Understanding the Tropical Biases in GCMs: Double

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Tropical Climate - From El Nino
to Sea Breeze
Review of last lecture
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Primary high and lows
Three-cell model. Mechanism for each cell
Two characteristics of temperature structure
Two characteristics of wind structure. Why does westerly
winds prevail in the extratropical troposphere? What cause the
jet streams?
• What drives the ocean currents? In the case of Ekman spiral,
what is the direction of surface current relative to surface
wind?
Vertical structure and mechanisms
Polar Cell (thermal):
Driven by heating at
50 degree latitude
and cooling at the
poles
Ferrel Cell (dynamical):
Dynamical response to
Hadley and polar cells
Hadley Cell (thermal):
Heating in tropics - forms
surface low and upper
level high - air converges
equatorward at surface,
rises, and diverges
poleward aloft - descends
in the subtropics
The tropical climate strongly affects the global
climate system
3 stages of research
Phenomena: structure, teleconnection,
scale-interaction, effects on
extreme events (disasters)
Mechanism: theory, observation
Prediction: GCM, statistical model
The TAO buoy array
Mean state: Sea surface temperature (SST)
Indo-Pacific
warm pool
Eastern Pacific
cold tongue
2 basic
regions
Mean state: Precipitation
Meiyu-Baiyu
front
Inter-tropical
convergence
zone (ITCZ)
Warm pool
South Pacific
convergence
zone (SPCZ)
GPCP Annual Mean Precipitation for 1979-2005 (mm/day)
4 basic
regions
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
thermal contrast between the
Indo-Pacific warm pool and
eastern Pacific cold tongue
• Interacts with underlying
Pacific Ocean with stronger
upwelling (cooling) in the east
Theories of tropical mean climate:
Ocean-atmosphere feedback mechanisms
SST gradient - trade wind
(Bjerknes) feedback
SST - Latent Heat Flux
feedback
SST - Shortwave Flux
feedback
From Lin (2007)
El Nino/Southern Oscillation (ENSO):
The dominant interannual variability
• 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)
ENSO: SST Change
Typical ENSO period is 3-7 years, but with
significant irregularity
The 1997-1998 El Nino event
The current La Nina event
Existing ENSO theories (not required)
(6) Stochastic forcing theory
(2) Delayer oscillator
theory
(1) Slow coupled mode theory
(4) Western Pacific
oscillator theory
(3) Advective-reflective
oscillator theory
(5) Recharge oscillator theory
From Lin (2008)
El Niño impacts on Peruvian fishing
• Large numbers of fish & marine plants die as
upwelling is reduced by weakened Ekman
transport from weakened trades
• Reduce Peruvian anchovy fish catch
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
The dominant intraseasonal variability:
The Madden-Julian Oscillation (MJO) and
convectively coupled equatorial waves
• In 1966, T. Matsuno conducted
a theoretical study of the
tropical atmosphere and found
the dominant equatorial waves
• 30 years later, Takayabu (1994)
and Wheeler and Kiladis (1999)
confirmed the existence of these
waves in the real atmosphere
Wavenumber-Frequency Spectrum of Tropical Rainfall
Symmetric waves
• Madden-Julian Oscillation (MJO): 30-60 day
• Kelvin wave
• Equatorial Rossby (ER) wave
• Westward inertial gravity (WIG) wave
Anti-Symmetric waves
• Mixed Rossby-Gravity (MRG) wave
• Eastward inertial gravity (EIG) wave
Real-time monitoring of the MJO and other waves
Vertical structure of the MJO and wave-heating feedback
mechanisms
Vertical heating profile
Column-integrated
diabatic heating has six
major components
(Mean state and higherfrequency modes affect
the MJO through the
nonlinear terms)
Global Impacts of the MJO
(Lin, J.L., G.N. Kiladis, B.E. Mapes, K.M. Weickmann, K.R. Sperber, W.Y. Lin,
M. Wheeler, S.D. Schubert, A. Del Genio, L.J. Donner, S. Emori, J.-F. Gueremy,
F. Hourdin, P.J. Rasch, E. Roeckner, and J.F. Scinocca, 2006, Journal of Climate)
Impacts of MJO on the number of Atlantic hurricanes
MJO westerly
phase
MJO easterly
phase
(From Maloney and
Hartmann 2001)
Monsoons
• A seasonal reversal of wind
due to seasonal thermal
differences between
landmasses and large water
bodies
• Orographic lifting often
enhances precipitation totals
A 3D view of the Indian monsoon
World’s Major Monsoon Systems
• Asian (Indian, Western North
Pacific, Eastern Asian)
• Australian
• Western African
• North American
• South American
Diurnal Variation I: Sea and Land Breeze
• Daily reversal of winds resulting
from differences in thermal properties
of land and sea
• During the day (night) land (water)
surfaces are hotter (cooler) than large
water (land) surfaces
• Thermal low develops over warmer
region - air converges into the low,
ascends, and produces clouds
Diurnal Variation II: Mountain and Valley Breeze
• mountain slopes heat/cool rapidly
• creates pressure differences
• day: air heats, rises upward
• night: air cools, sinks
Summary
• Mean state: The two basic regions of SST? The four basic
regions of precipitation. What is the Walker circulation?
• ENSO: What is the El Nino? What is the La Nina? What is
the Southern Oscillation? What is the ENSO?
• MJO and convectively coupled equatorial waves
• Monsoons: What is a monsoon? The 5 major monsoon
systems in the world.
• Diurnal variation: What are sea and land breezes? What are
mountain and valley breezes?
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