water cycle full Flashcards
what type of system is the hydrological cycle?
it is a closed system meaning it is finite and constant. it includes includes stores and transfers.
what is the hydrological cycle driven by?
it is driven by solar energy and gravitational potential energy.
1) solar energy- has water and turns from a liquid to a gas, rises into the atmosphere and cools and condenses to form clouds
2) gravitational potential- earth’s gravity pull is converted into kinetic energy and accelerates water through the cycle (falling as precipitation, flowing across the land, infiltrating and percolating)
what is the cryosphere and biosphere?
cryosphere- the ice system (ice sheets and glaciers)
biosphere- the living system (plants and animals)
what is a residence time?
how long water stays in a particular store
what is fossil water?
water that is no longer being naturally replenished. it may have been stored for a long time.
It is ancient, deep groundwater made from pluvial (wetter) periods in the geological past
what are the 7 water stores in order from largest to smallest with their percentage and residence times?
1) oceans (96.9%, 3600)
2) cryosphere (1.9%, 15000)
3) groundwater (1.1%, up to 10000)
4) rivers and lakes (0.01%, 2 weeks-10 years)
5) vegetation (0.0001%, 1 week)
6) atmosphere (0.001%, 10 days)
7) soil moisture (0.001%, 2-50 weeks)
what are the 5 flows/transfers in order from largest to smallest with their size, measured in km cubed per year
1) ocean evaporation (400,000)
2) ocean precipitation (370,000)
3) land precipitation (90,000)
4) evapotranspiration (60,000)
5) surface flow (runoff) (30,000)
explain the problem of the availability of water
2/3 of water is locked in the cryosphere and so is inaccessible. water is fundamental for human life and for crops to grow. with population growth we need to ask ourselves how we are going to have enough water, it is a finite resource.
water vapour is the most important greenhouse gas and is driving climate change
distinguish the different flows of the hydrological cycle
interception- the retention of water by plants and soils which is subsequently evaporated or absorbed by the vegetation
infiltration- the process by which water soaks into, or is absorbed by the soil
percolation- similar to infiltration but a deeper transfer of water into permeable rocks
throughflow- the lateral transfer of water downslope through the soil
groundwater flow- the very slow transfer of percolated water through permeable or porous rocks
surface run off- the movement of water that is unconfined by a channel across the surface of the ground
channel or river flow- takes over as soon as the water enters a river or stream, the flow is confined within a channel
explain the future for our water
- climate change is altering out budget, cryosphere is melting and increasing the proportion stored in the oceans
- global population is rising, scheduled to reach 10 billion by 2055, water supply will not be able to keep up with pop growth
- accessible water is a mere 1% of all the world’s freshwater and so it is a scarce resource needing careful management
what are the 5 implications for future water security
- conflict over water sources (transboundary)
- drought and famine
- environmental refugees
- price of water increases
- more use of technology (e.g. desalination = further carbon emissions
is water renewable or non-renewable?
- water is generally considered a renewable resource but humans extract fossil water which is not recharged
- fossil water and crois-here are two water stores that are claimed to be non-renewable
explain the water budget
the global water budget limits water available for human use due to:
- only small % available for human use (96% salt water)
- 2/3 freshwater locked in cryosphere (long residence time), have to melt it
- most of the rest of the water is in the soil, vegetation or deep underground (some is fossil water)
the water stores have different residence times
what are fluxes?
he rates of flow between stores. the greatest fluxes occur over the oceans.
what is the global water budget?
it takes its account all the water that is held in stores and flows of the hydrological cycle, only 2.5% is freshwater. only 1% of fresh water is accessible.
what are the inputs in the drainage basin?
precipitation patterns and types:
- orographic
- frontal
- convectional
vary in a number of different ways. by form (rain, snow, hail), amount, intensity, seasonality, distribution
what are the flows in the drainage basin?
interception infiltration surface runoff saturated overland flow throughflow percolation groundwater flow river or channel flow
what are the outputs in the drainage basin?
evaporation- the process by which moisture is lost directly into the atmosphere from water surfaces, soil and rock
transpiration- the biological process by which water is lost from plants through minute pores and transferred into the atmosphere
discharge (channel flow)- into another, larger drainage basin, a rivulet, lake or the sea, the amount depends on the amount of precipitation falling directly into the channel
what type of system is a drainage basin?
it is an open system. it has inputs and outputs.
what is orographic precipitation?
also known as relief rainfall, relating to mountains, orographic uplift is when the uplift of an air mass, because of an orographic obstruction, causes the cooling of the air mass, If enough cooling takes place, condensation can occur and form into orographic precipitation, especially mountains and hills
what is frontal precipitation?
caused when warm air meets cold air and forces the warm air to rise to then 3C’s, creating low pressure, rises and then forms rain through clouds, as seen in the UK
what is convectional precipitation?
caused by when moisture evaporates and rises when heated by the sun (3C’s), in warm areas
what is saturated overland flow?
surface run off caused when soil is saturated (full of water)
what are the different parts of the drainage basin?
source
watershed- the edge of highland surrounding a drainage basin which marks the boundary between two drainage basins
tributary
confluence - the point at which two rivers or streams join
mouth
what are the main physical factors that affect the drainage basin?
1) climate
- precipitation (type, intensity, duration and amount)
- temperature (climate zones)
this affects vegetation type
2) Soils (determines amount of infiltration and throughflow and therefore type of vegetation)
3) vegetation cover (hot and wet = dense)
- impact on amount of interception, infiltration and transpiration
4) geology (impact on subsurface processes such as percolation an groundwater flow and therefore on aquifers)
- indirectly, geology affects soil formation
5) relief (topography)
- impact amount of precipitation
- slopes affect amount of runoff
6) antecedent soil conditions
what physical factors affect the inputs of the drainage basin?
- precipitation is the major input into any drainage basin system. it varies according to its time and space according to its type and intensity linked to climatic season and weather systems
- the highest precipitation inputs into drainage basins are found in the tropics due to the ITCZ, the intense solar radiation fuels the convection off warm humid air, resulting in condensation and convectional rain, along the subtropical ridge
- lowest precipitation levels found in stable areas of high atmospheric pressure such as Quillagua in the Atacame desert which has 0.2mm a year of rain
- distribution of rain affected by continentality (distance from sea) as continental areas like the Gobi desert in Asia are from the moisture of maritime air masses
- relief affects rain with which high levels of rain happening where prevailing winds are forced to rise over high altitudes, as air is forced over the higher ground it cools, causing moist air to condense and fall out as rainfall.
explain the ITCZ and the rain seen here
inter-tropical convergent zone, this is where the sun is directly overhead with low pressure
- equator gets the most heating as the sun is directly overhead, more evaporation, more air rising, low pressure and high rainfall
- high pressure sinks at 30º so air sinks creating high pressure
- the ITCZ migrates north and south over the year as the earth tilts, creates a rain belt north and south of tropic, monsoon weather
- area of low pressure where the trade winds meet
explain the physical factors that affect the flow of interception in the drainage basin
- rate of interception dependant on two physical factors: precipitation and vegetation
- interception greatest when rain is lightest and of short duration as the dry leaves and stems have the greatest water storage capacity, if intensity increases then interception is less effective as more will drop off
- denser types of vegetation such as coniferous forests intercept more water than sparser deciduous trees, especially in winter when deciduous lose their leaves
explain the physical factors affecting infiltration
1) precipitation intensity - infiltration-excess overland flow will occur when rainfall intensity exceeds infiltration capacity, meaning that the water flows over the surface and infiltration is reduced
2) vegetation cover- roots help to break up the soil, increasing the infiltration rate
3) soil and rock type- infiltration rates will increase as porosity and permeability increase
4) water table depth- as the water table rises during prolonged rainfall the soil will become saturated, reducing infiltration
5) slope gradient- as gradient increases, more water will flow over the surface, reducing infiltration
what are the two types of surface run off?
saturated overland flow- when water accumulates in the soil until the water table reaches or ponds on the surface, forcing further rainwater to run off the surface
infiltration-excess overland flow- occurs when the rainfall intensity exceeds the infiltration capacity, so excess water flows over the ground surface. it will go into river channels, increasing floods
when is saturated overland flow common?
where there are thin soils of moderate permeability. concavities near a stream or riverbank often have high moisture levels and may produce saturated overland flow early in a rainstorm cycle
explain the physical factors that affect the rate of percolation and groundwater flow
the permeability of the rock and the angle of the rock strata as a steeper gradient will allow gravity to operate more effectively
- the rate of P and GWF increases with porosity, impermeable rocks like granite prevent any percolation
- areas with permeable rock will absorb the water and create groundwater storage and an aquifer (a permeable rock which shotes water)
- porosity relates to the total volume of pore spaces and is greatest in coarse-grained rocks such as sandstone, while pervious rocks such as limestone have joints and bedding planes along which water can flow
what are the 4 factors that affect the rate of evapotranspiration?
1) temperature- increases with temp, the main energy source is solar radiation so evaporation and temperature will increase with sunshine hours
2) wind- increases the rate by reducing the relative humidity and preventing saturation of the air
3) vegetation cover- transpiration will increase with increased vegetation cover, which will depend on the type of vegetation with a low albedo (reflectivity), such as dark forests will absorb more solar ration and so more evaporation
4) soil moisture content- will determine the amount of water available for transpiration. it is dependant on soil and rock permeability.
what is potential evapotranspiration?
the amount of evapotranspiration that could take place given unlimited supplies of water in an environment
what is actual evapotranspiration?
the amount of evapotranspiration that takes place given the actual water availability
what is evapotranspiration?
the total amount of moisture removed from a drainage basin by processes of evaporation and transpiration, they can represent a significant output
what are the 6 ways in which humans affect drainage basins?
- overabstraction
- river management (creation of dams and reservoirs)
- deforestation
- cloud seeding
- urbanisation
- changing of agricultural land use
explain the human factor affecting drainage basins: over abstraction through the use of the Aral Sea and china
- in some areas, groundwater is abstracted from aquifers faster than it is replaced, causing reduced groundwater flow and a lower water table
- groundwater rebound can occur as a result of reduced abstraction for industry, increasing the risk of groundwater flooding, seen in some of the conurbations like London
- over abstraction leads to salinisation, also leads to water table dropping
Aral sea
- filed were planted in order to make the soviet union completely self-sufficient in cotton consumer the rivers feeding into the Aral Sea
- the previous sea was the largest landlocked body of water but is now a pit of sand, salt and pollution
- the sand travels into the town through the winds as the salt sticks to the seabed, goes into the lungs pf the residents, tuberculosis has gone up as a result
China
- groundwater is used to irrigate more than 40% of china’s farmland and provides 70% of drinking water in the dry northern and north-western regions
- groundwater extraction is increasing by about 2.5 billion cubic metres/year and so ground water levels in the arid north china plain dropped by 1m between 1974 and 2000
explain the human factor affecting drainage basins of river management through the creation of dams and reservoirs, including glen canyon dam and the Aswan dam
- reservoirs and dams interrupt the natural flows of water by delaying the flows through a drainage basin and adding to the amounts lost by evaporation
- the chemistry of the water may be altered, water exiting may be higher in dissolved salts or have lower oxygen levels than would be the case for a free-flowing river
- the disruption of sediment transportation leads to areas further downstream suffering from sediment deprivation, affecting shoreline processes and biological productivity of coastal regions
- salinity levels within the reservoir can also rise as its water evaporates and lowering of water tables
glen canyon dam
- was constructed in 1963 and the erosion of sediment along beaches started because of the lack of incoming sediment , by the 1990s many beaches were at risk of disappearing, native species may not survive
aswan dam
- lake Nasser behind the Aswan dam in Egypt is estimated to have evaporation losses of 10 to 16 billion cubic metres every year. this represents a loss of 20-30% of the Egyptian water volume from the river nile
- estimated that 7% more water us evaporated from the world’s reservoirs than is actually used
explain the human factor affecting drainage basins of deforestation, including the amazon
- the removal of the dense canopy protecting the vital topsoil can have devastating consequences
- the clearing of forests for new roads and palm oil plantations and modern agribusinesses, disrupt the drainage basin cycle by accelerating natural processes, reduces evapotranspiration but increases infiltration and surface runoff
- the trees play an essential role in the hydrological cycle, through transpiration they add water into the atmosphere, contributing to the formation of clouds and therefore rain
- when forests are cut down less moisture goes into the atmosphere and rainfall declines, sometimes leading to drought
amazon
- deforestation affects everyone, moisture travels and results in rainfall on the other side of the world, ends up falling in Texas
- rainforests of borneo and the amazon have experienced severe droughts, made worse by deforestation
- soot impacts air quality and health, smoke producing forest fires reduce rainfall by disrupting clouds convection
- transpiration replenishes reservoirs
- more soil erosion and silt being fed into rivers
- a lowering of humidities
- more surface run off and infiltration
explain the human factor affecting drainage basins of cloud seeding, including china and UK
- the attempt to change the amount or type of precipitation by dispersing substances into the air that serve as cloud condensation nuclei ( hygroscopic nuclei)
- new technology and research claims to have produced reliable results that make cloud seeding a dependable and affordable water supply practise for many regions
- it can be done by ground generators or by planes/rockets
china
- china used cloud seeding in Beijing just before the 2008 Olympic Games to create rain to clear the air of pollution
USA
- it is used in the alpine meadows area in California to improve snow cover, and was used in 2015 in Texas to reduce the impact of drought
explain the human factor affecting drainage basins of urbanisation , including the UK
- urbanisation creates impermeable surfaces that reduce infiltration and percolation and increase surface run off and through flow through artificial drains, stream and river discharges often increase rapidly as a result
- building new storage reservoirs or abstracting more water from rivers and groundwater reserves, helps to satisfy the increasing water demands of expanding cities at the expense of natural water flows
UK
- urbanisation has increased flood risk in many towns and cities such as winchester and maidenhead (2014 floods) and Carlisle, York and Manchester (2015 floods)
- drains deliver rainfall more quickly to streams and rivers, increasing flooding
what are water budgets?
they show the annual balance between inputs (precipitation) and outputs (evapotranspiration) in any given area
how do you calculate a water budget?
precipitation = discharge + evapotranspiration + - change in storage
P = Q + E + - S
what are the different parts of a water budget?
monthly temperature potential evapotranspiration precipitation water surplus soil moisture utilisation soil moisture deficiency soil moisture recharge
explain the water budget of Southampton
mid December - mid april = water surplus
mid April- mid july = soil moisture utilisation
mid july-mid sep = soil moisture deficiency
mid Sep-mid December = soil moisture recharge
what is soil moisture surplus?
precipitation is greater than potential evapotranspiration, soil water store is full so there is a soil moisture surplus for plant use, runoff and ground water recharge. soil is at field capacity
what is soil moisture utilisation?
potential evapotranspiration higher than precipitation, more water evaporating from the ground surface and being transpired by plants than is falling as rain. water is also drawn up from the soil by capillary action. the water is gradually used up.
what is soil moisture deficiency?
deficiency of soil water and potential evapotranspiration is higher than precipitation. plants must have adaptations to survive and irrigation is needed
what is soil moisture recharge?
precipitation higher than potential evapotranspiration, soil water will start to fill again
what is field capacity?
the soil is now full of water and cannot hold any more. further rain could lead to surface runoff
what is potential evapotranspiration?
the amount of evaporation that occurs as a result of temperature and vegetation (time of year) as long as there is water available
what are water budgets influenced by?
climate type (tropical temperate or polar)
depending on the climate type, the impacts on soil and water availability will differ
explain the water budget in Cairo, Egypt
in Cairo’s desert biome it means that they have very little rainfall a year with only 24mm annually. they are constantly in a soil moisture deficiency. evapotranspiration of 1170mm a year. their low rainfall means the risk of failure of crops, dried soil making it hard to grow and rivers drying up. highest EP in July due to heat.
explain the water budget in Alaska
Alaska’s polar climate has led to 115mm of rain a year with an EP of 292mm and so they too suffer from deficits between may and September. their cold conditions and low rainfall means it is hard to grow crops and the soil lacks nutrients. highest EP in July due to heat.
what is infiltration capacity?
the max rate at which the water can be infiltrated
what are river regimes?
it indicates the annual variation of discharge of a river a particular point (in cumecs)
what is discharge?
the amount of water in a river at a particular point, measured in cumecs
what are simple regimes?
occur when the river experiences one main factor leading to a period of high and period of low discharge (e.g. snowmelt in summer or a rainy season/monsoon
what are complex regimes?
they are more likely to occur in longe rivers that cross more than one type of catchment which results in more than one factor influencing the pattern of discharge
explain why the Mekong has a complex regime
it has a very large parchment and therefore experiences different climates across the basin. it will have varying terrain and vegetate cover, adding to the complexity of the pattern. river passes through mountainous relief close to its source in the Tibetan plateau as well as tropical areas with dense vegetation cover and monsoon climate closer to its mouth,
explain the complexity of discharge
- much of the discharge is not from immediate precipitation or runoff as groundwater feeds rivers consistently over the year (if geology is permeable )
- there is often a lag time between when rain falls and when it reaches the river depending on many local and basin wide factors
explain the 6 factors that affect river regimes
1) the size of the catchment/basin = the larger it is the greater the discharge and complexity
2) precipitation- amount, pattern and intensity. seasonal maximum often reflected in regime. snow reduces discharge until it melts in spring/summer
3) temperature- control evapotranspiration which will be greatest in summer and reduce the discharge. glacier melt in summer increases discharge.
4) vegetation cover (influenced by climate and humans)- greater vegetation leads to less fluctuation in discharge as water is intercepted, stored, utilised and evaporated, wetlands can hold water and release it slowly into the river
5) human intervention- dam building changes flow and can regulate it, abstraction of water may lead to unexplained changes in the regime
6) geology and soil type- water stored in groundwater in permeable rocks steadily feeds the river as base flow all year and acts as a reservoir, reducing fluctuation. impermeable will fluctuate more (decreases more in summer/ periods of dry weather)
explain the river regime in the amazon
there is high rainfall all year round but has a drier season. some tributaries come from different ecosystems. over the year discharge is generally over 100,000 cumecs except from September-january.
explain the river regime in Yukon (northern canada and Alaska)
snowmelt in summer and high precipitation all year round but only falls as rain in summer. varying river regime with a large increase in discharge in June when snow melts and then goes back to down mid September. then slowly up in November.
explain the river regime in Murray-darling, Australia
monsoon climate in northern climate but temperate in the south but most lies in a rain shadow. much lower discharge with a range from around 300 cumecs to 1200 over the year. peak is in summer for monsoon climate
what are the different parts of a storm hydrograph?
The basin lag time is the time between peak rainfall and peak discharge (how long water takes to get to the river)
The rising limb is the slope upwards so the rising discharge
The recession limb is the decrease in river discharge as the river returns to its normal level.
peak discharge reached sometime after peak rainfall as water takes time to move over
The small bar graph measures the amount of precipitation.
The y axis is the discharge and the x axis is the hours from start of rain storm.
The actual space under the line graph goes from top to bottom: overland flow to through flow to base flow.
overland flow- water that came as surface runoff/overland flow
through flow- water that came through the soil
base flow- water that comes through the rocks (generally consistent, unless in drought)
when does a short and long lag time happen?
short lag time- urbanisation, no infiltration, a lot of surface run off so quickly reaches the river
long lag time- when less impermeable surfaces, low pop density, trees cause interception
explain a flashy hydrograph
- short lag time
- peak discharge high
- rising limb steep
- more likely to cause flooding
- rainfall reaches river quickly
explain a subdued hydrograph
- long lag time
- peak discharge low
- gentle rising limb
- rainfall reaches river slowly
- less likely to cause flooding
what are 5 factors that increase flood risk?
- urbanisation
- saturated ground
- high drainage density (lots of rivers)
- circular shape basin (all tributaries are equal distance they will all arrive at the same time)
- frozen ground
what are 5 factors that decrease flood risk?
- rural areas
- elongated shape
- low drainage density
- hot temperatures
- forested land
explain the different factors affecting a hydrograph for both flashy and subdued hydrographs.
drainage basin size
flashy: small, water will reach the channel rapidly as shorter distance to travel
subdued: large, water takes longer to reach channel as greater distance
drainage density
flashy: high, large number of surface streams per km squared, storm reacher reach channel quicker
subdued: low, small number of surface streams per km squared, water travels slowly through soil
drainage basin shape
flashy: circular, all tributaries are at equal distance, arrive at the same time
subdued: elongated, water will take long time to reach the channel from extremities of the drainage basin
rock type
flashy: impermeable, water cannot percolate into rocks, increased surface run off
subdued: permeable, water percolates through pore spaces and fissures
soil type
flashy: clay and thin soils, clay soils have low porosity and grains swell when they absorb water, low infiltration, thin soil becomes saturated quickly
subdued: sandy and thick soils, sandy soils have high porosity, deep soils allow for more infiltration
relief
flashy: high, steep slopes that promote surface runoff
subdued: low, gentle slopes that allow percolation and infiltration
vegetation
flashy: thin grass intercepts little water and low evapotranspiration more water reaches channel, low density, deciduous in winter
subdued: dense, forest and woodland, higher levels of interception, more water lost to evaporation
land use
flashy: urban, more hard surfaces less infiltration, drains carry water quicker
subdued: rural, vegetated surfaces, more infiltration, arrive to channel less quickly
urbanisation
flashy: urbanisation, producing impermeable surfaces, deforestation reduces interception, arable land, downslope ploughing
subdued: low pop density, few artificial impermeable surfaces, reforestation, pastoral land
explain the impact of urbanisation on hydrological processes
- construction work leads to the removal of the vegetation cover. this exposes the soil and increases vegetation cover
- bare soil is eventually replaced by tarmac, impermeable, high surface runoff
- high density of buildings means rain falls onto roofs that is swiftly fed into drains by gutters and pipes
- drains and sewers reduce the distance and time rainwater travels before reaching a channel
- urban rivers often have embankments to prevent flood, when flooded they can be more devastating
- bridges can restrain the discharge of floodwaters and act as local dams thus prompting upstream floods
explain the difficulty in saying whether dry ground increases or decreases flood risk
dry ground would normally have lots of pore spaces and encourage infiltration to occur until the soil becomes saturated again (unless the precipitation intensity is greater than the infiltration capacity)
however, it may have experienced high evapotranspiration which may be have drawn salts up through the soil via capillary action and left a salty crust on the surface, making it impermeable until washed away
explain the role of planners in managing land use
- in the uk planners must assess whether any proposed development will influence flood risk with the change in land use
- the national planning policy framework sets out strict guidelines
- economic development increases the likelihood of higher flow and a faster response time
- developers have to show that land use changes will not increase runoff beyond that estimated for a ‘Greenfield’ site
- in the development of large offices it will lead to lower ET, reduced infiltration, large volumes of poor quality runoff and wastewater discharge
- planners play a large role as many towns and cities are naturally prone to flooding because of their locations, number of people in urban areas and the huge amount of money invested in urban property
explain what sustainable drainage systems (SUDS) involve
these have been introduced to reduce runoff produced from rainfall.
examples include:
1) green roofs- vegetation cover planted over a waterproof membrane
2) infiltration basins- shallow depressions dug out to delay runoff and increase infiltration
3) permeable pavements- to delay runoff by using pas between pavement slabs
4) rainwater harvesting- collecting rainwater from roofs to be recycled, e.g. for irrigating gardens
5) wetlands- retention areas with marsh/wetland vegetation
what are the four types of drought in order of when they occur?
meteorological drought
agricultural drought
hydrological drought
socio-economic/famine drought
what is meteorological drought?
occurs when long-term precipitation is lower than normal
what is agricultural drought?
when there is insufficient soil moisture to meet the crops needs
what is hydrological drought?
deficiencies in surface and subsurface supplies (rivers, reservoirs, lakes and groundwater). can lead to salinisation in high temps
what is socio-economic/famine drought?
widespread failure of crops and natural vegetation, demand is greater than supply
what is the UN’s definition of a drought?
an extended period (season, year or several years) of deficient rainfall relative to the average for an area. it is a slow onset hazard.
what are the causes of meteorological drought?
1) global atmospheric circulation system and the blocking STR at 30 degrees North (Long term)
2) mid latitude blocking anticyclones (shorter term)
3) El Nino southern oscillation cycle (ENSO)
explain the difference between short term and long term precipitation deficients
shorter term may be mid latitude blocking anticyclones or sea surface temperature anomalies and long term trends may be those that are associated with climate change or the ITCZ
explain the role of global atmospheric circulation in influencing drought
areas 30º north and south are drought prone due to warm and clear skies through high pressure.
- sometimes the sub tropical ridge associated with the descending Hadley cell air blocks the high humidity, rain bearing air masses that arrive with the ITCZ.
- this is because it is dense and stable.
- this can cause severe droughts like in the Sahel and is generally worse 30º north
- climate change may increase this by reducing the temperature difference between land and sea
explain the role of mid latitude blocking cyclones in influencing drought
normally in the mid latitudes (UK) frontal precipitation is created in low pressure systems that form along the polar front where warm tropical air rises over cold polar air
- when the jetstream loops north of the UK, high pressure areas (anticyclones) from the subtropics can move northwards, this ‘blocking’ anticyclone is able to block the passage of the jetstream which brings depressions to the UK
- the stability of the anticyclones means that due to the sinking air and calm conditions they can persist and block weather systems from the west for up to two weeks, if this happens a lot it can cause drought
- from 2010-2012 England and Wales saw a prolonged period of below-average rainfall due to blocking anticyclones leading to:
- raging wildfires in NW Scotland as peat lands dried out
- in autumn 2011 farmers struggled to harvest crops
what are depressions
they move form west to east in the mid-latitudes as a result of the Coriolis force and their track is diverted by the polar front jet stream which is a very fast moving belt of air in the upper troposphere, these depressions are forced normally to the north but also to the south
what are the features of an anticyclone?
- upwards/rising air
- clockwise, light/calm winds
- high pressure
- dry weather
- very stable, not easily moved and can stay put for several weeks
what is the ENSO cycle?
It refers to the coupled ocean-atmospheric cycle where a large mass of warm water in the equatorial Pacific alters its position during the cycle. (normally happens at Christmas)
what are the normal conditions conditions in the Pacific basin compared to those during an el Nino?
normal conditions:
- trade winds blow from east to west along the equator
- the air pushes the warm moist air westwards to Indonesia and the cold dry air to South America
- the warmer air in the west results in low pressure and in the east the air cools and sinks creating high pressure
- warm, moist air rises, cools and condenses forming rain clouds
- thermocline, upwelling
el nino:
- the air circulation loop is reversed with air moving from west to east and so Peru gets wetter due to warm air and Indonesia gets dryer due to colder air leading to forest fires and droughts.
- the trade winds get weaker and warm surface water drifts across the Pacific towards S America leading to a drought in the west
- less upwelling of cold on east, cancels out normal temp difference
- it impacts the tropics the most
what happens in a la Nina ?
trade winds strengthen and so this leads to the ocean become colder with more upwelling in the east and warmer oceans in the west. this leads to enhanced conditions where water is colder than normal in the tropical part of the pacific. this can lead to severe drought conditions especially along the western coast of South America, usually follows after an el Nino event and there is very warm water moving east to west with very strong air circulation
explain what teleconnections are
it refers to the climate anomalies that relate to each other over large distances
ENSO cycles have been connected with climate anomalies across the world
teleconnections in el niño show dry weather in western pacific from June to January and eastern Pacific between the band of dry weather sees wet weather from June to April
- el nino has also created dryer conditions in south Africa and india
this is reversed in a la Nina where the band in western Pacific becoming wet and the band to the east becoming dry
- la nina has created dryer conditions in the Middle East and southern USA
areas wet in el niño will be dry in la Nina
give statistics on drought and food shortages as a result of enso
- Britain have pledged £45million of emergency support for 2.6 million people and food for up to 120,000 malnourished children in Ethiopia and elsewhere
- one of the worst affected areas is likely to be Malawi where the worst drought in almost 10 years is expected to cause further severe malnutrition
- unicef said it expected 8.5 million people in Ethiopia to need food aid and several million more in Somalia and Kenya
explain the 2015 el Nino
- the concentrated rain bands account for 40% of California’s water supply
- it reduces rainfall in their wet seasons and less rain means drier vegetation and drier air which makes forests more vulnerable to forest fires
- the 2015 el Nino was scheduled to be the strongest since 1997-98
- people wonder whether it would have brought significant rainfall to drought-plagued California
what is the relationship between volcanic eruptions and el ninos?
- large eruptions can trigger el Nino events
- el niños tend to occur one year after a large eruption as seen after pinatubo’s eruption of 1991. la Ninas also tend to be shorter after eruptions and el ninos longer
- the large amounts of sulfur dioxide can reflect sunlight. causing lower temperatures
