drought Management in DCs/LDCs
   
   1. 
      Management in DCs
      

• 
  Droughts disrupt lifestyle and economic activity, but access to capital makes in mere inconveniences in comparison
  
• 
  Reservoirs can be enlarged, extracted from rivers, improved distribution system by reducing leaks in pipes or linking supplies in different areas
  
• 
  Desalination plants – Saudi Arabia
  
• 
  1986-1993 drought in California – 55-75% of normal rainfalls, tensions between conservation, agriculture and urban land use
  
• 
  Water authorities ordered 25% reduction in water use in SF
  
• 
  Pasadena city – free installation of water saving devices
  

1. 
   Management in LDCs
   

• 
  Resources are less readily available. Improving water supplies only possible through international action e.g. Ethiopia.
  
• 
  UN Disaster Relief Office (UNDRO) and Red Cross
  
• 
  UN Education, Scientific and Cultural Organisation (UNESCO) involved in dam construction financed by World Bank
  
• 
  Short term measures – pump groundwater, which can lead to concentration of population and grazing animals around the new supply
  
• 
  Lake Kariba on Zambezi and Lake Nasser on Nile - damming
  

4. 
   Case Study of Droughts
   
   1. 
      Drought in the Sahel
      
      1. 
         Natural Causes
         

• 
  Semi-arid area, rainfall has high seasonality and variability. Possible reasons include El Nino (2003, International Institute for Climate Prediction found evidence of a link), also global dimming (air pollution from Western countries driving tropical rain belt in Sahara further south, 50% decline in rainfall in Sahel) causing cooling sea surface temperatures. Affected 1968-1974, Mauritania and Ethiopia
  
• 
  Feedback mechanisms – decreasing precipitation reduced plant growth, reducing evapotranspiration, decreasing moisture content in the atmosphere, reducing cloud cover, soil surface dries out and vegetation reduced, increasing ground heating.
  

1. 
   Anthropogenic Causes
   

• 
  Drought occurred concomitantly with increase in population and deleterious economic conditions. 1970s – fuel prices soared, higher demand for wood for cooking, rapid harvesting of trees. Diminishing crop yields, less land used for crops, reducing soil moisture. Plowing caused surface crusts, preventing infiltration.
  
• 
  Droughts are getting more frequent – now every 5 years or less in Ethiopia. 1984-85 killed a million Ethiopians
  
• 
  LDC governments are more inactive, not economically able to develop mechanisms for drought mitigation and alleviation. International relief plays a more vital role
  
• 
  UN Office for Emergency Operations in Africa (UNOEOA) channelled US$4.5 billion from 35 countries, 47 NGOs to northern Africa, saving 35 million
  
• 
  Supplying aid: increase in volume of goods may not be efficiently handled due to inefficient or non-existing transport infrastructure. Grain to Ethiopia in 1980s. Isolated area = no transport network.
  
• 
  LDCs may be corrupt or inefficient, siphoning aid for export or black market
  
• 
  Inefficient administration slow relief aid with paperwork
  

1. 
   Drought in Australia
   

• 
  Farmers rely on technology and efficient land management. Maximise return, minimise permanent damage
  
• 
  Land management -> moisture conservation and crop protection, mulching, tillage, contour banks, minimise runoff. Catchments cleared and compacted to maximise runoff into farm dams used for irrigation during droughts
  
• 
  Pasture is set aside in spring to be used for hay when dry. Grain is held over to supplement feed. When persistent drought, stocks are culled or shipped overseas
  
• 
  Technological advances – aerial spraying of insecticides, aerial seed sowing, monitoring, computer management models to maximise crop yields
  

5. 
   Droughts and Desertification
   

• 
  Desertification refers to land degradation, loss of soil fertility. Often caused by droughts. Semi-arid and sub-humid areas face highest risk.
  
• 
  Sahel – desertification led to expansion of the south edge of the Sahara, largely attributed to overexploitation of resources by an increasing population – accelerated soil erosion and deforestation
  
• 
  Removal of vegetation cover causes loss of fertile topsoil and surface crusting, reducing productivity and degradation. Sahel, also human activity like nomads and herds, built near water, exacerbates overgrazing and compaction of soil
  
• 
  Irrigation from groundwater causes water table to rise – Murray Valley in SE Australia, rose by nearly 10m from 1962-1974. Once saline groundwater within 1-3m of ground surface, salt crusts accumulate, degrading the land.