Global Effects of Deserts

Deserts cover over 1/3rd of the land surface, thus desert weather has global and regional climate effects

Dust Production

• Natural• High winds over deserts

• Human contribution- draining lakes in arid areas for irrigation- vehicle traffic breaking up soil structure and

crusts in deserts (recreational, military, construction vehicles)

- desertification (devegetation – overgrazing, firewood)

- dryland agriculture

Saharan Dust Transport

Number of day in July-Sept. 1987 & 1988 with aerosols.

Sahara and Empty Quarter è dust productionAmazon and Central Africa è biomass burning

Mongolian Dust Transport

• Largest dust storms in spring• Still vigorous winds associated with jet, and the ground is

drying out prior to summer

The more dust in the atmosphere the more dramatic the sunset

Fine Mineral Dust Concentration (μg m-3)7 July 1993

Fine Mineral Dust ConcentrationGreat Smoky Mountains National Park

Dust Effects on Downwind Places

• Human health• Bacteria/fungus transport

• Visibility – surface and air transport• Climate change• Radiative effects – local surface and atmospheric

temperature and global climate• Impact on greenhouse gases• Precipitation – chemistry and microphysics

• Biogeochemistry of global ecosystems• Soil formation

Human Health

• Small particles enter the lungs and cause asthma, bronchitis, emphysema, and other damage

• Pathogens (fungi, bacteria) can be transported by dust – either across oceans or more locally• example – fungus is carried by dust storms in Arizona

and causes “valley fever” - ~30 deaths per year• Psychological Effects

The coastal air is robbed of humidity by this thirsty invader and fills with static electricity. As itenvelopes desert and littoral alike, the Santa Ana creates a weird atmosphere of impending doom.

Raymond Chandler - Red Wind

those hot dry [winds] that come down through the mountain passes and curl your hair and make your nerves jump and your skin itch. On nights like that every booze party ends in a fight. Meek little wives feel the edge of the carving knife and study their husbands' necks. Anything can happen.

Great Green Wall

The Wall’s Effect on Dust in Beijing

Visibility

• Airport closures and aircraft accidents• Traffic accidents

Climate Change

• Radiative effects on temperature

Radiative Effects

• Affect the total heating of the atmosphere and surface, and the vertical distribution of the heating in the atmosphere• Can have local effects on weather, and possibly global effect

Clear sky

Saharan dust

Afghan dust

Aerosols and their interactions with radiation and moisture are the largest uncertainty in climate projections today

IPCC AR5

Climate Projections

Climate Change

• Radiative effects on temperature• Greenhouse gases • Iron from dust enters ocean• Phytoplankton growth increases•More CO2 used by biomass• Reduction in atmospheric CO2

Dust Deposition in the Ocean (mg m-2 y-1)

THE BIOLOGICAL PUMP

•The biologic pump leads to a transfer of CO2 and nutrients to the deep ocean

Climate Change

• Radiative effects on temperature

• Greenhouse gases

• Effects on precipitation- subsidence- condensation nuclei- stability (enhances/suppresses convection)- muddy precipitation

•Smoke/dust can warm the levels where clouds typically form and cool surface

•This increases stability and inhibits convection

•Plants may reduce transpiration in smoky/dusty conditions reducing moisture available for cloud formation

Biogeochemistry of Global Ecosystems• Amazon Basin – Saharan dust is an important soil

nutrient – 13 million tons per year• Hawaiian rainforests – sustained by nutrient-rich

desert dust.• West Indies/Caribbean – much of the soil originates

as Saharan dust.• Iron in dust can cause blooms of toxic algae, or red

tides, that kill fish, marine mammals, birds.• Coral reef mortality in Caribbean – linked to

Saharan dust

Dynamic Effects of Deserts on Other Atmospheric Processes• Tropical storms, hurricanes• Elevated mixed layers• Heat-low strength• Monsoon circulation

Sahara Air Layer (SAL)

•A very dry and dusty elevated layer of air advected westward over the North Atlantic every 2-5 days

•High wind velocities relative to the North Atlantic background –results in large wind shear

•Can be 6000 m deep (the height of the African boundary layer)

SAL / Tropical Cyclone (TC) Interactions

•Promotes waviness in the trades

•Can lead to organized convection and TCs

•TCs embedded in SAL generally stay weak

•Dry air•High wind shear

•TCs emerging from SAL can rapidly intensify

Dynamic Effects of Deserts on Other Atmospheric Processes• Tropical storms, hurricanes• Elevated mixed layers• Heat-low strength• Monsoon circulation

The Elevated Mixed Layer (EML)

EMLs contribute to severe weather development in the Great Plains

Dynamic Effects of Deserts on Other Atmospheric Processes• Tropical storms, hurricanes• Elevated mixed layers• Heat-low strength• Monsoon circulation

Heat Lows and Monsoons•Monsoonal circulation results

from seasonal changes in solar radiation

•As land is heated in the summer a heat low forms

• The large amount of land in the tropics and sub-tropics in the NH makes the NH heat lows and monsoons very strong

• The heating of the huge Sahara can remotely influence the strength of the Asian monsoon

Monsoon Circulation over N. Africa• Strong summer heating creates

low-pressure over west-central N. Africa drawing moisture from tropical Atlantic• Wet summer monsoon

• Winter cooling creates high pressure in northwest Sahara Desert enhancing flow of the northern trade winds• Dry trade winds inhibit

precipitation

• Similar circulations develop over Asia and southwestern US