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Climatic Causes of Aridity

Cause Type of Desert

§ Planetary-scale circulation effects –subtropical deserts

§ Orographic effects – rain-shadow deserts§ Geographic remoteness from moisture

sources – continental-interior deserts§ Coastal effects – cool, coastal deserts

Simplification

In reality, there are often multiple causes for a place

being a desert.

Planetary-Scale CirculationEffects

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Global Circulation: A Closer Look

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Effects of Heating and CoolingHow does heating or cooling of the atmosphere change the surface pressure?

Global Circulation

What causes this subsidence?

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Convergence and Divergence

§ On the previous slide is there convergence or divergence occurring at 30 N?

Convergence, Divergence, and Vertical Motion

§ Does convergence or divergence lead to rising motion?

§ It depends on where in the atmosphere the convergence or divergence occurs

§ Why?

Convergence, Divergence, and Vertical Motion

§ Upper level convergence or low level divergence leads to sinking air

§ Upper level divergence or low level convergence leads to rising air

A Lid on Vertical Motion: The Tropopause

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July Average – Rub Al Khali Desert, 20o Lat.

Subsidence in Relation to High Pressure Centers

Subtropical high-pressure

centers

Wind and Sea Level Pressure

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Sea-Level Pressure Averaged Over Longitude

12 N

30 N

12 S

30 S12 S

30 S

Simplification, but useful

(subsidence from subtropical high’s

depends on season and longitude)

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12 N

30 N

12 N

30 N

12 S

30 S

Orographic Effects

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Orography Has Multiple Effects on Desert Precipitation

§ Decreased rainfall downwind of mountains (rainshadow effect to be described shortly) causes deserts

§ Mountains within deserts enhance the precipitation causing “altitude oases”.

§ Mountains generate long waves in westerlies, and subsidence to the westof lee-side trough

Rainshadow process

§ Explain how mountains can cause conditions to be more arid on downwind side – less precipitation, hotter drier.

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Rainshadow Process

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Questions

§ Is the formation of cloud over a mountain enough to cause a rainshadow?

§ Or, do we need precipitation to reach the ground to create a rainshadow?

Summary - Why Rainshadows Cause Deserts

§ Leeside air has less water vapor because of upstream condensation and precipitation –lower specific humidity

§ Drier air is mixed in from aloft, as air passes over mountain

§ Air is warmer because of latent-heat release§ Higher temperature and lower specific

humidity both contribute to lower relative humidity – more desiccating air

§ Subsidence causes temperature lapse rates to be more stable

North American Orography

and Rainshadow

Deserts

Elevationshaded > 1500 m

contours interval = 500 m

Wind direction

XX

X

X

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South American Orography and Rainshadow Deserts

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Reminder About Wind

Directions

Easterly TradesWesterlies

South American Orography and Rainshadow Deserts

Wind direction

Wind direction

Monte Desert – Andes Rainshadow

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Some Other Deserts That Exist Partly Because of the Rainshadow Effect Tropical

Island Rainshadows

Elevationlight gray > 2 kmdark gray > 3 km

Hawaii

Ambient = 65-75 cm/year

Example of Altitude Oases

(The Sahara)

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Highest Mountains in Central Sahara Maximum Observed 24-h Rainfall (mm)

Sea breeze hitting the mountains in Oman

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Another Orographic Effect

Long Waves in the Westerlies and Aridity

ObservedStationary

WavesN. AmerAsia Europe

Subsidence

Subsidence in Relation to Upper Level Trough

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Global Model Experiment To Illustrate Mountain Effect

§ Simulate a year of weather with existing mountains

§ Repeat the simulation without mountains.§ Subtract the annual-total precipitation§ But, all effects of mountains are involved –

long waves, rain shadow, blocking of moisture

§ Model used was coarse resolution, so results are approximate

Precipitation Difference –

Gray where less precip.

with the mountains

light – 1 mm/daydark – 2 mm/day

Great Basin Desert

Asian Deserts

Summary of the Effects of Mountain Waves in the Westerlies§ Effects limited to middle and high

latitudes, and winter§ Reduced precipitation region extends far

downwind of mountains

Continental Interior Deserts

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Cause§ These deserts are “distant” from major water-

vapor sources (e.g., evaporation from oceans), and may be near the middle of continents.

§ During the transit of the air over large distances, much water vapor gets precipitated out, causing the air to be dry when it gets to the interior of continents.

§ Need to consider air trajectories (path), when deciding whether a location is “distant” from moisture sources.

§ Source regions for water vapor – best is oceans, worst is dry land and frozen ground.

Many Deserts Are Located Where a High Percentage of the Precipitation Moisture Originates Over Land – These

Areas are Not Supplied With Much Moisture

Referring to Previous Figure

§ Some deserts apparently receive plenty of moisture – SW North America, Australia.

§ Thus, the amount of moisture in the air is only part of the story.

§ The dynamics are another factor. § That is, the air may be humid, but if there

is subsidence the air does not saturate.

Example Continental Interior Deserts

§ High Plains of North America§ Northeast Africa§ Central Asia

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Air Currents Shifting From Continental Interior to Oceans

Can Be Responsible For Drought

For example the High Plains of the

U.S. frequently experiences

extended drought –WHY?

Drought rainfall - % normal Top of PBL wind– non-drought years

Top of PBL wind – drought years Months - flow from base of Rockies

Coastal Deserts

Coastal deserts – the aridity is related to some aspect of coastal dynamics

(i.e., not a subtropical desert that just happens to be on a coast)

Coastal Dynamic Effects That Can Increase Aridity

§ 1) Contrasting friction of the air flowing near the ground along the coast

§ 2) North-south mountains on west side of continents in the subtropics can cause H’s to be stationary along the coast

§ 3) Heating of mountains near the coast can cause subsiding air over the adjacent coastline

§ 4) Cold ocean current near the coast– air that moves onshore is stable because it has

been cooled at low levels

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1) FrictionContrast

Effect

2) Coastal mountains blocking

high pressure centers

3) Subsidence Induced by Mountains Near the

Coast

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4) Cold Currents – UpwellingN. Hemisphere Example

Cold Ocean Current Effects

Lima

Antofagasta

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Temperature Profile Near Coast

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Coastal Desert Precipitation

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Dynamic “Feedback” Mechanisms That May Cause

or Perpetuate Deserts

What is a “Feedback”?§ More than a “cause and effect”.§ Perturbation

Effect

Amplifiedperturbation (positive feedback)

Effect

Perturbation

Reducedperturbation (negative feedback)

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Vegetation – Albedo Feedback

This feedback is based on the fact that vegetation is generally less reflective than bare desert surface

Vegetation–Albedo Feedback§ Vegetation is generally less reflective than bare desert

surface.§ Say that some vegetation dies because of drought, it is

overgrazed, it is burned off, or it is smothered by sand or dust

§ Albedo will increase§ The surface will not absorb as much solar radiation, and

the surface and lower atmosphere will be heated less. § This will make the atmosphere less unstable.§ There will be less convective rainfall.§ This will reduce the vegetation amount even more.§ And so on – a positive feedback.

But…§ This argument says that drought leads to

the surface being heated less.§ But don’t we normally think of droughts as

being hotter than normal?

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Another Vegetation Feedback –Vegetation-Transpiration Feedback§ Some vegetation dies because of drought, etc.§ There is less transpiration of water vapor and less

evaporative cooling.§ The air near the surface will become hotter, and

the lapse rate more unstable.§ There will be less water vapor released into the air

to produce convective rainfall.§ Less rainfall means even less vegetation.§ And so on – another positive feedback.

Soil-moisture Feedback§ A drought causes the soil to become drier.§ This means that less water vapor is evaporated into the

atmosphere.§ This reduction in the source of water vapor may reduce

precipitation.§ A positive feedbackOr§ A rainy period causes the soil to be moist.§ More water vapor is evaporated into the atmosphere.§ More precipitation results§ A positive feedback

Discussion§ Whether this feedback works this way

depends on the degree to which evaporated water vapor is recycled locally as precipitation, or whether local precipitation depends on water vapor originating far upwind.

§ The importance of local water “recycling” -many studies show that irrigation in arid areas increases the amount of rainfall (and severe weather).

Comments

§ More on this vegetation feedback later in the discussion of desertification (e.g., deforestation)

§ The following slides gives examples of how this feedback may work.

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§ Modeling study of western Sahara rainfall – model was run with existing dry surface,

and only small amounts of rainfall simulated.

– surface was made wet, and the simulated rainfall lasted for weeks and kept the ground moist.

Evidence of Soil-Moisture Feedback § Another modeling study – A spring drought over North America was prolonged throughout the summer because of the dry land surface.

§ Data study, 65 years in N. America – warm, dry springs followed by hot, dry summers; cool and wet springs were followed by wet summers.

§ Data study, in Sahel of Africa – single convective event wetted a path on the ground, and many other events followed the same path.

Dust-Radiation Feedback§ Drought causes desiccation of the surface and the loss of

vegetation.§ This leads to wind erosion and elevation of dust into the

atmosphere.§ The dust will absorb, reflect and scatter incoming solar

radiation, and absorb outgoing long-wave radiation.§ If this causes cooling and subsidence, or if it changes the

vertical temperature profile and stabilizes the atmosphere to convection, there will be less precipitation.

§ This will continue the cycle, a positive feedback.

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Discussion of Dust-Radiation Feedback

§ There is much uncertainty about the radiative effects of dust, and how it affects the atmospheric temperature.

§ The effect depends on the particle sizes, the elevation of the dust in the atmosphere, mineral composition of dust particles, albedo of underlying surface

§ It is speculated that a positive dust-radiation feedback was responsible for the generation of the Thar Desert of India

Real-World Situation

Drought Decrease in vegetation

Higher albedo, more-stable lapse rates, less precip

Less transpiration, less water vapor, less precip

Dessication of soil, elevated dust, maybe more stable lapse rates

?

Upwelling – Coastal-Desert Feedback

§ Hot desert surface next to cold ocean current causes a geostrophic wind to develop parallel to the coast.

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Hot SurfaceCold Surface Development of a thermal (heat) low

Sea-Breeze Circulation - Day

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Combining the Coriolis and the Pressure-Gradient Forces

The Geostrophic

Wind

LH

Geostropic Wind

Upwelling – Coastal-Desert Feedback

§ Hot desert surface next to cold ocean current causes a geostrophic wind to develop parallel to the coast.

§ This coast-parallel wind enhances the upwelling and makes the water colder.

§ This colder water helps maintain the cold-water hot-land temperature contrast and strengthens the wind.

§ This effect helps maintain the cold water, and its effect on the adjacent desert.

Vegetation-Substrate Feedback (does not involve atmosphere in feedback)

§ Some vegetation dies during a drought§ The lack of foliage will make the soil less able to

absorb rain water, and more water will run off.§ This will cause topsoil to be lost through erosion.§ The loss of topsoil will mean that remaining

vegetation will be less likely to survive…and erosion will increase.

§ A positive feedback

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Dust-Biogeochemical Feedback(one example of many)

§ Dust is elevated into the atmosphere over the desert.§ This dust is often transported over the ocean, where some

of it settles out.§ All dust contains some iron, and the iron increases the

production of phytoplankton. § This increased biomass in the ocean would reduce the

amount of CO2 in the atmosphere and ocean, and reduce greenhouse warming.

§ This may increase deserts in some areas, and increase dust production – a positive feedback. Negative feedbacks elsewhere.