What is the basic idea behind the dust transportation?
This concept is an interaction between the atmosphere,
geosphere and pedosphere. It also includes the anthrosphere, hydrosphere and
biosphere as flow on effects. A process occurs between these spheres. This
process is when eroded nutrient-rich dust particles are taken into the
atmosphere and transported in the air. The dust is transported via different
atmospheric circulation currents to the Amazon rain-forest. This dust acts as a
nutrient rich blanket over the rain-forest floor which is quickly consumed by
the vegetation both on the forest floor, and also through the roots of the tall
canopy trees. The process is crucial for the growth and sustainability of the
Amazon rain-forest. This shows the complex and unique interaction between the
world’s most barren desert and the world’s most abundant rain-forest.
Why is the dust transportation beneficial
for the Amazon?
The Saharan soil
contains levels of phosphorus that the amazon needs so it can continue to
grow. This is because the Amazon loses some of its natural phosphorus source to
run off and floods due to its high precipitation and water levels. The 'Sahal'
is a semi-arid landscape in the southern portion of the Saharan desert. When
the Sahal has heavy rain there is more dust transported to the amazon, this is
because the Sahara desert was previously a huge lake full of marine life, which
has contributed to phosphorus reserves over many years. "Specifically the dust picked up from the
Bodélé Depression in Chad, an ancient lake bed where rock minerals composed of
dead microorganisms are loaded with phosphorus. Phosphorus is an essential
nutrient for plant proteins and growth, which the Amazon rain forest depends on
in order to flourish. The phosphorus that reaches Amazon soils from
Saharan dust is an estimated 22,000 tons per year, about the same amount as
that is lost from rain and flooding" (E. Gray, 2015). This phenomena can be seen by Figure 1 below showing the dust transportation from a NASA Satellite in a 3-D perspective.
Figure 1: Titled - NASA Satellite Tracks Saharan Dust to Amazon 3-D
Figure 1: Titled - NASA Satellite Tracks Saharan Dust to Amazon 3-D
Wind Principles: basic systems and currents of the atmosphere:
Low Pressure and High Pressure Systems:
Wind
always flows towards the lowest pressure. In a low pressure system the lowest point of pressure is the
centre of the system. Warm, moist air is pulled into the low pressurised centre
and rises. At a certain height the water vapour in the air condenses and the
air then precipitates and becomes a cloud after changing state. The direction
of a low pressure system is clock-wise in the Southern Hemisphere with the
Coriolis force and Pressure Gradient Force. This can be seen in Figure 2 below.
Figure 2: Diagram showing - Direction of air flow in Low and High pressure systems
A high
pressure system’s most pressurised point is its centres. This means the cold,
dry descending air flows into the top of the system and is forced downwards to
escape the high pressure and find the relatively lower pressure outside the
system. The air is transported outwards towards the exterior of the system. The
direction of a high pressure system is anti-clock-wise in the Southern
Hemisphere with the Coriolis force and Pressure Gradient Force, (UCAR.com, (2016), Center for Science Education).
Coriolis Effect:
The Coriolis Effect is due to the Earth's shape which creates a rotational force that acts on the Earth’s atmosphere. The effect causes the atmospheric conditions to be deflected to the right in the Northern Hemisphere and be deflected to the left in the Southern Hemisphere.
Walker Circulation and El Nino/La Nina:
The walker circulation is system traveling from South America
towards Australia. This causes many affects as the atmospheric currents
interact with the ocean currents and create El Nino and La Nina patterns which
are different versions of the walker circulation. La Nina is a strong walker
circulation and El Nina is a weak walker circulation. When La Nina and El Nino
are not there you have a normal walker circulation. A normal walker circulation
happens as follows:
-There is warm water in the Western Pacific Ocean and cold water
in the Eastern equatorial Pacific Ocean. The area of warm water evaporates into
the atmosphere above it and causes a warm moist area and a LP system, this
creates rain as the air becomes full of moisture from evaporation so it has to
release it by cloud formation and rain.
-Since the air has now lost its moisture by precipitation it is
now dry. This causes it to head Eastwards across the Pacific Ocean away from
the LP system
-While travelling to the cold water of the Eastern Pacific Ocean
the air cools and descends. This creates a HP area in the Eastern Pacific Ocean
as the cold air particles are close together increasing the pressure. This also
causes the upwelling effect of nutrients as the cold water circulates around,
causing good fishing conditions. Because air always wants to get back to LP the
air that has just settled over the HP system then travels back to the Western
Pacific Ocean. This whole process then repeats and can be seen by Figure 3 below.
Figure 3: Diagram of the normal Walker Circulation conditions, and of the El Nino conditions
El Nino is a variation of the walker circulation, where the wind
that would usually flow from East to West becomes very weak and loses its
energy. This means that there is no wind to create the processes that would
separate the Pacific Ocean into warm and cold areas, so the water temperatures
and climate reverse. The effects of this is comparatively warm Eastern waters
and comparatively cold Western waters. This also causes the upwelling to cease
until the normal winds return which affects South American fishing. La Nina is
another variation of the walker circulation where the winds become much
stronger than they usually are, this doesn't reverse the HP and LP systems, but
just intensifies them (Study.com, ND (No Date), Vol: Chapter 9, Lesson 12 - El Nino, La Nina and Walker Circulation).
Radiation:
Wind circulation is powered by the Sun’s uneven heating of the
Earth’s surface, and by the Earth’s rotation. The uneven heating causes giant
circulation patterns. This is because the spherical shape of the Earth means
that the tropics will receive more radiation than the poles. The higher in
latitude means the less amount of short wave radiation will be absorbed by the
surface. As mentioned before the LP and HP systems effect the air in different
ways. The only reason the LP systems exist is due to having warm spots for
moisture to evaporate off the oceans into the atmosphere, which is due to the
uneven distribution of the sun's radiation. This effects wind circulation
because this creates cold areas and warm areas in the atmosphere, and the Earth
counters this by creating a number of different convection currents and cells
to attempt to distribute all heat evenly around the Earth.
How does the dust transport itself from the Sahara Desert to the Amazon?
The dust from the Sahara Desert has different processes that transfer it to the Amazon rain-forest. The three primary processes are the Trade Winds (Alize Maritime), Harrmattan Wind and the Haboob Storm.
Haboob storm:
Figure 4: Titled - Thunderstorm Creates Haboob Dust Storm over the Sahara
"The term Haboob is taken from the word Habb meaning Wind in Arabic" (WeatherOnline.co.nz, ND (No Date), 'Haboob'). The Haboob is a strong wind or storm that occurs in arid locations. The formation of the Haboob storm occurs when anti- cyclone conditions create a low pressure system, this is common in the 'Sahal' landscape which is a semi arid landscape south of the Sahara Desert. It is common here because it is close enough to the equator to be warm, and it has not experienced full desertification so it still has some moisture in its atmosphere. In terms of this case study of dust transportation to the Amazon this Haboob storm is an important process. The storm often forms in South-Sudan and works its way up towards the Sahara, (Figure 5 below).
Figure 5: Titled - Map of Regions in the Saharan Desert and Sahal Desert
"When the storm collapses and begins to release precipitation, wind gusts outward from the storm", (Wikipedia.org, (September 2016), 'Haboob'). "When this downdraft of cold air reaches the ground, it blows dry, loose silt and clay (collectively, dust) up from the desert, creating a wall of sediment that precedes the storm cloud. This wall of dust can be up to 100 km wide and several kilometres in elevation. At their strongest, Haboob winds often travel at 35–100 km/h" (Wikipedia.org, (September 2016), 'Haboob'). This wall of sediment can be by Figure 6 below showing a Haboob storm in Khartoum, Sudan. This wall of cold air picks up dust sediment and suspends it in the air. The dust wall continues over the Sahara Desert picking up the nutrient rich particles. The fuel behind this process is the coldness of the air, but as it traverses northward across the Sahara Desert it slowly begins to lose its energy because the air starts to warm up. Once the air is too warm and has lost all its energy it settles back down in the Sahara Desert region.
Figure 6: Title - Haboob Storm in Khartoum, Sudan.
Trade Winds (Alize Maritime):
There are two sets of trade winds, one in the Northern-hemisphere and one in the Southern-hemisphere. The trade winds are "prevailing winds that meet at the Intertropical-Convergence zone", ((March 2008), NOAA Ocean Service Education Website). "This is a narrow zone near the equator where northern and southern air masses converge, typically producing low atmospheric pressure", (The Columbia Electronic Encyclopedia, Vol:6, (2012)). To understand the trade winds we must first explain the westerlies, because the trade winds are the returning Westerly winds. The westerlies are a current of air that starts from the equatorial region and is flowing to higher latitude both North and South of the equator. Due to the Coriolis Effect the air has left the equator and has been dragged to the right, therefore in the northern hemisphere this Westerly wind is traveling in a SE direction. This is important to know in terms of the trade winds because now we know where the air is coming from. The air from the westerlies ends up in the NE and SE around the mid 30-60 degree latitude. This air then descends and blows back towards the equatorial region, in the Northern-hemisphere the air will blow from the NE to the SW and settle at the equator. It is at this stage that the Trade Winds are over the equator and the Sahara Desert. While the dust was suspended in the air over the Sahara by the Haboob storm they get caught by the North-Easterly Trade Winds. These are the winds that blow back towards the equatorial region from the Northern hemisphere in a NE to SW direction at approximately 30 degrees latitude, just like the Sahara Desert at approximately 25 degrees latitude.
Figure 7 and 8: Titled - Direction of Trade Winds
The Trade Winds which have now picked up the dust particles from the Haboob storm flow along the 30 degrees latitude and slowly continue their NE to SW direction until they hit the inter tropical convergence zone (Figure 7, top left). Once the air reaches this zone it changes its direction from going to the SW to just heading West (Figure 8, bottom left). This is a strong warm current that transports the Trade Wind air full of dust straight West. This inter-tropical convergence zone co-insides exactly with the Amazon rain-forest. This is the final flow of the dust as it is carried along the inter-tropical convergence zone until it is over the Amazon, then the dust that falls out of the air through precipitation etc reaches the Amazon ground surface and makes the rich nutrient blanket for the vegetation.
Harrmattan Wind:
Figure 9 (right): Titled - Harmattan Dust Storm, Niamey, Niger - June 26, 2012
The Gulf of Guinea has low pressure because it is in the inter-tropical convergence zone which means it is warm due to heating from proximity to the equator, and also it is on Africa’s West-coast leading into the Atlantic Ocean. This combination of warm air and moisture coming off the Atlantic Ocean causes a low pressure system to form. The Harrmattan wind full of dust from the Sahara is coming towards this low pressure and it is leaving a high pressure system, the Sahara Desert. The meeting of these pressure systems creates the Harrmattan wind as "the cool Gulf of Guinea air (SW direction) undercuts the warm Trade Winds (NE direction)". This causes the warm Harrmattan wind to flow on top of the cold air due to density, thus creating the huge wall of dust. This Harrmattan air then travels past the Gulf of Guinea and dissipates. The dust dropping out of the air is picked up by the Trade Winds and taken to the Amazon in the same way that the dust that falls out of the Haboob does after it dissipates.
Hadley Circulation Cells:
Hadley Circulation cells are giant currents in which warm equatorial air flows toward the poles and cold polar air streams back to replace it. This cycle is important to the dust transportation because the cells overlap with the inter-tropical convergence zone which is carrying the nutrient rich sediment.The replacing warm air with cold polar air happens first at the equator.
Hadley Circulation cells are giant currents in which warm equatorial air flows toward the poles and cold polar air streams back to replace it. This cycle is important to the dust transportation because the cells overlap with the inter-tropical convergence zone which is carrying the nutrient rich sediment.The replacing warm air with cold polar air happens first at the equator.
-The warm moist air created by low pressure (LP) conditions flows out the top of the system after being drawn inwards at the equator.
-The air flows away from the equator. The further away it gets it is losing heat because it is receiving less short wave radiation the further it gets from the equatorial heating zone, and is not being warmed by the LP system.
-This air also gets colder the higher it gets, becoming colder and dryer, because the atmosphere decreases in temperature as you gain altitude.
-Due to the rapid cooling of the air it begins to descend, this is because warm air is less dense because the molecules are further apart and have more energy through vibration, so it rises above cold air.
- Some of this descending air flows over the desert belt at 25-30 degrees latitude. This air flow is what created the deserts either side of the equator to start with, as the lack of moisture in the dry air caused desertification of the area.
-Once the air sinks close enough to the surface it will rapidly heat up again due to being near the Earth’s surface. This is because the surface absorbs high levels of short wave (SW) radiation and emits it back out again as long wave (LW) radiation.
-The surface heats the air back up again, therefore making it rise again.
-This is a continuous cycle of heat by the equator; loss of heat by flow away from the LP system and re-gain of heat through proximity to the surface thus creating a flowing cell called a Hadley Cell.
