atmosphere–ocean interactions
TRANSCRIPT
Atmosphere–OceanInteractions
11. Monsoon Systems
Saturday, September 28, 13
Outline1. What are monsoons?
2. The Asian monsoon
3. The global monsoon
4. The role of atmosphere–ocean coupling
5. Simulating monsoons
Saturday, September 28, 13
Monsoons1. The term monsoon comes from seasonal variations in
winds, though it is often extended to precipitation
2. Monsoons arise from seasonal reversals in heating and temperature gradients between land and ocean
3. Monsoon climate regimes affect approximately three billion people
4. Understanding monsoons is important for agricultural planning and flood and drought mitigation
5. Monsoons vary on a variety of timescales: biweekly, intraseasonally (30–60 days), interannually (e.g., ENSO), and interdecadally
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The Asian Monsoon:
East Asian monsoon
Western North Pacific monsoon
Precipitation
South Asian monsoon
Annual cycle of precipitation
Wang et al., Marine Geology, 2003Saturday, September 28, 13
South Asian Monsoon1. Strongest and best known of the regional monsoons
2. Carries moist air from the Indian Ocean to South Asia
3. Accounts for approximately 80% of the rainfall in India
4. Driven by strong heating over the Tibetan Plateau, which reverses the low-level pressure gradient
boreal winter boreal summer
Saturday, September 28, 13
East Asian Monsoon1. Carries moist air from South Asia to East Asia
2. Affects approximately one-third of global population (China, Korea, Japan)
3. Driven by temperature differences between Asia and the Pacific Ocean
boreal winter boreal summermonsoonrainband
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Western North Pacific Monsoon1. An oceanic component of the Asian monsoon system
2. Important for tropical cyclone tracks in the Pacific
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The Strength of the Monsoon
Webster, 2006
Large-scale vertical wind shear
Saturday, September 28, 13
The Strength of the Monsoon
Webster, 2006
Large-scale vertical wind shear
Saturday, September 28, 13
The Strength of the Monsoon
Webster, 2006
Large-scale vertical wind shear
large differences in low-level winds may affect atmosphere–
ocean interactionsSaturday, September 28, 13
Webster, 2006
The Coupled MonsoonHeat transport in the ocean
out of phase with solar heating
MODEL ESTIMATES
OBSERVATIONAL ESTIMATES
Northward during late autumn and winter, southward during
spring, summer and early autumn
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Webster, 2006
Indian Ocean Dipolecoldwarm more rain over East Africa
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Webster, 2006
SST GRADIENT DRIVES EASTERLIES
EASTERLIES PUSH UP SSHIN THE WEST
DOWNWELLING WAVES ENHANCE THE ANOMALY
EKMAN TRANSPORT ASSOCIATEDWITH MONSOON ONSETREVERSES THE ANOMALY
Saturday, September 28, 13
Webster, 2006
The Coupled Annual Cycle
WARMESTSSTs
UPWELLINGIN WEST IO
ASIANMONSOON
AUSTRALIANMONSOON
Saturday, September 28, 13
Webster, 2006
The Coupled Annual Cycle
WARMESTSSTs
UPWELLINGIN WEST IO
ASIANMONSOON
AUSTRALIANMONSOON
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Webster, 2006
The Asian Monsoon
Saturday, September 28, 13
Webster, 2006
The Asian Monsoon
Saturday, September 28, 13
Webster, 2006
The ocean moves energy from summer hemisphere to
winter hemisphere
Atmosphere moves energy from winter hemisphere to
summer hemisphere
Saturday, September 28, 13
Webster, 2006
The Asian Monsoon
S�T (t) = ⇢wCw
ZZH(t)
@T (t)
tdxdy
S�H(t) = ⇢wCw
ZZT (t)
@H(t)
@tdxdy
Saturday, September 28, 13
Webster, 2006
The Coupled Annual Cycle•without ocean transport, buoyancy gradient would be large...•...ocean transport is driven by atmospheric circulation...•...atmospheric circulation is driven by the surface fluxes associated with the buoyancy gradient•monsoon seasonal cycle modulated by negative atmosphere–ocean feedback associated with Ekman transport
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Interannual Variability
Webster, 2006Saturday, September 28, 13
Interannual Variability
Webster, 2006
Why?
Saturday, September 28, 13
Webster, 2006
Asian monsoon variabilityfirst tied to ENSO in 1920s
ENSO
El Niño ~ below averageLa Niña ~ above average
Saturday, September 28, 13
Webster, 2006
Asian monsoon variabilityfirst tied to ENSO in 1920s
ENSO
El Niño ~ below averageLa Niña ~ above average
Accounts for about 40%of monsoon variance
Saturday, September 28, 13
Webster, 2006
Asian monsoon variabilityfirst tied to ENSO in 1920s
ENSO
El Niño ~ below averageLa Niña ~ above average
Strength of relationshipvaries with time
Accounts for about 40%of monsoon variance
> –0.8 < –0.2 > –0.8
Saturday, September 28, 13
Webster, 2006
Asian monsoon variabilityfirst tied to ENSO in 1920s
ENSO
El Niño ~ below averageLa Niña ~ above average
Strength of relationshipvaries with time
Accounts for about 40%of monsoon variance
> –0.8 < –0.2 > –0.8
biennial variability?
Saturday, September 28, 13
Webster, 2006
The Asian Monsoon
Saturday, September 28, 13
Webster, 2006
Biennial VariabilityWARM SSTs LEAD
TO STRONG MONSOONSTRONG MONSOON ENHANCES
MIXING/EVAP, COOLING SST
REVERSAL OF E–W GRADIENTLEADS TO STRONG MONSOON
COOL SSTs LEADTO WEAK MONSOON
Saturday, September 28, 13
Webster, 2006
The Asian Monsoon
Tendency for weak monsoon to follow strongmonsoon and vice versa
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Intraseasonal Variability
Webster, 2006Saturday, September 28, 13
Goswami, 2005
active breakmonsoon
inactive activeMJO
Intraseasonal Variability: MJO?
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Webster, 2006
Intraseasonal Variability: MJO?
Saturday, September 28, 13
Webster, 2006
OHT respondsto high-frequency
wind forcings
Coupled Intraseasonal VariabilityOCEAN HEAT TRANSPORT
Saturday, September 28, 13
Webster, 2006
OHT respondsto high-frequency
wind forcings
Coupled Intraseasonal VariabilityOCEAN HEAT TRANSPORT
Saturday, September 28, 13
The Coupled Monsoon1. How is the upper ocean temperature anomaly
maintained from year to year? Is the dipole involved?
2. Cold Indian Ocean » weak monsoon » less ocean heat transport » warm Indian Ocean » strong monsoon...
3. Weak monsoon circulation reduces upwelling along east coast of Africa and increases upwelling along the west coast of Sumatra (cold in the east, warm in the west) – strong monsoon has opposite effect
4. Dipole and its influence on equatorial waves introduces slow dynamics to the system, enhancing and prolonging the SST patterns that regulate the monsoon
Saturday, September 28, 13
Webster, 2006Saturday, September 28, 13
The Global Monsoon1. Monsoon climates characterized by seasonal reversal of
winds. This reversal is global, not just regional
2. Regional monsoons are coordinated primarily by the annual cycle of solar heating
3. Tied to the Hadley and Walker circulations (the ‘lateral’ and ‘transverse’ monsoons)
Trenberth, 2006Saturday, September 28, 13
The Global Monsoon
Webster, 2006
NH MONSOON
SH MONSOON
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Asia-Australian Monsoon1. In addition to Asian monsoon, Australian monsoon
2. Associated with the seasonal migration of heating between the northern and southern hemispheres
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North America
South America
Africa
Other Regional Monsoons
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Wang & Ding, Dyn. Atmos. Oceans 2008
EOF analysis: annual cycleof precipitation and winds
solstice mode: annualcycle with maximum in July
and minimum in January
equinox mode: annualcycle with maximum in April
and minimum in October
semi-annual mode: maximain April and October
First two modes explain84% of total variance!
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Wang & Ding, Dyn. Atmos. Oceans 2008
solstice mode: regional monsoons
equinox mode: oceans cool more slowly
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Wang & Ding, Dyn. Atmos. Oceans 2008
combining principal components, get a representation ofglobal land–ocean monsoon system annual cycle
The Global Monsoonboreal monsoon
austral monsoon
solstice modeequinox mode
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Wang & Ding, Dyn. Atmos. Oceans 2008
Global Monsoon Domain
(provided annual mean precipitation > 300 mm)
MPI = annual range of precipitation
annual mean precipitation > 0.5
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Simulating the Asian Monsoon
Sperber et al., Clim. Dyn. 2012
the multimodel mean outperforms all individual models
observations
range of individual model performanceSaturday, September 28, 13
Simulating the Asian Monsoon
Sperber et al., Clim. Dyn. 2012
differences amongobservations
CMIP5 error structure similar to earlier models (CMIP3)
Saturday, September 28, 13
Simulating the Asian Monsoon
Sperber et al., Clim. Dyn. 2012
wind simulated better than rainfall
reanalyses
mon
soon
rai
nfal
l
monsoon winds
Saturday, September 28, 13
Simulating the Asian Monsoon
Sperber et al., Clim. Dyn. 2012
wind simulated better than rainfall
reanalyses
some improvement in newer models, particularly for rainfall
mon
soon
rai
nfal
l
monsoon winds
Saturday, September 28, 13
Simulating the Asian Monsoon
Sperber et al., Clim. Dyn. 2012
newer models slightly better, but amplitude is still too weak
monsoon rainfall during El Niño
Saturday, September 28, 13
Simulating the Asian Monsoon
Sperber et al., Clim. Dyn. 2012
leng
th o
f mon
soon
sea
son
start of monsoon season
wide range of performance simulating onset and duration
newer models generally better
Saturday, September 28, 13
Simulating the Global Monsoon
Lee and Wang, Clim. Dyn. 2012
some problem areas:
•Eastern Indian Ocean•Bay of Bengal•equatorial western Pacific•tropical Brazil•Maritime Continent •Philippines•high-elevation areas like the Andes and the Tibetan Plateau
Saturday, September 28, 13
Simulating the Global Monsoon
Lee and Wang, Clim. Dyn. 2012
•models tend to underestimate solstice mode, especially for East Asia and South China Sea•models tend to overestimate the equinox mode, with greater spread•multi-model mean captures monsoon intensity well, except over East Asia and W North Pacific
multi-model mean again better than individual models
(equinox mode)(annual mean)
(solstice mode) (intensity)
multi-model mean
Saturday, September 28, 13
Simulating the Global Monsoon
Lee and Wang, Clim. Dyn. 2012
newer models again perform slightly better
multi-model mean
annu
al c
ycle
annual mean precipitation intensity
best four models
Saturday, September 28, 13
Simulating the Global Monsoon
Hsu et al., J. Geophys. Res. 2013
(global monsoon area)(global monsoon precipitation)(global monsoon intensity)
multi-model mean reproduces the global monsoon, but the inter-model spread is large
CMIP5 projects future increases in area, precipitation, and intensity
Saturday, September 28, 13
Monsoons1. Seasonal variations in winds and precipitation due to
seasonal variations in heating and T, p gradients
2. Can be defined either regionally or globally
3. Monsoons arise from coupled interactions between the atmosphere and ocean
4. Coupled models are often able to capture the basics of monsoon dynamics, but the multi-model ensemble mean generally performs ‘better’ than any individual models
5. Model representations of the monsoon have improved over the past decade
Saturday, September 28, 13