Climatic Causes of Aridity: Climatic Causes of Aridity


Cause Type of Desert: 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: Simplification In reality, there are often multiple causes for a place being a desert.


Planetary-Scale CirculationEffects: Planetary-Scale Circulation Effects


Schematic of Large-Scale Regimes: Schematic of Large-Scale Regimes subsidence


The Vertical Circulation, Based on Data: The Vertical Circulation, Based on Data (Sine-Latitude to Reflect Area Coverage on Sphere) Variance in circulation Circulation strength


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


Subsidence in Relation to High Pressure Centers: Subsidence in Relation to High Pressure Centers


Slide10: Subtropical high-pressure centers Climate of the Horizontal Wind and Sea-level Pressure


Sea-Level Pressure Averaged Over Longitude: Sea-Level Pressure Averaged Over Longitude


Slide12: 12 N 30 N 12 S 30 S 12 S 30 S Simplification, but useful (subsidence from subtropical high’s depends on season and longitude)


Slide13: 12 N 30 N


Slide14: 12 N 30 N 12 S 30 S


Orographic Effects: Orographic Effects


Orography Has Multiple Effects on Desert Precipitation: 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 west of lee-side trough


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


Rainshadow Process: Rainshadow Process


Question: Question Is the formation of cloud over the mountain enough to cause a rainshadow? Do we need precipitation to reach the ground?


Summary - Why Rainshadows Cause Deserts: 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 DesertsElevationshaded > 1500 m contours interval = 500 m: North American Orography and Rainshadow Deserts Elevation shaded > 1500 m contours interval = 500 m Wind direction


Slide25: X X X X


South American Orography and Rainshadow Deserts: South American Orography and Rainshadow Deserts


Reminder About Wind Directions: Reminder About Wind Directions Easterly Trades Westerlies


South American Orography and Rainshadow Deserts: South American Orography and Rainshadow Deserts Wind direction Wind direction


Monte Desert – Andes Rainshadow: Monte Desert – Andes Rainshadow


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


Tropical Island RainshadowsElevationlight gray > 2 kmdark gray > 3 km(Speculate why the precip minimum is where it is?): Tropical Island Rainshadows Elevation light gray > 2 km dark gray > 3 km (Speculate why the precip minimum is where it is?) Hawaii Ambient = 65-75 cm/year Minimum


Example of Altitude Oases(The Sahara): Example of Altitude Oases (The Sahara)


Highest Mountains in Central Sahara: Highest Mountains in Central Sahara


Maximum Observed 24-h Rainfall (mm): Maximum Observed 24-h Rainfall (mm)


Slide40: Sea breeze hitting the mountains in Oman


Another Orographic EffectLong Waves in the Westerlies and Aridity: Another Orographic Effect Long Waves in the Westerlies and Aridity


Orographic Waves in the Westerlies: Orographic Waves in the Westerlies DOWN UP


ObservedStationary Waves: ObservedStationary Waves N. Amer Asia Europe Subsidence


Global Model Experiment To Illustrate Mountain Effect: 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 mountainslight – 1 mm/daydark – 2 mm/day: Precipitation Difference – Gray where less precip. with the mountains light – 1 mm/day dark – 2 mm/day Great Basin Desert Asian Deserts


Summary of the Effects of Mountain Waves in the Westerlies: 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: Continental Interior Deserts


Cause: 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: 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: 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: Example Continental Interior Deserts High Plains of North America Northeast Africa Central Asia


Air Currents Shifting From Continental Interior to Oceans Can Be Responsible For Drought: 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?


Slide55: 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 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: 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


1) FrictionContrast Effect: 1) Friction Contrast Effect


2) Coastal mountains blocking high pressure centers: 2) Coastal mountains blocking high pressure centers


3) Subsidence Induced by Mountains Near the Coast: 3) Subsidence Induced by Mountains Near the Coast


4) Cold Currents – Upwelling N. Hemisphere Example: 4) Cold Currents – Upwelling N. Hemisphere Example


Coastal mountains block high pressure centers: Coastal mountains block high pressure centers


Cold Ocean Current Effects: Cold Ocean Current Effects


Slide64: Lima Antofagasta


Temperature Profile Near Coast: Temperature Profile Near Coast


Coastal Desert Precipitation: Coastal Desert Precipitation


Dynamic “Feedback” Mechanisms That May Cause or Perpetuate Deserts: Dynamic “Feedback” Mechanisms That May Cause or Perpetuate Deserts


What is a “Feedback”?: What is a “Feedback”? More than a “cause and effect”. Perturbation Effect Amplified perturbation (positive feedback)


Slide73: Effect Perturbation Reduced perturbation (negative feedback)


Vegetation – Albedo Feedback: Vegetation – Albedo Feedback This feedback is based on the fact that vegetation is generally less reflective than bare desert surface


Slide76: 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…: 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?


Another Vegetation Feedback – Vegetation-Transpiration Feedback: 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: 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 feedback Or A rainy period causes the soil to be moist. More water vapor is evaporated into the atmosphere. More precipitation results A positive feedback


Discussion: 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: Comments More on this vegetation feedback later in the discussion of desertification (e.g., deforestation) Examples of how this feedback may have produced deserts.


Evidence of Soil-Moisture Feedback: 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


Slide83: 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: 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.


Discussion of Dust-Radiation Feedback: 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: 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: Upwelling – Coastal-Desert Feedback Hot desert surface next to cold ocean current causes a geostrophic wind to develop parallel to the coast.


Slide90: Hot Surface Cold Surface


Sea-Breeze Circulation - Day: Sea-Breeze Circulation - Day


Combining the Coriolis and the Pressure-Gradient Forces: Combining the Coriolis and the Pressure-Gradient Forces The Geostrophic Wind


Slide93: L H Geostropic Wind


Upwelling – Coastal-Desert Feedback: 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.


Temperature Profile Near Coast: Temperature Profile Near Coast


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


Dust-Biogeochemical Feedback(one example of many): 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.