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West Africa Meteorology Lecture notes3


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This is an introduction to West Africa Meteorology

This is an introduction to West Africa Meteorology

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  • 2. 5.3 EL-NINO SOUTHERN OSCILLATION (ENSO) AND LA-NINA5.3.1 El-Nino• El-Nino (EN), Spanish name for “the little boy” refers to the “Christ child”.• This name is given therefore to a phenomenon which is usually noticed around Christmas time along the coast of Ecuador and Peru and extending across central tropical Pacific Ocean.• El-Nino is a global coupled ocean-atmosphere phenomenon and can be defined as periodic sustained sea-surface temperature anomalies (increase) of magnitude greater than 0.50C across the Central Pacific Ocean. It occurs between 800W to 1500E and 50N to 50S• When this condition is met for a period of less than five months, it is classified as El-Nino conditions and if the anomaly persists for five months or longer, it is classified as an El-Nino episode.• Every three to seven years, an El-Nino event may last for many months. Historically, it has occurred at irregular interval of 2-7 years and has usually lasted for one to two years.• Fig. 47 gives the annual mean global sea-surface temperatures.5.3.2 Southern Oscillation• The atmospheric signature, the Southern Oscillation (SO) reflects the monthly 2
  • 3. Fig. 47: Annual Mean Global Sea Surface Temperatures 3
  • 4. or seasonal fluctuation in the air pressure difference between Tahiti and Darwin, Australia.• Under non El-Nino (or normal) conditions a high pressure exists around Tahiti while low pressure prevails at Darwin, Australia.• However, in El-Nino condition the reverse is the case for low pressure exists at Tahiti while high pressure at Darwin.• This sea-saw of pressure fluctuation is termed Southern Oscillation (SO) and its linkage with El-Nino gave birth to the name EL-Nino Southern Oscillation (ENSO).• These effects were first described in 1923 by Sir Gilbert Walker from whom the Walker Circulation, an important aspect of the Pacific ENSO phenomenon, takes its name.5.3.3 La-Nina• La-Nina (female child in Spanish) refers to an anomaly of unusually cold sea surface temperatures found in the eastern tropical Pacific Ocean.• It is the opposite to or counterpart of El-Nino and occurs roughly half as often as El-Nino.• When the conditions (i.e. sustained sea-surface temperature anomalies (decrease) of magnitude greater than 0.50C across central tropical Pacific Ocean 4
  • 5. is met for a period of less than five months, it is classified as La-Nina conditions.• If the anomaly persists for five months or longer, it is classified as La-Nina episode.• La-Nina condition often follows (but not always) the El-Nino, especially when the latter is strong.• La-Nina conditions typically last for 9-12 months but may persist occasionally for as long as 2 years.5.3.4 Atmospheric Circulation Under Normal Condition• Under ordinary (i.e. normal or non El-Nino) condition, there is a general upwelling of cold water around the coast of Peru (Fig. 48).• High pressure there exists over Tahiti around the South Eastern Pacific Ocean and lower pressure prevails at Darwin, Australia near Indonesia in the western Pacific Ocean (Fig. 49).• This produces easterly trade winds along the equator and the winds promote upwelling and cooler ocean water in the eastern Pacific.• The trades are part of a circulation that typically finds rising air and heavy rain over the western Pacific and Indonesia and sinking air and generally dry weather over the eastern Pacific Ocean around Peru & Ecuador. 5
  • 6. Fig. 48: Walker Circulation underNormal Condition 6
  • 7. Fig. 49: Typical Walker CirculationSystem under Normal Condition 7
  • 8. 5.3.5 Atmospheric Circulation under La-Nina Conditions• In La-Nina conditions the above mentioned easterly trade winds become exceptionally strong, water along the equator in the eastern Pacific Ocean become quite colder than normal.• Higher than normal pressure exists over Tahiti around south eastern Pacific Ocean and lower than normal pressure prevails at Darwin, Australia and near Indonesia in the western Pacific.• The stronger easterly trade winds strongly drive the surface water westwards.• The surface water becomes progressively much warmer going westward and leads to stronger rising air and heavier rain over western Pacific and Indonesia and stronger sinking air and drier than normal weather over eastern Pacific.5.3.6 Atmospheric Circulation under El-Nino Conditions• The first signs of an El-Nino are:  Rise in air pressure over the Indian Ocean, Indonesia and Australia in Western Pacific.  Fall in air pressure over Tahiti and the rest of the Central and Eastern Pacific Ocean.  Trade winds in the South Pacific weakens or head east. 8  Warmer air rises over Peru, causing rain in the Northern Peruvian deserts.
  • 9. Fig. 50: Typical Walker CirculationDuring an El-Nino Condition 9
  • 10.  Warm water spreads from the West Pacific and the Indian Ocean to the east Pacific and eventually reach the South American Coast.  Height of the ocean surface drops over Indonesia and rises in the eastern Pacific forcing the thermo-cline lower near south America and preventing upwelling.5.3.7 Impacts of El-Nino Southern Oscillation (ENSO) on Global Climate5.3.7.1 Effect on Precipitation and Wind Patterns• During El-Nino year, tropical rains usually centered over Indonesia shift eastwards, influencing atmospheric wind patterns worldwide.• This shift in the large tropical rain clouds alter the typical pattern of the Sub- Tropical Jet stream location, storm tracks and monsoons, producing unseasonable weather over many regions of the world.• From June to August drier conditions occur in parts of South-East Asia, India and West Africa (weak monsoon). In Northern Australia there is dramatic increase in bush fires and worsening haze and decreasing air quality and also in Queensland, Inland Victoria and Tasmania.• In North America, typical winters are warmer than normal in the upper Midwest States and the Southwestern US are wetter than normal. 10
  • 11. • East Africa (Kenya, Tanzania, White Nile basin) experiences the long rain from March-May, a wetter than normal situation.• Drier than normal condition also occurs from December-February in the South Central Africa (Zambia, Zimbabwe, etc.)• In the Atlantic, cases of double El-Nino event have been linked to severe famine related to extended failure of monsoon rains.• For example, during 1982-1983 El-Nino event, some of the abnormal weather patterns observed included:  Drought in Southern Africa, Southern India, Sri-Lanka, Philippines, Australia, Southern Peru, Cuba, US Gulf State.  Drop in fish catch for few months over Peru and Ecuador.  Hurricane in Tahiti and Hawai.  Flooding across the Southern United States. Effect on Temperature• In March, April and May, El-Nino causes warmer conditions in most of the tropics. The North-western coast of North America is also warmer than usual.• Between June and August, the heat signal is very clear in India, West Africa and Eastern South America. Summer in East Asia and Eastern Canada is often somewhat cooler than normal. 11
  • 12. • In September-November, the effects of El-Nino are strongest.• El-Ninos are of different intensities and the most intense El-Nino is that of 1998. The table below gives global impact of five major El-Nino events.5.3.8 Impacts of ENSO and La-Nina on West African Precipitation• West African precipitation is associated with the summer monsoon just like India and Asia though of lesser degree.• Therefore, the following impacts are noted: Good positive Equatorial Atlantic sea-surface temperature (SST) gives enhanced rainfall over south of 10°N but reduced rainfall in the Sahel. El Nino reduces Sahel rainfall while La Nina increases Sahel rainfall. Cold SSTs off Guinea coast implies wetter Sahel while warm SST off Guinea coast implies drier Sahel. This however is not applicable everywhere in West Africa. For instance, westermost Sahel is better with tropical North Atlantic SSTs.• The impacts of five major El-Nino events over Sub-Saharan Africa are shown in the table below: 12
  • 13. The impacts of five major El-Nino events over Sub-Saharan AfricaRegion/Year 1877 – 1878 1899 – 1900 1972 – 1973 1982 – 1983 1997 – 1998Sub-Saharan Moderate Intense Intense Intense No Wx eventAfrica Drought Drought Drought Drought as Drought 13
  • 14. 6.0 DROUGHT6.1 INTRODUCTION• Drought is a normal part of climate that occurs in almost all the regions of the world. It is an extended period of months or years when a region notes a deficiency in its water supply.• There is no universally agreed definition of it but a definition which may be generally accepted is “severe water shortage” which requires a further definition of “shortage” or alternately the specification of the amount of water needed.• Water needs depends on the types and numbers of animal and plants communities using the water so that the concept of drought cannot be divorced from the use to which water is put.• Thus, if the minimum water need for a giving period of time is met by rainfall of a given amount, drought may be said to occur whenever the rainfall during that time is less than that amount.• The severity of drought is linked to the amount by which rainfall falls short of that requirement.• It can have a substantial impacts on the ecosystem and agriculture of the affected region. Although droughts can persist for several years, even a short, 14 intense drought can cause significant damage and harm the local economy.
  • 15. 6.2 CAUSES• Generally, rainfall is related to the amount of water vapour in the atmosphere, combined with the upward forcing of the air mass containing that water vapour. If either of these are reduced, the result is a drought.• This can be triggered by an above average prevalence of high pressure systems, winds carrying continental, rather than oceanic air masses (i.e. reduced water content), and ridges of high pressure areas form with behaviours which prevent or restrict the developing of thunderstorm activity or rainfall over one certain region.• Oceanic and atmospheric weather cycles such as the El Nino-Southern Oscillation (ENSO) of the Americas along the Pacific coasts and Australia make drought a regular recurring feature worldwide.• Human activity can directly trigger exacerbating factors such as over-farming, excessive irrigation, deforestation, and erosion adversely impact the ability of the land to capture and hold water.• While these tend to be relatively isolated in their scope, activities resulting in global climate change are expected to trigger droughts with a substantial impact on agriculture throughout the world, and especially in developing nations.• Overall, global warming will result in increased world rainfall. Along with 15
  • 16. 6.3 TYPES/STAGES OF DROUGHT• As drought persists, the conditions surrounding it gradually worsen and its impact on the local population gradually increases.• Droughts go through three stages before their ultimate cessation:  Meteorological Drought is brought about when there is a prolonged period with less than average precipitation. Meteorological drought usually precedes the other kinds of drought.  Agricultural Droughts are droughts that affect crop production or the ecology of the range. This condition can also arise independently from any change in precipitation levels when soil conditions and erosion triggered by poorly planned agricultural endeavours cause a shortfall in water available to the crops. However, in a traditional drought, it is caused by an extended period of below average precipitation.  Hydrological Drought is brought about when the water reserves available in sources such as aquifers, lakes and reservoirs falls below the statistical average. Like an agricultural drought, this can be triggered by more than just a loss of rainfall. For instance, Kazakhstan was recently awarded a large amount of money by the World Bank to restore water that had been diverted to other nations from the Aral Sea under Soviet rule. Similar circumstances also place their largest lake, Balkhash, at risk of completely 16 drying out.
  • 17. • These three droughts would lead to Socio-economic Drought which is associated with the demand and supply aspect of economic goods. This type of drought mainly occurs when the demand for an economic good exceeds its supply as a result of weather related shortfall in water supply.6.4 IMPACTS OF DROUGHT• The impacts of drought can be economic, environmental or social. Drought has a complex web of impacts that spans many sectors of the economy and reaches well beyond the area experiencing physical drought.• This complexity exists because water is significant to society’s ability to produce goods and provide services. The impacts are commonly referred to as direct and indirect.• Direct impacts include reduced crop, rangeland and forest productivity; increased fire hazards, reduced water levels, increased livestock and wildlife mortality rates and damage to wildlife and fish habitat. The consequences of these direct impacts illustrate indirect impacts.• Indirect impacts include a reduction in income for farmers and agriculturists, increased prices for food and timber, unemployment, reduced tax revenues because of reduced expenditures, foreclosure on bank loans to farmers and businesses, mitigation and disaster relief programmes. 17
  • 18. Economic Impacts• Many economic impacts occur in agricultural and related sectors, including forestry and fisheries, because of the reliance of these sectors on surface and subsurface water supplies.• In addition to obvious losses in yields in crop and livestock production, drought also brings increased problems with insects, plant and forest diseases and wind erosion.• The incidence of forest and rangeland fires increases substantially during extended drought which in turn places both human and wildlife populations at higher levels of risk.Environmental Impacts• Environmental losses are the result of damages to plant and animal species, wildlife habitat, air and water quality, forest and range fires, degradation of landscape quality, loss of biodiversity and soil erosion.• Some of the effects are short-term and conditions quickly return to normal following the end of the drought. Other environmental effects linger for some time or may even become permanent.• Wildlife habitat may be degraded through the loss of wetlands, lakes and vegetation. 18
  • 19. • The degradation of landscape quality, including increased soil erosion, may lead to a more permanent loss of biological productivity of landscape.Social Impacts• Social impacts involve public safety, health, conflicts between water users, reduced quality of life and inequities in the distribution of impacts and disaster relief.• Many of the impacts identified as economic and environmental have social components as well. Population mitigation is a significant problem in many countries, often stimulated by a greater supply of food and water elsewhere.• Mitigation is usually for urban areas within the stressed area, or to region outside the drought area.6.5 GLOBAL REALITIES• Global realities of drought could be noted in the disappearing of lakes, shrinking of seas, etc. as are given below:Lake Chad Lake Chad affecting Chad, Niger, Nigeria and Cameroon has shrunk by 95% from 25,000km2 in the mid-1960s to 1350km2 in 2001, corresponding with increased irrigation (local agricultural water use quadrupled between 1983 and 1994) and decades of depressed rainfall. 19
  • 20. Lake Nakuru• Lake Nakuru in Kenya has shrunk in area since the 1970’s from 48 to less than 37km2 today. Nearby forests are being cleared for farmland to feed a fast growing population, causing soils to erode and wash into the lake.• Failed urban sewage system and unregulated industrial effluent have polluted the lake. Heavy metals in the water, including lead, zinc, mercury, copper and arsenic, have been linked to flamingo die offs.Dead Sea• Dead Sea (in Israel, Jordan and Palestine) at 417 meters below sea level is the lowest place on earth and is falling by up to 1meter per year.• Much of the water that would otherwise feed it is tapped by Israel and Jordan for irrigation.• The Sea has shrunk in length since the early 1900s, from over 75 to 55 kilometers long and has split in two, with the southern basin turned into evaporation ponds for potash extraction.• The salty lake could disappear entirely by 2050, along with the 90 species of birds, 25 species of reptiles and amphibians, 24 species of mammals, and 400 plant species that live on its shores. 20
  • 21. Dal Lake• Lake Dal in India has shrunk from 75 km2 in 1200AD to 25km2 in the 1980s, to smaller than 12 km2 today.• Over the last decade the lake has dropped 2.4 meters in height. Other lakes in the Kashmir Valley are facing similar problems.Yangtze River Basin• Yangtze River Basin is in China. More than13,000 km 2 of lakes in the middle and lower reaches of the Yangtze River were lost over the last 50years, resulting in the loss of about 500 million m3 of water storage capacity.• More than 800 lakes disappeared entirely. This loss of water retention has contributed to increasingly severe flooding.Lake Balaton• Lake Balaton, the central Europe’s freshwater lake in Hungary is shrinking likely because of climate change.• A series of warm and dry years elevated rates of evaporation above precipitation, causing the lake’s shores to contract.• Balaton’s water quality has deteriorated since the mid-1940’s, with major fish kills and algal bloom from eu-trophication beginning in the 1960s.• These were just a few of drought cases over the world. 21
  • 22. 6.6 METHODS OF DEALING WITH DROUGHT The impacts of drought can be reduced through preparedness and mitigation. The components of drought preparedness and mitigation plan include prediction, monitoring, impact assessment and response.• Prediction can benefit from climate studies which use coupled ocean/atmospheric model, anomalous circulation patterns in the ocean and atmosphere, soil moisture, assimilation of remotely sensed data into numerical prediction models, and knowledge of stored water available for domestic, stock and irrigation uses.• Monitoring exists in countries which use ground-based information such as rainfall, weather, crop conditions and water availability. Satellite observations complement data collected by ground systems. Satellites are necessary for the provision of synoptic, wide-area coverage.• Impact Assessment is carried out on the basis of land-use type, persistence of stressed conditions, demographics and existing infrastructure, intensity and area extent, and its effect on agricultural yield, public health, water quantity and quality, and building subsidence.• Response includes improved drought monitoring, better water and crop management, augmentation of water supplies with groundwater, increased public awareness and education, intensified watershed and local planning, 22 reduction in water demand, and water conservation.
  • 23. • Drought preparedness and mitigation can be accomplished with the following practices: soil and water conservation, and herd management.• For soil and water conservation, we have; Crop rotation, Contoured row crops, Terracing, Tillage practices, Erosion-control structures, Water retention and detention structures, Windbreaks and shelterbelts, Litter management, and Reclamation of salt-affected soil, rain water harvesting, and river transversement.• For Herd management, we have; reduction in herd number, strategic weaning of calves, herd segregation, parasite control, optimizing use of drought affected paddock, attention to contaminated water supply. 23
  • 24. 7.0 Early Warning System ● requires reliable seasonal or climate prediction ● must have clear and positive impact on the socio-economy ● greater impact with interdisciplinary cooperation Climate Prediction: Processes and Application Areas (From 24
  • 25. 7.1 FORECASTING METHODS The general concept of empirical methods is outlined below but attention is devoted to the methods developed by Omotosho (1990, 1992 and Omotosho et al 2000), starting with the vertical wind shear method (Omotosho,1990) 25
  • 26. Schematic u-component profiles in West Africa during transition periods (a, b and c) and peak rainy season (d) (From Omotosho, 1992). The low level wind shear is denotedby ∆Ul and mid-level shear by ∆Um. Sample time series of the shears are shown next page for 1980 and 1988 at Kano, Nigeria. 26
  • 27. 27
  • 28. A sample Onset map for West Africa is shown below. The blue colour depicts areas where both conditionshave been satisfied for 3 consecutive weeks, implying rainfall onset of rainy season is only 2 weeks away fromthe date of the map while all areas under the green colour will have onset in 3 weeks and so on. Typicalforecasts for Ghana and Cote d’Ivoire are shown next page. 28
  • 29. 29
  • 30. Sudden Wind Direction Change Scheme at 700 (____), 400 (------) and 300 hpa (………) over Kano, Nigeria.(Omotosho, 1992). Winds at 400 and 300hPa, which have been persistently westerlies suddenly change to easterlies (marked S in the figures). Sharp oscillations continue for a few weeks before winds atall levels finally stabilise to easterly direction 30
  • 31. EQUIVALENT POTENTIAL TEMPERATURE METHOD (Omotosho et al, 2000) Based on equivalent potential temperature, Өe● very conservative parameter ● most appropriate for convection studies (storms accounts for ≥ 85% of West African rainfall). ● Given as Өe = Ө exp [Lq/Cp Tv], and its saturation component Өes = Ө exp [Lqs/Cp T] ● Variations in Өe better manifested in Өe (and Өes). It can be shown that Өe - Өes ~ q - qs and that, for pre-rainy season moisture build-up q > 0 > qs Өe - Өe > 0 31 ● Accounts for day-to-day, hence year- to-year variations in moisture
  • 32. Time-series of equivalent potential temperature anomalies 32
  • 33. Prediction Methodology Required conditions: 1. Onset --- Өe > 0 or Өe - Өes > 0 for 10 – 15 days 2. Cessation --- Day of max(Өe - Өes) 3. Annual Rainfall a) ∑ Өe for 3 months b) ∑ (Өe - Өes) for 3 months c) No. of days btn Өe > 0 and 1st RR ≥ 5 or 10mm 4. Monthly Rainfall: as (a) and (b) in 3, but for 1 month only 2 and 3 give forecast lead time of greater than 90 days but 1 and 4 have 15- 40 days lead time 2008 predictions for Nigeria – given on next page. 33
  • 34. 34
  • 35. Fog prediction scheme. The development considered various factors favourable to fog formation but focused finally on requirement for calm or low wind speed |V| (left figure), temperature fall ∆T between 1800 and 2100 (right figure). The conditions for fog are that ∆T > 1.8C and |V| < 3m/s between mid-night and 0600UTC to allow for good radiative cooling. Three fog seasons were considered. The final Fog Diagram is on next page. 35
  • 36. THE FOG DIAGRAM FOR IKEJA (Omotosho and Ogunjemiyo, 1989) 36