atmosphere–ocean interactions

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

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

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

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

Western North Pacific Monsoon1. An oceanic component of the Asian monsoon system

2. Important for tropical cyclone tracks in the Pacific

The Strength of the Monsoon

Webster, 2006

Large-scale vertical wind shear

Webster, 2006

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

Webster, 2006

Indian Ocean Dipolecoldwarm more rain over East Africa

Webster, 2006

SST GRADIENT DRIVES EASTERLIES

EASTERLIES PUSH UP SSHIN THE WEST

DOWNWELLING WAVES ENHANCE THE ANOMALY

EKMAN TRANSPORT ASSOCIATEDWITH MONSOON ONSETREVERSES THE ANOMALY

Webster, 2006

The Coupled Annual Cycle

WARMESTSSTs

UPWELLINGIN WEST IO

ASIANMONSOON

AUSTRALIANMONSOON

Webster, 2006

The Coupled Annual Cycle

WARMESTSSTs

UPWELLINGIN WEST IO

ASIANMONSOON

AUSTRALIANMONSOON

Webster, 2006

The Asian Monsoon

Webster, 2006

The Asian Monsoon

Webster, 2006

The ocean moves energy from summer hemisphere to

winter hemisphere

Atmosphere moves energy from winter hemisphere to

summer hemisphere

Webster, 2006

The Asian Monsoon

S�T (t) = ⇢wCw

ZZH(t)

@T (t)

S�H(t) = ⇢wCw

ZZT (t)

@tdxdy

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

Interannual Variability

Webster, 2006Saturday, September 28, 13

Interannual Variability

Webster, 2006

Asian monsoon variabilityfirst tied to ENSO in 1920s

El Niño ~ below averageLa Niña ~ above average

Webster, 2006

Accounts for about 40%of monsoon variance

Webster, 2006

Strength of relationshipvaries with time

> –0.8 < –0.2 > –0.8

Webster, 2006

Strength of relationshipvaries with time

> –0.8 < –0.2 > –0.8

biennial variability?

Webster, 2006

The Asian Monsoon

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

Webster, 2006

The Asian Monsoon

Tendency for weak monsoon to follow strongmonsoon and vice versa

Intraseasonal Variability

Goswami, 2005

active breakmonsoon

inactive activeMJO

Intraseasonal Variability: MJO?

Webster, 2006

Intraseasonal Variability: MJO?

Webster, 2006

OHT respondsto high-frequency

wind forcings

Coupled Intraseasonal VariabilityOCEAN HEAT TRANSPORT

Webster, 2006

OHT respondsto high-frequency

wind forcings

Coupled Intraseasonal VariabilityOCEAN HEAT TRANSPORT

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

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

Asia-Australian Monsoon1. In addition to Asian monsoon, Australian monsoon

2. Associated with the seasonal migration of heating between the northern and southern hemispheres

North America

South America

Africa

Other Regional Monsoons

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!

solstice mode: regional monsoons

equinox mode: oceans cool more slowly

combining principal components, get a representation ofglobal land–ocean monsoon system annual cycle

The Global Monsoonboreal monsoon

austral monsoon

solstice modeequinox mode

Global Monsoon Domain

(provided annual mean precipitation > 300 mm)

MPI = annual range of precipitation

annual mean precipitation > 0.5

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

differences amongobservations

CMIP5 error structure similar to earlier models (CMIP3)

wind simulated better than rainfall

reanalyses

monsoon winds

wind simulated better than rainfall

reanalyses

some improvement in newer models, particularly for rainfall

monsoon winds

newer models slightly better, but amplitude is still too weak

monsoon rainfall during El Niño

start of monsoon season

wide range of performance simulating onset and duration

newer models generally better

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

•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

newer models again perform slightly better

multi-model mean

annual mean precipitation intensity

best four models

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

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

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