What are the general characteristic of the tropics?
Easterly (trade winds), deep convection, monsoon and hurricanes
What drives the large-scale atmospheric circulation?
The tropics have a surplus of radiative energy, and the extra-tropics a deficit. Heat is therefore transported from the tropics to the extra tropics.
Why do air parcels that move out of tropics overshoot to the east on the NH?
Because of Earth’s angle momentum conservation.
Do tropical easterlies extract or return angular momentum? And in the mid-latitude westerlies?
At the tropical easterlies the atmosphere is slower than the surface momentum, which results in these easterlies. In the mid-latitudes the atmosphere moves faster than the surface, causing the overshooting and also westerlies.
By what is global atmospheric circulation driven? Explain how it works.
Pressure-Gradient force.
- Warmer tropical air expands and this the same pressure level is found at higher altitudes in the tropics compared to the poles.
- This preddure gradient drives winds which become westerlies due to Coriolis effect.
- Strongest winds form at strong pressure gradients, and this strong temperature gradients (Polar Jet Stream)
The ITCZ migrated with solar annual cycle. What does this drive?
It drives monsoon rainfall.
Explain how the Hadley-Walker circulation works.
At cold pool, evaporation takes places - > cooling. Condensation takes place in the warm pool, so the latent heat is being released. Therefore, it strengthens the circulation. Also, cold pool low clouds have more albedo. Warm pool clouds go to high altitudes, and therefore there is more backward radiation over the full column -> More warming.
High clouds have a warming effect, low clouds have a cooling effect and therefore strengthen the circulation. -> higher temperature gradient!! Clouds influence radiation, which determines the temperature distribution, which determines the wind patters, which then again have an influence on the clouds and therefore, again, radiation.
-> Strengthening of gradients and thus circulation
What does the Southern Oscillation of ENSO refer to?
It refers to the negative correlations in sea-level pressure between Darwin and Tahiti.
What is the timescale of occurrence El Niño and La Niña?
2-5 years
How does the Hadley-Walker circulation interact with El Niño and La Niña?
There is an interaction between the ocean circulation and the atmospheric circulation (Walker circulation). Weakening of the Walker results in less upwelling in the east -> warmer east pacific. Therefore, there is a smaller temperature gradient between the West and the East pacific, which leads to further weakening.
Bjerknes ocean atmosphere feedback!
Explain El Niño.
El Nino creates warm temperatures in Peru. El Nino are warming years and La Nina are cooling years. Tropical regions are very much influenced. North America as well. Europe and Eurasia less. E.g. with El Nino we have seen an increase in forest fires in 2023.
El Nino refers to the warm sea surface temperatures. Fish catch could be bad towards Christmas -> therefore the name. The Southern Oscillation, indicates the pressure difference between Darwin and Tahiti. La Nina is an amplified normal state. Warmer warm pool and cooler cool pool. During El Nino, the warm pool is colder, and the cold pool is warmer.
Shift of upflow to central Pacific. Strong upflow over Horn of Africa.
What drives El Niño?
Seasonal cycle. In spring, trade winds are weakest giving a “window” to tup towards El Nino state. It will always develop first in spring. In April we can predict what will have in the 9 months ahead. Early spring (April may) we have the first outlooks for the winter.
El Niño effect on tropics:
- Dry warm pool & wet central-eastern Pacific
- In JJA (June, July, August) mostly limited to tropical pacific region
- In DJF (December, January, February) stronger impact further away: Africa, US
- Very important for rainfall in Africa and California.
La Niña effect on tropics:
- Wet warm pool & dry central-eastern Pacific (thunderstorms migrate)
- Downflow over horn of America
- In JJA mostly limited to tropical Pacific region
- In DJF stronger impacts, further away: Africa, US
- Very important for rainfall in Africa and California
How does the Jetstream behave during El Niño?
Moves southward. Because of this hurricanes wil occur less likely.
Explain why there are periods of enhanced (active) and reduced (breaks) rainfall during the Indian monsoon.
Indian summer monsoon (low pressure area shift due to ITCZ). Flow from ocean to the land. In winter flow away from the land towards the equator, In summer from the ocean to the land. It has a weird circulation around the equator, due to Coriolis.
How will El Niño change in a changing climate?
- Its impact will change: Warmer air
holds more water vapor which intensifies
heavy rainfall & drought extrêmes
associated with ENSO - Its dynamics might change: very
uncertain whether we go to more El
Nino’s or more La Nina’s
