The gases in our atmosphere allow the sun’s energy (shortwave radiation) to pass through and heat the earth’s surface. The earth in turn radiates the energy back to the atmosphere in the form of long-wave infrared radiation. This process causes the net warming of the earth and atmosphere and is sometimes referred to as the natural greenhouse effect. The glass of a greenhouse works in a similar way, letting in short-wave radiation but preventing long-wave radiation from escaping and thus the air in the greenhouse is warmer that the air outside. The major greenhouse gases are Water Vapor (H2O), Carbon Dioxide (CO2), Methane (CH4), Ozone (O3) and Nitrous Oxide (N2O).
This natural greenhouse effect enables the earth to maintain an average of 15 degrees Celsius; without this effect the surface temperature would drop drastically to -18 degrees Celsius and the earth would be unable to support life as we know it. Humans have however altered the chemical composition of the atmosphere by releasing large quantities of greenhouse gases - primarily carbon dioxide, methane, and nitrous oxide. These gases are trapping more energy and reflecting it back to earth. This is having an effect on global atmospheric temperatures. This is often referred to as the human-induced greenhouse effect.
The weather is the state of the atmosphere, as defined by various weather elements, for a specific place and time. Weather refers to short-term (minutes to weeks) variations in the atmosphere. Climate on the other hand is usually defined as the average weather for a particular place and is described in terms of the mean and variability of the weather elements over a long time period. The classical period is 30 years, as defined by the World Meteorological Organization (WMO).
The climate of the world varies from one decade to another, and a changing climate is natural and expected. However, there is a concern that the human industrial and development activities of the past two centuries have caused changes over and above natural variation... What is climate change? Climate change is the natural cycle through which the earth and its atmosphere are going to accommodate the change in the amount of energy received from the sun. The climate goes through warm and cold periods, taking hundreds of years to complete one cycle. Changes in temperature also influence the rainfall, but the biosphere is able to adapt to a changing climate if these changes take place over centuries. Unfortunately, human intervention is currently causing the climate to change too fast. (Climate models predict that the mean air temperature over South Africa will increase by an estimated 2°C over the next century.)
Plants and animals may not be able to adapt as quickly to this rapid climate change as humans can, and therefore the whole ecosystem is in danger. What causes climate change? The global climate system is driven by energy from the sun. Several gases in the atmosphere act to trap the energy from the sun, thus warming the earth. These gases are called greenhouse gases and the process is the greenhouse effect. Without this there would be no life on earth. Human activities over the last 200 years, particularly the burning of fossil fuels (oil, coal, natural gas) and the clearing of forests, have increased the concentration of greenhouse gases in the atmosphere. This is likely to lead to more solar radiation being trapped, which in turn will lead to the earth's surface warming up - called the enhanced greenhouse effect. How does a changing climate influence South Africa? Higher temperatures will influence the rainfall, but it is still uncertain how the annual rainfall will change. It could increase in some parts of the country, and decrease in other parts. Water resources Human and animal health Maize and wheat Grazing livestock Forestry The coastal zone Fisheries Biodiversity What can we do to slow the process down? The enhanced greenhouse effect can be slowed down by following two guidelines: (1) Increase sinks and (2) decrease sources of greenhouse gases. A sink is a process which removes greenhouse gases from the atmosphere. For example: growing a tree where one did not previously exist provides a sink for carbon dioxide, because the tree extracts carbon dioxide for photosynthesis. A source is a place or activity from which greenhouse gases are emitted. This can be a process such as coal burning or a location such as cultivated fields.
The Kyoto Protocol The Kyoto Protocol is a legal instrument that is separate from, but related to the Climate Change Convention. Countries ratifying the Protocol have mainly the following obligations: (1) Developed countries are obliged to ensure that their greenhouse gas emissions do not exceed the amounts assigned to them. (2) Climate change policies must be implemented. (3) Energy efficiency must be enhanced. (4) Emissions in the waste and transport sectors must be limited and/or reduced. (5) Sinks for greenhouse gases must be protected. (6) Market instruments that are counter productive to the aims of the Protocol should be phased out. (7) Sustainable forms of agriculture and relevant research must be promoted. All these activities must be undertaken in such a way that potentially adverse effects on developing countries are minimised. The future of climate change issues in South Africa are on the moment mainly in the government's hands.
Drought is not easily defined and often depends on who you speak to. The South African Weather Service defines drought on the basis of the degree of dryness in comparison to normal or average amounts of rainfall for a particular area or place and the duration of the dry period. This is what is termed a meteorological drought. Less than 75% of normal rainfall is regarded as a severe meteorological drought but a shortfall of 80% of normal rainfall will cause crop and water shortages which will ultimately affect social and economic factors. Normal rainfall for a particular place is calculated over a 30-year period using for example rainfall figures from 1961 to 1990. Other climatic factors such as high temperature, high wind, low soil moisture and low relative humidity can significantly aggravate the severity of drought conditions and these additional factors should also be taking into account.
Currently the South African Weather Service makes use of two drought indices Percentage of Normal Rainfall The percentage of normal rainfall is a simple calculation that is aimed at a general audience and is used fairly effectively when comparing conditions within specific regions or for particular seasons or time periods. One of the biggest disadvantages of the Percent of Normal index is that many people misunderstand it. A normal is a rainfall average that is calculated over at least 30 years, but this average is not necessarily the same as the median (middle) value of the rainfall for the 30-year period. One very wet year or one very dry year could result in the average value being either more than or less than the median value respectively. The rainfall value that may be calculated as being 75% of the normal will indicate a meteorological drought, but this value may in actual fact be quite close to the median value and so may not be a true reflection of the water deficit being experienced by the place in question. Deciles The rainfall deciles are used in the monthly Climate Summary publication issued by the South African Weather Service. This index requires rainfall data for long periods of time. The monthly rainfall distribution over a long period of time (usually more than 30 years) is divided into tenths of the distribution. Each of these 10 categories is called a decile. By definition, the fifth decile is the median (middle) rainfall amount and is not exceeded by 50% of the rainfall occurrences over the entire record of the station. The deciles index is a more useful index in assisting decision-makers to determine where financial assistance has to be provided in times of drought.
The disadvantage of the index is that it compares the rainfall deficit in the current month with rainfall for the same month in the history of the station and does not consider the cumulative effect of rainfall deficit. What is the Standardised Precipitation Index (SPI) The Standardised Precipitation Index (SPI) is an index based on the probability of rainfall for any time scale and can assist in assessing the severity of drought. The SPI can be calculated at various time scales which reflect the impact of the drought on the availability of water resources. The SPI calculation is based on the distribution of rainfall over long time periods (preferably more than 50 years). The long term rainfall record is fit to a probability distribution, which is then normalised so that the mean (average) SPI for any place and time period is zero. SPI values above zero indicate wetter periods and values less than 0 indicate drier periods. The SPI values that will be adopted at the South African Weather Service are the same as those developed by McKee, Doesken and Kleist in 1993 (for more information, refer to the preprints of the 8th Conference on Applied Climatology, pp 179 - 184, held in January 1993 in Anaheim, California). These values are as follows: Greater than 0 - Wet (50% occurrence) 0 to (0.99) - Somewhat Dry (34.1% occurrence) (1) to (1.49) - Moderate Drought (9.2% occurrence) (1.5) to (1.99) - Severe Drought (4.4% occurrence) Less than (2) - Extreme Drought (2.3% occurrence) Why the move to the Standardised Precipitation Index (SPI)? Neither the Percent of Normal nor the Decile drought indices are able to assist decision-makers with the assessment of the cumulative effect of reduced rainfall over various time periods. Neither of these indices can describe the magnitude of the drought compared with other drought events. The Standardised Precipitation Index can alleviate both of these principal shortcomings of the other indices, while at the same time being less complex to calculate than some of the other drought indices not in use at the South African Weather Service.
The highest worldwide temperature was recorded in AlAziziya, Libya measuring 57.7 ºC on 13 September 1922 and in South Africa at Dunbrody (Sundays River Valley in Eastern Cape) measuring 50.0 ºC on 3 November 1918. The hottest place in South Africa is Letaba (Limpopo Province) with a mean annual temperature of 23.3 ºC and an average annual maximum temperature of 35.0 ºC. The lowest worldwide temperature was recorded in Vostok, Antarctica at -89.2 ºC on 21 July 1983 and in South Africa at Buffelsfontein near Molteno (Eastern Cape) measuring -18.6 ºC on 28 June 1996.
The coldest place in South Africa is Buffelsfontein near Molteno (Eastern Cape) with a mean annual temperature of 11.3 ºC and an average annual minimum temperature of 2.8 ºC. The highest worldwide monthly rainfall occurs at Cherrapunji, India (9300 mm) and in South Africa at Matiwa in the Limpopo Province (1510 mm measured in January 1958). The highest worldwide 24 hour rainfall was measured at Fac Fac, Reunion Island (1825 mm) and in South Africa at St Lucia in KwaZulu-Natal (597 mm measured on 31 January 1984). This was rainfall associated with the passage of tropical cyclone Domoina. In South Africa the highest ever rainfall in one year was measured at Woodbush in the Limpopo Province (1950 mm measured in February 2000). The wettest place in South Africa is Matiwa with an average annual rainfall of 2004 mm (calculated over a 60-year period). The driest Place in South Africa is Alexander Bay in the Northern Cape with an average annual rainfall of only 46 mm.
The windiest place in South Africa is Cape Point (Western Cape) which experiences only 2% of all hours in the year with calm conditions. The average wind speed is 6.9 m/s with 42.1% of the wind speeds greater than 8 m/s. The strongest wind gust ever in South Africa occurred at Beaufort West (Western Cape) on 16 May 1984 and measured 186 km/h. Which is the windiest place Cape Town or Port Elizabeth? This is a difficult question to answer. What magnitude of wind is more noticeable than others? Possibly we can resolve the issue by referring to the Beaufort Wind Scale. If one uses the Beaufort Wind Scale, wind only really becomes felt by human beings when the wind speed exceeds 1.5 m/s. Cape Town experiences wind of 1.6 m/s or more on 95.6% of the days of the year and Port Elizabeth on 95.7% of the days of the year. The Beaufort Wind Scale classifies any wind greater than 8 m/s as a Fresh Breeze.
In Cape Town a fresh breeze is experienced on 18.7% of the days in the year (68 days of the year) and in Port Elizabeth on 20.2% of the days in the year (73 days of the year). How often, and during which months, does the South Easter usually blow in Cape Town? South Easterly winds are experienced throughout the year, but are not that frequent when compared to winds blowing from other directions. South Easterly winds blow about 3% of the time in Cape Town, mainly in August (average wind speed 5 m/s) and October (average wind speed 6 m/s). The strongest South Easterly winds occur in the summer months of December and February. During these months the average speed of the South Easterly winds is 7 m/s, but they occur less frequently during these months. How windy is South Africa compared with the windiest place on earth? The windiest place on earth is the coastal part of Antarctica. Cape Denison on Commonwealth Bay in Antarctica has an average wind speed of 20 m/s. The windiest place in South Africa is Cape Point with an average wind speed of 6.9 m/s.
This is a quick summary of the average data by month for a large number of cities as found in the publication The World Weather Guide, E A Pearce and C G Smith, Hutchinson and CO Publishers, 1984
What is the difference between averages and normals?
A normal of temperature, precipitation or any other weather element is defined as the arithmetic average of the observed values of that element. A normal is strictly for 30 years, whereas an average can be computed over any time span. There is an international agreement that normals should be from a 30-year period from 1961 to 1990.
To calculate the normal daytime maximum temperature for Pretoria on say April 15, we would add together all the daily maximum temperatures on this day over the 30-year period from 1961 to 1990, divide the total by 30, to get the normal. Normal values are supposed to be representative, not necessarily what should occur.
What are period averages?
Period averages are averages of climatological data computed for any period of at least ten years starting on 1 January of a year ending with digit 1.
What are climatological standard normals?
Climatological standard normals are averages of climatological data computed for the following consecutive periods of 30 years:
1 January 1931 to 31 December 1960
1 January 1961 to 31 December 1990
The rainfall climate of South Africa is one of great variability. Seasonal rainfall percentage deviations since 1960 demonstrate wide fluctuations about the long-term average and it is in this context that large rainfall deficits must be assessed. Between July of 1960 and June of 2004, there have been 8 summer-rainfall seasons where rainfall for the entire summer-rainfall area has been less than 80% of normal. A deficit of 25% is normally regarded as a severe meteorological drought but it can be safely assumed that a shortfall of 20% from normal rainfall will cause crop and water shortfalls in many regions accompanied by social and economic hardship.
All but the south-western and southern regions of South Africa rely on summer rainfall, which normally falls between October and March, the summer season. Rainfall is heaviest in the east and decreases westward. For convenience the rainfall season is taken to run from July until June of the following year, but rainfall outside of the summer season is usually insignificant.
The consequence of rainfall being confined to six months of the year is that most crops can only be grown during this period. Similarly, the recharging of water resources is also confined to these crucial six months. When the seasonal rainfall is seriously below normal, crop yields are poor and ground and dam water levels fall dangerously low. Should these conditions occur in swift succession, as in the periods from 1964 to 1970, 1991 to 1995 and again from 2002 to 2005, there is insufficient time for natural resources and the economy to recover from each rainfall-deficit period.
Graph showing percentage-of-normal rainfall for Gauteng - bad rainfall years have a medium term effect
Simultaneous to low rainfall are cloud-free skies and high temperatures. The effect of abnormally high temperatures is an increase in evapotranspiration as well as stress on plants whilst further depleting surface-water reserves through evaporation.
The most serious impact, other than dwindling water supplies, is the effect on staple crops and, ultimately, commercial crops. In 1992/1993, undoubtedly one of the most widespread droughts of the last 45 years, maize had to be imported to South Africa (as well as the rest of southern Africa). The knock-on effect of crop failure could be seen in the population drift from rural areas into the cities, farm labour lay-offs and farm closures as well as an increasing indebtedness in the agricultural sector.
Graph showing percentage-of-normal rainfall for Freestate - maize crop impact
Other serious impacts brought about by drought are the devastating veld fires which destroy large areas of grazing at a time when grass is in short supply. Commercial timber and orchards are also prone to damage at such times. In 1992 there were several huge fires which destroyed thousands of hectares of grassland. In one of the worst events, during August, at least nine people perished. In 1994, a combination of unusually strong winds and very dry conditions saw large areas of grazing and timber destroyed. Six people died in one such fire in July of that year. Again, in July of 2002, Mpumalanga was devastated by fires that destroyed 24,000 ha of pasture and left four people dead and damages amounting to more than R32 million.
Severity of Recent Droughts
It is very difficult to look at the entire summer-rainfall region and deduce that drought affected all of these areas equally. On the contrary, some of the provinces in South Africa appear to suffer more harshly than others at times of rainfall deficit.
The impact of ENSO on South Africa
Although the southern part of Africa generally receives below-normal rainfall during El Nino years and La Nina usually brings normal or above-normal rainfall, it cannot be accepted as a rule. Southern Africa can be divided into numerous rainfall regions, each region having a different correlation with ENSO. Also, ENSO explains only approximately 30% of the rainfall variability, which means that other factors should also be taken into account when predicting seasonal rainfall. For example: The 1997-98 El Nino was the strongest on record, but not all of South Africa received below-normal rainfall. Some regions had an abundance of rain because of moist air that was imported from the Indian Ocean. One should be careful not to make a general rule for rainfall and temperature changes in ENSO years over southern Africa.
Does El Nino always cause drought in South Africa?
No. Although most El Nino years have been associated with below-normal rainfall, the impact of El Nino is often reduced by the sufficient groundwater and soil moisture content carried over from previous seasons.
Can ENSO be forecast?
Yes. SSTs are used to measure the state of the ocean (and ENSO) and can be forecast up to 9 months ahead with good skill. Computer models are used for this and the first indication of ENSO influencing the October-to-March (summer) rainfall season can be forecast as early as the preceding May. IMPORTANT: An El Nino/La Nina forecast is NOT a rainfall forecast.
If you would like to know more about ENSO please follow these links to other ENSO sites:
Anomaly: The deviation from the mean. To calculate SST anomalies, the long-term mean for a specific point in the ocean is subtracted from the current value. A negative value indicates that the current value is cooler (smaller) than usual, while a positive value indicates that the current value is warmer (larger) than usual.
For example: The Nino 3.4 value for December 2003 is 26.9 °C. The long-term mean for the Nino 3.4 region is 26.5 °C
Anomaly = current value – mean
Anomaly = 26.9 °C - 26.5 °C = 0.4 °C
What happens in the ocean during ENSO?
The Pacific Ocean is a huge mass of water which can control many climate features in its region, since changes in the ocean result in characteristic changes in the atmosphere which, in turn, alter climate and weather patterns across the globe.
During normal years (non-ENSO years) relatively cold water occurs along the west coast of South America , an effect increased by upwelling of cold water along the Peruvian coast. The cold water then flows westward along the equator to Australia and is heated by the tropical sun. These normal conditions make the western Pacific about 3°C to 8°C warmer than the eastern Pacific.
During La Nina years, the upwelling off the Peruvian coast is enhanced and the SSTs in the Nino regions become cooler than usual. During El Nino years, the area of warm water (usually over the western tropical Pacific near Australia ) cools down and the warm water is displaced eastward to the central Pacific. The upwelling off the Peruvian coast is suppressed and the SSTs in this region become warmer than usual.
Measuring the ocean
Sea-surface temperature (SST) anomalies are (amongst other) used to measure the state of the global oceans. The long-term mean for each location over the oceans is calculated from a long record of SST data for these specific locations. The mean is then subtracted from the current value. If the ocean is warmer than usual, it will have a higher SST value than the mean and therefore a positive anomaly. On the other hand, a colder-than-usual ocean surface will give rise to a negative anomaly. El Nino events are associated with positive SST anomalies, while La Nina events are associated with negative SST anomalies.
What happens in the atmosphere during ENSO?
The atmospheric circulation between high and low pressure regions in the tropical Pacific is known as the Walker Circulation. The easterly trade winds are part of the low-level component of the Walker circulation. During normal years (non-ENSO years), the trade winds move over the warmer sea bringing warm, moist air towards the Indonesian region. This moist air rises to high levels and travels eastward before sinking over the eastern Pacific Ocean . The rising air is associated with a region of low air pressure, towering cumulonimbus clouds and rain. High pressure and dry conditions accompany the sinking air. (Figure: Normal conditions in the Pacific Ocean and the atmosphere .)
During La Nina years, the Walker Circulation operates in the same way as described for normal years, but because of the larger area of colder water off the South American coast and the displacement of warmer water to the west, the atmospheric pattern also shifts accordingly. During El Nino years, the Walker Circulation is altered due to the changes in the Pacific Ocean . The lifting and sinking of air - and therefore rainy and dry conditions - move with the warmer and colder SSTs to form the pattern depicted in the figure Conditions in the Pacific Ocean and the atmosphere during El Nino .
Measuring the atmosphere
The Southern Oscillation Index (SOI) gives a simple measure of the strength and phase of the difference in sea-level pressure between Tahiti (in the mid-Pacific) and Darwin (in Australia ). This difference is given in terms of an index. The typical Walker circulation has an SOI close to zero, while a strong negative value usually indicates that the oscillation has entered an El Nino phase. A strong positive value usually indicates a La Nina phase.
What is El Nino?
El Nino is the warming of sea-surface temperatures in the equatorial Pacific Ocean which influences atmospheric circulation, and consequently rainfall and temperature in specific areas around the world.
El Nino is translated from Spanish as the boy child. Peruvian anchovy fishermen traditionally used the term - a reference to the Christ child - to describe the appearance of a warm ocean current off the South American coast around Christmas. Over the years the term El Nino has come to be reserved for the sequence of changes in the circulation across the Pacific Ocean and Indonesian archipelago when warming is particularly strong. Approximately 14 El Nino events affected the world between 1950 and 2003. Amongst them was the 1997/98 event, by many measures the strongest thus far this century, although South Africa escaped the impact of it to some extent.
What is La Nina?
La Nina is the cooling of sea-surface temperatures in the equatorial Pacific Ocean which influences atmospheric circulation, and consequently rainfall and temperature in specific areas around the world.
La Nina, Spanish for the girl, is the opposite of El Nino. SSTs in the equatorial Pacific become cooler than normal, giving rise to the term cold event. This situation is reflected by negative SST anomalies. The Walker circulation intensifies and the SOI consequently becomes positive during this event.
What about ENSO?
The changes in the Pacific Ocean are represented by the term El Nino/La Nina, while changes in the atmosphere are known as the Southern Oscillation. Because these two cannot be separated, the term ENSO is often used. ENSO refers to both El Nino and La Nina.
The scientific definition for ENSO
A scientific definition was recently developed to help scientists to identify ENSO events. When the three-month running mean of the SST anomalies in the Nino 3.4 region are greater than or equal to 0.5°C, there is a good chance of an El Nino event taking place. When the anomalies are smaller than or equal to -0.5°C, there is a good chance of a La Nina event taking place. Take note, however, that strong ENSO events (which are more likely to affect our seasonal climate) have a larger SST anomaly and normally last for a period much longer than three months.