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  Key Idea 4 continued: What are the causes of low-pressure and high pressure hazards?  
     
 

In this lesson: High pressure system formation and associated hazards of drought in tropical climates (or drought, frost and fog in temperate climates.)

  
     
 

  • In High Pressure Systems - Low Rainfall, High Evaporation Rates, Drought and sometimes High Winds.
     

  • Causes – these hazards are associated with anticyclonic conditions which are due to the continued persistence of the subtropical high pressure over continental areas limiting the ITCZ zone to lower latitudes (nearer the Equator) than is normal for the time of the year. Global warming is now recognised as also a contributory factor.
     

  • Triggering secondary hazards of a falling water table, loss of vegetation, wild fires, soil erosion – desertification.

  
     
  Classnotes  
     
  Descending Limb of the Hadley Cell creates a zone of permanent  High Pressure over the Sahara Desert  
 

 
  Areas dominated by high pressures are always dry. A long term high pressure in a region which is not expecting it will lead to a drought i.e. a lack of the expected rainfall. There is no one definition of a drought it varies from place to place. In drier places drought are often longer term climatic hazards (rather than a short term weather hazard) as they may last several years. the expected riany season may fail to arrive for several years consecutively so that the hazard to people gradually intensifies..  
     
  In some years a 'Blocking' super big High Pressure spreads out from the usual Azores 'High' over the Sahel too.  
 

 
  In the Sahel and the dry Savanna of northern Nigeria there is usually rainfall in the high sun season (summer). however, some years a 'Blocking High' keeps the ITCZ 'out' from advancing as far north leading to drought.  
 

 
  The High Pressure over the Azores spreads and in some years is much spreading across North Africa.  This is a 'blocking high' preventing the usual northward spread of the ITCZ over the west African in the northern hemisphere summer. (also see below)  
     
     
  El Nino Events - can cause droughts in Australia, Indonesia, southern Africa and Brazil.  
     
  The Location of the Sahel Region  
 

 
  Variability of Rainfall.  
  Rainfall is not only relatively low in the Sahel, nor is it only very seasonal (falling in the high sun 'summer' season) - it is also very variable. The total amount can vary massively from one year to another. Also the amount received can vary markedly over a relatively short distance.  
     
  Periodic climate change  
  We have seen that the periodic changes are linked to changes in sea surface temperatures in the Gulf of Guinea (and further into ocean atmosphere links like El Nino). The changes gradually build up over a number of years.  
 

 
  Source: Woolliscroft  
     
  The northern hemisphere summer Trade Winds off the sea are weaker When the Gulf of Guinea is warm, the Intertropical Convergence Zone shifts south away from the Sahel reducing the African monsoon that draws moist air into the Sahel. Longer term changes in rainfall from decade to decade are forced by changes in sea-surface temperature in the western Indian and tropical Atlantic oceans. When these areas are cool, Sahel rainfall increases.  
 

 
  Source: http://oceanworld.tamu.edu/resources/environment-book/desertificationinsahel.html   
  The sea surface temperature in the Gulf of Guinea may be warmer and so the heat difference between the hot land and the cooler sea is reduced so the 'monsoonal ' type trade winds are weaker. The result is  a drought in the northern Savanna / Sahel region.  The vegetation dies back and so less water is released by transpiration. More sunlight is reflected back off the surface.  The air is drier and convectional uplift is not fed by latent heat released as moisture is not condensing. The drought is intensified.

Rains return when the Gulf of Guinea is cooler, and rainfall shifts north watering the Sahel. Evaporation from the land increases, less sunlight is reflected, and the African monsoon strengthens

  
     
  The oceanic forcing of Sahel rainfall is amplified by land-atmosphere feedbacks. As the land dries out, there is less vegetation, less evaporation from the land, and more sunlight is reflected from the land. These processes further weaken the monsoon. This positive feedback also involves land degradation due to human interactions with the land.  
 

 
  Source: http://oceanworld.tamu.edu/resources/environment-book/desertificationinsahel.html  
     
  Human causes of the drought hazard - desertification  
  Drought as a slow onset hazard going through stages of from meteorological to agricultural to hydrological to socio-economic drought. Explain how the physical and huamn causes and effects are completely interlinked.  
 

 
  Source: http://www.eea.europa.eu/themes/water/water-resources/figures-and-maps/drought-impacts-1/image_preview  
     
  Talk through the diagram below to show human action can affect the ecosystem leading to desertification which in turn changes the environment having adverse hazardous effects for people.  
 

 
     
  Diagram to explain the desertification process:  
 
A natural ecosystem with a seasonally dry climate able to support some drought resistant trees like Park Savanna with baobab trees. Perhaps a longer period of wetter years has 'greened' the environment.
During the wetter years harvests boomed and population growth followed. this created an increased demand for farmland and fuelwood so deforestation occurs.
Nomadic pastoralists like the Turaegs or Fulani tribes graze the land with camels, sheep and goats. Sedentary settled tribes like the Hausa grow crops like millet. The increased population puts pressure on the land especially in drought years. Overgrazing and overcultivation are occurring.
 
The land  stripped of its vegetation cover is degraded and when the summer rains come they cause severe soil erosion and gullying. In dry season wind erosion blows the dusty top soil away.
The albedo of the surface is changed.  The surface is light coloured rock which reflects the insolation. The atmosphere absorbs the reflected heat and air temperatures rise. The hot air can hold more vapour so there are few clouds and little likelihood of rain. Desertification has occurred.
 
     
  Natural Causes of High Pressure hazards, Man-made causes of High Pressure hazards: Direct and Indirect Hazards  
  Complete the Dustbin game. Then try to redraw the dustbins as a table seeing how many labels you can remember for each box.  
  Scores: 1 label per box = rather pathetic; average of 2 labels / box = weak; 3 = average; 4 = good; 5+ = almost superhuman  
  You can think of labels of your own of course (as long as they are agreed as relevant.  
   

Click here for full screen version

  
  Resource made using: http://classtools.net/widgets/dustbin_5/7HYBn.htm  
     
  The connection between Wet and Dry Years in The Sahel and Hurricane magnitude.  
 
African easterly waves vary from year to year, and these variations have been found to have an impact on the seasonal hurricane frequency. Increases in rainfall over the western Sahel have been associated with more frequent and stronger waves, and with the occurrence of more intense Atlantic hurricanes.

 

Source: http://www.meted.ucar.edu/meteoforum/tropwaves/print_version/04-African_wave_model.htm#a6

 
     
  Essays:  
 

Assess the relative importance of the physical and human causes of hazards associated with high pressure systems in your chosen climatic region. (20) Legacy Jan 2009 and Jan 2007

 

Assess the relative importance of direct and indirect impacts brought by high pressure hazards in either Tropical or Temperate regions. (GeogOnline)