Aspects of the Asian Monsoon system

from Webster et al., JGR, 1998

Monsoon regions are the largest regions where precipitation exceeds evaporation.

This excess water must come from somewhere …

…….which defines the scale of the monsoon.The monsoon is intrinsically inter-hemispheric with the

winter hemisphere being the source of moisture

Longitudinal & latitudinal sections through monsoon regionof specific humidity (color contours) and winds (vectors)

Longitudinal & latitudinal sections through monsoon regionof specific humidity (color contours) and winds (vectors)

Impacts of Elevated Heat SourcesImpacts of Elevated Heat Sources

Note the dominating effect of the Himalayas

Changes in Tropospheric Temperature in Monsoon Region

Temperature changes much larger over Himalayas than elsewhere.

Impact of East African Highlands

Without the Himalayas & the East AfricanHighlands, South Asia would be a desert like North Africa

Average Sea Level Pressure for N.H. winter months

Storms are generated inregions of strong land-seatemperature contrast andare guided by the axis ofwesterly flow.

from Webster et al. (in press)

Tends to produce anomalous monsoon of opposite sign in following years

Biennial cycle

from Webster et al. (in press)

Anatomy of a

biennial cycle

All India Rainfall Index

Note the tendency for weak monsoon years to be

followed by strong monsoon years

El Niño years are generally associated with weak summer monsoons

Interannual forcing of the ocean

(from Webster et al., in press)

less warm

warm cold

less cold

Interstadial? Stadial?

The monsoon represents a viable mechanism for transmitting

“abrupt change” into the tropical oceans

less warm

warm cold

less cold

Precessional Max? Precessional Min?

The dynamical response of the monsoon/ocean coupled system to altered summer insolation could play a major role in determining climate sensitivity over ice age cycles

What’s actually available to record the dynamics

on these various timescales?

All India Rainfall Index

Note the tendency for weak monsoon years to be

followed by strong monsoon years

El Niño years are generally associated with weak summer monsoons

Aspects of the Asian Monsoon system

from Webster et al., JGR, 1998

Gershunov et al. 2003

Indian Monsoon/ENSO

correlation over

the 20th century

Gershunov et al. 2003

-0.5

0

0.5

1840 1880 1920 1960 2000

-0.4

-0.2

0

0.2

0.4

0.61840 1860 1880 1900 1920 1940 1960 1980 2000

-1

-0.5

0

0.5

11750 1800 1850 1900 1950 2000

Zonal coral indexSeychelles Bali

SON Anomalies

from Abrams (unpublished)

Fossil coral samples from the uplifted reefs of Sumatraprovide at least anecdotal evidence for zonal mode events over theHolocene

Intense oxygen minimum

zones off Oman and

Pakistan sporadically preserves

annual laminated sediments

Deep sea sediment cores from the Arabian Sea record the

millennial scale dynamics of Indian Monsoon

from Staubwasser et al. 2002

Some indicators suggest a secular change in Indian

monsoon intensity over the past few centuries.

Anderson et al. 2002

from Rostek et al.,1997

Arabian Sea record of

temperature over the last

40,000 years.

from Altabet et al., 2002

Arabian Sea Record of denitrification

from Higginsonet al., 2005

Expanded view of a

single abrupt transition

from Barrows et al. 2005

Distribution of temperature

observations from the

Last Glacial Maximum

from Barrows et al. 2005

East-west gradients

reduced

Distribution of temperature

observations from the

Last Glacial Maximum

Distribution of sediment cores that have been

analyzed for a varietyof geochemical temperature proxies

K. Dahl, PhD dissertation, 2005

K. Dahl, PhD dissertation

SST in the Arabian Sea was roughly 3.5 degrees

cooler during the last ice age and cross basin gradients

were reduced.

The loess plateau of central China

Wind-blown loess from a satellite photo

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An et al. 1991

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Jin et al. 2007

Speleothems?….Ice cores….?

VERY PROMISING FRONTIER AREA FOR EXPLORINGMONSOONAL DYNAMICS ON INTERANNUAL-ICE AGE TIMESCALES

Oman speleothem record

of monsoonal

precipitation

Last 800 years (Burns et al. 2002)

Last 11000 years (Fleitmann et al. 2003)

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Speleothem record of central India

Singha et al. 2007

Expanded view of Holocene section of Dongge Cave Record

Wang et al. 2005

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Wang et al.

Chinese speleothem record of S.E. Asian Monsoon variability

Aspects of the Asian Monsoon system

from Webster et al., JGR, 1998

from Webster et al. (in press)

Summary• Interannual/decadal: coral records that capture the Indian Ocean zonal mode show a

complex pattern of temporal correlations that seems to vary in conjunction with--but in opposite ways to--the high frequency (ENSO) and low frequency (ENSO-like) behavior of the Pacific. The observations suggest that some process external to the Indian Ocean is missing from the current thinking on the Indian Ocean zonal mode.

• Millennial--the strength of the Asian monsoon is modulated on kyr timescales by the (orbitally controlled) changes in seasonality, but there are also “abrupt changes” that are temporally connected to the dramatic shifts in high latitude Northern Hemisphere climate observed in Greenland ice cores. These changes in Asian monsoon intensity are communicated at least to the northern tropical ocean basins. An unknown question involves the extent to which these changes are felt in the open ocean.

• Glacial--SSTs were uniformly lower, but cross-basinal gradients were relaxed across not only the Arabian Sea, but also across the entire I.O. The implication here is that monsoon dynamics play an important role in governing global climate sensitivity. (W

Tectonic reconstruction of the Eocene (45 MA)

Tectonic reconstruction of the early Oligocene (35 Ma)

Tectonic reconstruction of the middle Miocene (20 Ma)

Tectonic reconstruction of the late Miocene (7 Ma)

Tectonic reconstruction of the late Miocene (7 Ma)

Model effects of increasing elevation of the Tibetan Plateau

Final stages of monsoon development

Greenhouse to Icehouse• Over the course of the last ~60 million years, the Earth drifted from a state of

hothouse climate to one that features (as it does today) permanent ice in both hemispheres. The transition was not smooth. There are several prominent steps, as well as apparent oscillations, along with at least one dramatic “aberration”.

• Classes of explanations can be broken down into categories of

--Continental configuration --Carbon cycle change--Ocean heat transport to the poles (changes in oceanic “gateways”)--Any combination of these aspects of the system (e.g. the direct and indirect

effects of continental drift on carbon cycling)

Ocean-wide compilation of isotopic variability (from Zachos et al. 2000)

Combining minor element and oxygen isotopic approaches to resolvethe timing and magnitude of Cenozoic ice growth

Lear and Elderfield, Science, 2000

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from Miller et al., Science 2005

Sea level estimates overthe last 100 million years

from Pearson and Palmer, 2000, Nature

If one combines the pH reconstructions with some othermeasure of the carbon chemistry of the ocean (e.g. alkalinity, TCO2), then the pCO2 of the atmosphere can be calculated.

How does a large ice sheet grow from scratch?

A large land mass near the poles is a good way to start

from Crowley’s book

Hysteresis of ice cap growth

Ice sheets that fall below a critical size are inherentlyunstable. They will decay away rapidly.

The explanation for this behavior comes from theeffect of ice albedo on the regional temperature depression

White Mtns., Ca

Once established,large ice sheets can createtheir own climate, to someextent. So it makes senseto focus on this majorcomponent of the system,as a way of thinking about“forced response”.

Hysteresis of ice cap growth

Ice sheets that fall below a critical size are inherentlyunstable. They will decay away rapidly.

When an ice cap is near its bifurcation point, in principle, itdoesn’t require much change in heating to make a big effect(in either direction)

Models show that, for any continental geometry, the continental interiors are usually below freezing in the winter, unless atmospheric CO2 is unusually high

1200 ppm CO2

Modern seasonalcycle (300 ppm CO2)

from Bush andPhilander, 1997,Paleoceanography

Evidence for widespreadglaciation exists only in discreteepisodes of Earth history. Theindicators for carbon cyclingappear to vary on roughlythe same timescales

Geometry of the landmasses during the Ordivician (~430 MA)

Shallow carbonate platforms could precondition thecarbon cycle to large excursions.

from Kump et al. 1999

One indirect measure(or proxy) of atmospheric CO2

might be found in the carbon isotoperatios of organic matter.All else equal, the ratios of 13C/12C in organics seemsto depend on how muchdissolved carbon dioxide thereis in the water.

13C is thedeviation of 13C/12C ratios awayfrom some standard, in this case,average seawater carbon

Pagani et al. Science 2005

Most recent compilation oforganic carbon isotopic variabilityduring the early Cenozoic

Isotopic compositionof bulk boron isfixed (residence time~20 million years)

Carbonate (forams)mostly take up thecharged species ofdissolved boron.