Aspects of the Asian Monsoon system from Webster et al., JGR, 1998.
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Transcript of Aspects of the Asian Monsoon system from Webster et al., JGR, 1998.
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.