Climo after Exam #2

Wettest time of the year…

corresponds to the location of the ITCZ

• Tropical rainforest

• Tropical wet and dry

• Monsoon

Why does the ITCZ remain in the NH near South America and Africa?

• North Atlantic Ocean is warmer than the South, NH has a larger landmass that heats up faster to which persistent north-heavy heating created a “thermal equator” on average

• ITCZ position doesn’t change very much year to year, but tropical climates can still experience substantial year to year fluctuations in precipitation.

Is there typically a stronger or weaker monsoon during El Niño?

Weakened monsoon

Dry Climate

• Desert

• Steppe

• Subtropical high, large masses, rain shadow

• 26 degrees N → Influence subtropical high but no completely dominated

How has land use affected this dry/humid boundary?

• Vegetation dies out (no moisture)

• Western Nebraska - wet climate

• Sahara (driest)

• Land use as pushed this boundary wetter, making central U.S. wetter

North of the Sahara, when is the west season?

• Mid-latitude cyclone — winter

South of the Sahara, when is the west season?

Northward migration of the ITZC affects it (brings moisture from the Atlantic Ocean) — Summer

Sahel…

is in a transition zone between dry and wet climates

What should determine whether the Sahel has a relatively wet year, vs. dry year?

• Over time, there is multi decay variations

• not primarily ITCZ affects —- more north vs. south movement

• Ocean temperatures

• Atmospheric circulation

• Land-atmospheric feedbacks

Sahel Precipitation (near subtropical high)

• Amount of precipitation tied to strength to African Easterly Jet

• SSTs have influence

• tend to have reduced precipitation during El Niño years

ITCZ placement

• Colder (going right and up)

• Warm (going left and down)

La-Nina vs. El-Nino

La-Nina - wetter in Sahara

El Niño - dryer in Sahara

Desertification - Has land use made the Sahel drier?

• yes it has

• Land use pressure on edge of desert

• Severely degraded the soil

• reduced vegetation cover

• decreased land’s ability to retain water

How is the summertime surface energy balance alternated due to the changes in land use?

• Rn - ^ (net radiation)

• H-^ (Sensible Heat Flux)

• LE - down (Latent Heat)

• S - down (Specific Heat)

• Drier climate, doesn’t get precip

Modeling the effect of Sahel desterificiation

• found modest decreases in ET and precipitation, primarily in early part of rain season

Future Changes

• likely changes in Sahel precipitation over the next century

What is a factor that would tend to increase Sahel precipitation with 21st century warming?

• Intensification of the West African Monsoon (WAM) system

• This will be the Sahel

• the numerical products in the Sahel contribute, strengthen, regional circulation changes

• Increase SST (more water vapor)

What is a factor that would tend to decrease Sahel precipitation with 21st century warming?

• Increasing the sea surface temperatures

• Regions that tend to get dry will get drier

Which things should change the climate substantially over millions of year?

• Pollution, greenhouse gases (carbon dominated by what’s being stored)

• position of the continents

• Volcanic eruptions (average out)

• Greenhouse eras (some percent ice sheets)

• Feedbacks from ice sheets or lack thereof

Ice sheets only…

exist when land is in polar position

• Ice sheets will exist when land is in polar position (WRONG…. no land in polar position = no continental ice)

Ocean Circulation

• large impact on the global climate

• can be marked different

• Unlikely to play the dominate role in the shift between icehouse and greenhouse eras (does not mean there is no impact)

CO2 Variations

• varies a lot over millions of years

• Carbon storage in the earth is remarkable (hence fossil fuel extraction)

• Chemical weathering helps explains this

CO2 variations (continued)

• changes over millions of years - attributed to chemical weathering feedback

• In warmed eras, there is greater global precipitation and vegetation which increase chemical weathering and slowly reduces CO2 over millions of years (leads to a cooler environment) (negative feedback on temperature)

• Related uplift weathering hypothesis, which argues that this process is enhanced when tectonic activity has resulted in more fresh rock

Why did the climate cool since then?

• increasing chemical weathering reducing CO2

Ice Returns

• 35 million years ago - ice on Antarctica

• 7 million years ago - ice on Greenland

• Mountain glaciers in last 4-7 million years

• 2.7 million year ago: ice ages begin

Milankovitch Orbital Cycle

Obliquity:

• 41,000 year cycle

• Tilt (± 1 degree)

• Higher tilt favors melting

Precession:

• 23,000 year cycle

• timing of the year with close pass and distance pass positions of the orbit

• Close pass during NH summer favors melting

Eccentricity:

• 100,000 year cycle

• When maximized, the different between close pass and distance pass is maximized

• Higher eccentricity favors melting, but the effect is tiny

Melting

• Lack of summer melting is key for growth

• Feedbacks are important as well as orbital forcing

• Carbon Dioxide is lower during ice ages - providing a positive feedback

• Ice albedo feedback is another key positive feedback

Ice ages have gotten larger in the last million years or so, relative to the earlier period.. why?

• glaciers began sticking more efficiently to hardened bedrock allowing them to grow thicker rather than breaking into the ocean

• Increase in greenhouse gases

Ice covers..

• 7% of earth, 25% land

• Global temperature ~ 4 degrees C colder than today

• 25% less biomass

Sun Spots

• small increase in solar radiation during periods with more sunspots

• 11 year cycle not particularly important

• Forcing is ~ 1% and applied over a short time period

• Over longer periods of time, the solar effect can be greater

The Little Ice Age to Present

• Little Ice Age is linked to reduced solar radiation during the Sporer and Maunder sun spot minimum

• Warming since this has been due to solar forcing (most in early 20th century) and increase in carbon dioxide (later period)

Feedbacks over the Next Century

• Water Vapor (positive)

• Snow and ice cover (positive)

• Clouds

• Thermohaline circulation (THC) (negative, continues to weaken)

• Vegetation cover and fertilization

Where is there greater warming?

• at mid to high latitude land mass locations

• Global precipitation will increase for certain areas are harder to explain

• Sea level rise is expected to be modest in the short one, but there is quite a bit if uncertainty.

Can climate change cause an event?

• NO event is caused by climate change, but it is of interest whether a warming climate makes the even meaningfully more or less likely