Transcript Understanding the Tropical Biases in GCMs: Double

What Are the El Nino and La Nina?
Review of last lecture
• Tropical cyclone genesis: Western Pacific has the highest
averaged number per year. 6 necessary conditions. 4 stages.
• Tropical cyclone structure: 3 major components, rotation
direction of inflow and outflow, location of maximum wind and
rainfall, 3 feedbacks
• Tropical cyclone intensity scale. Category 1: 74mph, category 5:
155mph
• Tropical cyclone destruction: 4 reasons? Which side has the
most intense destruction?
• Trends and variability in tropical cyclone activity
Tropical Climate
Video: Chasing El Nino
https://www.youtube.com/watch?v=TYR3GcSLHco
The TAO buoy array
Tropical mean state: Sea surface temperature (SST)
Indo-Pacific
warm pool
Eastern Pacific
cold tongue
2 basic
regions
Tropical mean state: Precipitation
Inter-tropical convergence
zone (ITCZ)
Strong rainfall
(heating)
Weak rainfall
GPCP Annual Mean Precipitation for 1979-2005 (mm/day)
Tropical mean State: Walker Circulation
• An atmospheric circulation
cell oriented along the
equator with rising motion in
the west and sinking motion
in the east, which is induced
by the SST contrast between
the Indo-Pacific warm pool
and eastern Pacific cold
tongue
Ocean upwelling
• is an oceanographic phenomenon that
involves wind-driven motion of dense,
cooler, and usually nutrient-rich water
towards the ocean surface, replacing
the warmer, usually nutrient-depleted
surface water.
• Equatorial upwelling: Due to Coriolis effect
• Coastal upwelling: Due to Coriolis effect
Tropical mean State: Ocean-atmosphere
feedback
• Interacts with underlying
Pacific Ocean with
stronger upwelling
(cooling) in the east,
leading to a positive
feedback.
El Nino/Southern Oscillation (ENSO):
The 4-year oscillation
• El Nino: Very warm sea surface
temperature over central and eastern
tropical Pacific, which occurs every 3-7
years. The Walker Circulation becomes
disrupted during El Niño events, which
weakens upwelling in eastern Pacific.
• La Nina: the opposite condition to El
Nino
• Southern Oscillation: The atmospheric
oscillation associated with the El NinoLa Nina cycle.
• The whole phenomena is now called
El Nino/Southern Oscillation (ENSO)
Typical ENSO period is 3-7 years, but with
significant irregularity
The dramatic impacts of ENSO around the globe
Flood in Lakeport, California as a result
of the 1998 El Nino event
Bushfire in Australia as a result of the
1998 El Nino event
Disastrous effects of 1982-1983 El Nino:
1.Australia-Drought and devastating brush fires
2.Indonesia, Philippines-Crops fail, starvation follows
3.India, Sri Lanka-Drought,fresh water shortages
4.Tahiti-6 tropical cyclones
5.South America-Fish industry devastated - decrease in
nutrients off Peru- fewer fish (anchovy)
6.Across the Pacific-Coral reefs die
7.Colorado River basin-Flooding, mud slides
8.Gulf states-Downpours cause death, property damage
9.Peru, Ecuador-Floods, landslides
10.Southern Africa-Drought, disease, malnutrition
Land-sea Contrast: Seasonal “Monsoon”
Seasonal temperature
distributions:
• T over land > water in summer
• T over land < water in winter
The Seasonal “Monsoon”
• A seasonal reversal of wind
due to seasonal thermal
differences between
landmasses and large water
bodies
• Orographic lifting often
enhances precipitation totals
Extratropical Climate
Mean State
• Westerly winds and jet streams
• A constant Polar Vortex:
transport/mixing, important for
ozone hole
The Pacific/North American Oscillation (PNA):
The way El Nino/La Nina affect U.S.
The Strengthening/Weakening of Polar Vortex
• North Pole: Arctic Oscillation (AO)
• South Pole: Antarctic Oscillation (AAO)
Arctic Oscillation Index
Arctic Oscillation Index: Prediction
Tropical-extratropical Interaction
• Zonal mean (3 cells)
• ENSO
• The interactions are mutual
Summary
Tropical climate:
• Mean state: The two basic regions of SST? Which region has
stronger rainfall? What is the Walker circulation?
• Mean state: Two types of ocean upwelling, ocean-atmosphere
feedback
• El Nino and La Nina: Which region has warm SST anomaly
during El Nino? 4-year period.
• Land-sea contrasts: seasonal monsoon
Extratropical climate:
• Mean state: westerly winds, polar vortex
• What is the primary way El Nino affect extratropics? (PNA)
• The oscillations associated with strengthening/weakening of polar
vortex: AO, AAO