Air Masses and the ITCZ

Air masses and the inter-tropical convergence zone

The inter-tropical convergence zone (ITCZ) is where the trade winds converge; it is an area of uplift of air. The air that converges on the ITCZ does not have the differences in temperature and density that are typical of convergence at the polar front, and therefore there are no weather fronts. Equatorial weather, despite this, is far from placid. Heating the tropical air can cause instability and periods of heavy rainfall, which is of vital importance to some tropical regions. This rainfall occurs when mT (maritime Tropical) air is drawn in over the land but, when the source region is a continent, dry weather persists. This conflict between mT and cT (continental Tropical) is played out each year over the west coast of Africa.

Transcript

Air Masses and the ITCZ

Global wind circulation and the pattern of ocean currents are important determinants of climatic patterns but to get a fuller understanding of day-to-day weather some knowledge of the Origin and characteristics of air masses is essential.

The source regions for these air masses are usually places where surface geography is fairly uniform, such as over deserts, oceans, large plains and ice-covered areas. Air that is slow-moving or gently subsiding over these areas can acquire uniform temperature and humidity characteristics. These large volumes of air with relatively uniform characteristics are called air masses.

Air moving from the source regions of the air masses influences the weather of adjoining regions. Air moves from these source regions in response to pressure differences (pressure gradient force), to areas of lower pressure.

The air tends to retain its characteristics of heat and humidity for some time but is gradually modified by the earth's surface and by solar insolation. Because the earth's surface plays such an important part in the modification of the air, the lower layers are affected first. An air mass moving equatorward from the poles is heated in its lowest layers. This makes the air unstable because the temperature gradient is steepened and convective cloud is likely to develop. Conversely air moving poleward is cooled in the lowest layers lessening the temperature gradient and making the lower air more stable.

Air masses and the inter-tropical convergence zone

The inter-tropical convergence zone (ITCZ) is where the trade winds converge; it is an area of uplift of air. The air that converges on the ITCZ does not have the differences in temperature and density that are typical of convergence at the polar front, and therefore there are no weather fronts. Equatorial weather, despite this, is far from placid. Heating the tropical air can cause instability and periods of heavy rainfall, which is of vital importance to some tropical regions. This rainfall occurs when mT (maritime Tropical) air is drawn in over the land but, when the source region is a continent, dry weather persists. This conflict between mT and cT (continental Tropical) is played out each year over the west coast of Africa.

The tropical Hadley circulation which transfers energy to middle latitudes is maintained by the release of latent heat of condensation in the ITCZ. The trade winds feed into this zone of convergence which is a zone containing clusters of deep convective clouds which produce heavy precipitation.

Clouds in the ITCZ form clusters which are separated by cloud-free areas. These cloud clusters form where the weak trade-wind inversion is broken through and convection occurs to great heights, forming towering cumulonimbus clouds. The cloud clusters are 100-1000 km across and within these are convective cells 10-100 km across with individual cumulonimbus clouds of 1-10 km across. The clusters move slowly westwards within the ITCZ producing heavy precipitation.

From January to July the ITCZ moves northwards across West Africa bringing cloudy humid conditions. As the moist air surges northwards, rainstorms develop. This moist air gives rise to little rainfall at its northern edge where it meets dry subsiding cT air from the Sahara Desert. This air overlies the mT air giving stability and dry conditions. Further to the south, where the airstream is deepest, belts of cumulonimbus clouds give stormy conditions with heavy rain.

The ITCZ doesn't migrate much over the ocean but over land, the position reflects the latitude of maximum insolation.

In winter the cT air over the Sahara extends southwards, almost to the coast. This allows the dry Harmattan wind (northeast trades), blowing from the north to desiccate the Sahel zone. The Sahel region of Africa lies between the Sahara Desert to the north and the Savannah to the south. By 2050, the region’s population is expected to more than double, to 450 million, and temperatures there are expected to rise to about 3°C above their 1950 level.

ITCZ migration and its effect in West Africa.

The marginal lands bordering the Sahara Desert have suffered since the 1970s from a combination of overpopulation (of humans and livestock) and drought which has desiccated large tracts of land. During the 1950s greater than average rainfall encouraged the extension northwards of agriculture and the increase in livestock numbers. But about average rainfall in the 1960s and drought from the 1970s right through until 2007 has brought about southward migration as desertification has claimed marginal lands. In 1975 alone over one million people migrated southwards in western Africa. There has been severe starvation and millions of animals have died (about 80% of the 1960’s livestock total has died).

The climate of the Sahel has been variable over the last 100 years and runs of dry or wet years are a part of the natural variability of the Sahel's climate. However, the length of regional droughts is now statistically significant and points to the region going through a climatic change which will lead to the southward extension of the Sahara Desert.

During the drought years, there has been an equatorward shift in the ITCZ of some 200-300 km. The shift does vary from place to place but this shift has caused the failure of the rains in many Sahel countries. The subtropical high-pressure zone has also drifted south causing strong subsidence over the Sahara and stronger, dry north-east trade winds into the Sahel.

In West Africa, the length and severity of the dry season is the most important factor influencing agriculture. Humid conditions, where there is no dry season (for example, Lagos) enable cultivation year-round. Crops such as sweet potato, yams and cassava are important subsistence crops under these conditions. Once the dry season extends to 3-5 months (for example, at Minna, Zinder) grain crops such as maize become important and, for dry seasons as long as 6 months, drought-resistant crops such as millet, sorghum, cotton and groundnuts are grown. Areas with a dry season of more than 7-8 months (for example Timbouctou and Agades) are limited to short-term drought-resistant crops including millet and sesame. In areas with dry seasons of more than six months, cattle and goat rearing is more common and these are kept for the most part by nomadic pastoralists.

Current crop yields are at least 50% below what they could be, were farmers to use the best available seeds, as well as the optimal irrigation and fertilizer schemes that are suitable for the region. Today, only 4% of the Sahel’s cropland is equipped with any kind of irrigation infrastructure, and the average application rate for fertilizer is 10 kilograms per hectare - one-tenth of the global average.

The Sahel is already prone to unpredictable rainfall, soil degradation and erosion - all of which are expected to worsen with global warming. Farming approaches that enable crop growers to retain enough rainwater to bridge the dry season (by collecting run-off, reducing evaporation or improving infiltration) need to be scaled up to avoid crop failures.

Burkina Faso, for instance, has increased its yields of millet and sorghum by up to 500% by adopting the Zaï technique. Here, pits are dug in the soil to concentrate water and nutrients, and seeds are placed in the pits rather than scattered over the soil. Other water-harvesting techniques include terracing, mulching (the use of bark and other plant matter to reduce evaporation and enrich the soil), no-till farming (whereby farmers use hoes or other tools to remove weeds instead of ploughs) and contour stone bunds (the building of rock walls that prevent the soil from being washed away).

Agroforestry, combining agriculture and forestry, can also increase food production and resilience to climate change. In Niger and Mali, trees grown at the edge of fields or among crops increase the fertility and moisture of the soil by producing leaf litter and shading, among other benefits. The development of new varieties of cereal that are locally adapted and drought-tolerant could help to improve agricultural production as the effects of climate change intensify.

Hunger and malnutrition are widespread in the Sahel. As droughts and other weather extremes make it even harder for farmers to produce the crops and livestock needed to sustain the growing population. The Region has seen conflict and terrorism increase. If climatic conditions worsen, millions of people could die in famines, and there is likely to be unprecedented levels of migration, including to Europe, through North Africa and across the Mediterranean Sea. West Africa is the origin of more international migrants than any other region of Africa. A 2017 study predicted that millions of people could be forced to leave the Sahel by the end of this century.

To tackle the long-term problem of drought there are several possible responses:

(i) accept the situation and be prepared for large-scale migration out of the Sahel.

(ii) attempt to increase rainfall by cloud-seeding.

(iii) plant trees and shrubs to improve atmospheric humidity, by transpiration.

(iv) develop new sources of water for improved irrigation techniques.

(v) attempt to change the customs of the inhabitants (for example, reduce cattle herds, introduce grazing control, re-settlement, land reform, improved farming practices, family planning and educate girls to secondary school level).

In addition, security in the area needs to improve to combat terrorism which is a factor that stunts development and inhibits change.

There are several intergovernmental environmental projects aimed at reclaiming desiccated land for productive use, but it is too early to say if these offer any medium to long-term hope for agriculture in the Sahel.

Many of the possible responses are difficult to implement for economic or social reasons.

Population growth has been outstripping food supply in the Sahel since the 1970s. Millions of people in the Sahel zone have had to depend on food aid for years at a time. Relief efforts, such as those provided by the United Nations’ World Food Programme, struggle to meet the demand for emergency food assistance. World Bank data shows that stunting in children (in which a child’s height is more than two standard deviations below the World Health Organization’s median for that age group) currently affects 17% of children under five in Senegal and 42% in Niger.

Because of the lack of clean water and sanitation services, the region has the world’s highest mortality from waterborne diseases: around half a million people here die from diarrhoeal disease and cholera each year.

Women in the Sahel are among the least empowered in the world. Many don’t have a say in their own basic life choices. Niger and Chad have the world’s lowest median age of marriage among girls (around 16 years old) and together with Mali, among the highest rates of adolescent births. In these three countries, 36-40% of women give birth before they are 18 years old. Such women are more likely than older mothers to deliver a preterm or low-birth-weight baby and to experience complications during childbirth. Women in Niger have on average 7.6 children, and in some northern Nigerian states, more than 8. These large families are relying on ever-smaller plots of land on which to grow food and graze their animals, mainly cattle and goats.

Rapid population growth is undermining the quality of both primary and secondary education. It means that fewer resources are available per child. In Niger and northern Nigeria, it is increasingly common for schools to teach half the children in the morning and half in the afternoon, because there are too many students and not enough teachers.

Violence in the Sahel is rising rapidly, too. As population growth forces farmers of crops such as sorghum and millet to expand their fields, leading to conflict with herders. According to the World Bank, Niger’s arable land per capita shrank from 1.4 hectares in 1996 to 0.8 in 2016. Most of this change is through population growth.

In the meantime, scientists continue to study the climate of West Africa to better understand the cause of the variability in rainfall.

One discovery in 1972 by J.M. Walker was the identification of another cell-like circulation in West Africa, but this operates in an east-west direction. Convection of air to high altitudes, caused by intense heating, is followed by movement westwards in the subtropical jet stream and then descent. These cells play a role in the weather and climate of this tropical zone not just here but also in the Pacific where, as part of the El Niño Southern Oscillation (ENSO) circulation (covered in the chapter on Climate Change), they play a critical role.

Image result for walker cell circulation black and white

Another piece of research suggests that there is a link between sea surface temperatures (SSTs) and periods of drought. One theory is that abnormally warm temperatures in the southern Atlantic weaken the equatorial low pressure and hence weaken the airflow over West Africa. There appears to be a strong correlation between SSTs and drought but how the two are linked is still a matter for further research.

The variability of rainfall

The variability of rainfall is the extent to which the precipitation of individual years varies from the average value. Not surprisingly hot desert areas lying near the Tropics have the highest variability, with some years having no rainfall and others with 3 or 4 times the average.

Monsoon regions also have a high variability because of the nature of monsoon rainfall, which depends on the strengths of regional air circulation. The rainfall in Bombay has ranged from as low as 600 mm in 1918 to 3400 mm in 1933. On the other hand, wet equatorial climates have a low variability because of high convectional rainfall produced by the inter-tropical convergence zone.

Even in the British Isles, which tends to have a low variability of rainfall, there can be extreme years. The period between May 1975 and August 1976 gave England and Wales the driest 16-month period since weather records began in 1727. This drought-like so many others did not have one single cause but was a result of several factors. One major cause was a blocking anticyclone. This blocking anticyclone was a complete anticyclonic cell that broke away from the Azores anticyclone and established itself in higher latitudes. Once established a blocking anticyclone can persist for weeks, diverting the rain-bearing fronts to the north and south of it.

In the last few years, several rainfall events have caused widespread flooding in the UK. In winter 2013/14 a succession of storms hit the UK leading to record rainfall and flooding in many regions including the south-east. December 2015 was similar when Storm Desmond hit the north-west causing widespread flooding and storm damage. In November 2019 record rainfall for a three-day period triggered over 100 flood warnings and caused widespread flooding in the Sheffield and Doncaster areas. The Met Office climate predictions suggest these types of unprecedented rainfall events will become more commonplace in the 21st century, with the climate we are now experiencing.

Climate change models predict that Eurasia and North America will experience greater rainfall variability and this could be accompanied by higher winter rainfall. Certainly, in recent years we have seen rainfall records, for 2 and 3 day periods, broken regularly.

Around the world, in 2019 there have been devastating floods including in Iran, Iraq, Afghanistan, Syria, East Africa, India, Paraguay, Bolivia and France. While many countries recorded their highest ever temperatures any many regions suffered extensive wildfires.