Global Monsoon: Concept and InterdecadalCentennial Variations

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Global MonsoonConcept and Interdecadal-Centennia
l Variations

• Bin Wang
• University of Hawaii
• Acknowledge contribution from
• Q. Ding, J. Liu, W. Soon
• IAP/LASG, Beijing 12-26-2007

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Topics of discussion

• Concept of the GM
• Metrics for precipitation climatology
• How to measure the strength of the GM?
• Change of the GM in the past 56 years
• Centennial variation of the GM in response to
  external forcing

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Monsoon Conventional Defination
Monsoon domain by Khromov (1957) and Ramage (1971)

• Tradiitonal definition is based on surface wind
  field (Ramage 1971)
• Prevailing wind direction shifts at least 120
  degree between January and July
• Average of the frequencies of the prevailing wind
  direction in January and July gt40

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Remarks

• Monsoon climate is not only characterized by
  annual reversal of prevailing surface winds
  (Ramage 1971) but also by a contrasting rainy
  summers and dry winters (Webster 1987).
• Precipitation should be emphasized as it plays an
  essential role in determining atmospheric general
  circulation and hydrological cycle and in linking
  external radiative forcing and the atmospheric
  circulation.
• A broader perspective of monsoon a precipitation
  and wind response of the global coupled
  atmosphere-land-ocean system to annual variation
  of the solar forcing.
• A global perspective is imperative and
  advantageous

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Latent heat and latent energy transport

• All seven regional monsoons are coordinated by
  the annual cycle of the solar radiative heating.
• Conservation laws apply to global atmosphere.
• Trenberth and Stepaniak (2004) depict monsoon
  system as global-scale persistent overturning of
  the atmosphere that varies with season.

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Teleconnections associated with Asian-Australian
Monsoon
(Webster et al 1998)
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Tropical-Extratropical linkage Summer
Circumglobal Teleconnection (CGT)
Ding and Wang05
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Regional monsoon systems interact with each other
and with global oceans

• A strong South Asian summer monsoon tends to be
  followed by a strong Australian and weak eastern
  African monsoon (Meehl 1997).
• Indian monsoon-East Asian monsoon (Kripalani
  1997)
• South American monsoon and the African monsoon
  are possibly related (Biasutti et al. 2003).
• Teleconnection exists between East Asian-western
  North Pacific summer monsoon and North American
  summer rainfall (Wang et al. 2001 Lau and Weng
  2002).
• Continental monsoons are interactive with
  surrounding oceans. Sahel drying is a response to
  warming of the South Atlantic relative to North
  Atlantic SST Southern African drying is a
  response to Indian Ocean warming (Hoerling et al.
  2006).

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What is the global monsoon?

• Global Monsoon
• Dominant Mode of Annual Variation of the Global
  Tropical Circulation
• Bin Wang and Qinghua Ding
• Dynamics of the Atmosphere and Ocean (2007
  Special issue)

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What are the major modes of the tropical
precipitation and low-level circulation in
response to annual variation of solar radiative
heating forcing?
Decomposition of a climatological time series
Extract major modes of climatology Construct a
metrics for defining GM and assessing climate
models performances. Understand the physical
processes behind the response of the tropical
hydrological cycle and the coupled climate system
to the solar radiative forcing on the annual
cycle and orbital time scale.
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MV-EOF modes of the climatological monthly mean
precipitation and 850hPa winds
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Physical interpretation
JJAS minus DJFM
MVEOF1
Solstitial mode
Equinoctial Asymmetric modes
AM minus ON
MVEOF2
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Monsoon season MJJAS vs NDJFM
Combined PCPC1x71 PC2 x 13.
the boreal summer (austral winter) consists of
MJJAS, the boreal winter (austral summer)
consists of NDJFM The annual range can be
defined by the local summer-minus-winter , i.e.,
MJJAS minus NDJFM in the NH and NDJFM minus
MJJAS in the SH.
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Global Monsoon Domain Precipitation
Annual range exceeds 150 mm and MPI exceeds 50.
Similar definition can be constructed for wind
field by using similar formula and criteria.
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Dynamic consistency between the precipitation and
circulation
the monsoon precipitation domain the regions in
which MPI greater than 0.5 and the annual range
greater than 300 mm the monsoon wind domain the
regions in which MWI is greater than 0.5
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• Summary
• The global monsoon consists of a solstitial mode
  and an equinoctial asymmetric mode, both reflects
  the response of the coupled climate system to
  external solar forcing.
• The monsoon precipitation domain can be
  delineated by a simple monsoon precipitation
  index (MPI) annual range exceeding 300 mm and
  the MPI exceeding 50.
• Strong monsoon strong annual reversal in lower
  tropospheric winds and a wet summer-dry winter
  contrast. Weak monsoon a wet summer-dry
  winter contrast but weak annual reversal of
  winds.

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• Metrics for Gauging
• Precipitation Climate Change
• long-term mean
• Solstitial mode (JJAS-DJFM)
• Equinoctial asymmetric mode (AM-ON)
• Global monsoon domain

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Global monsoon precipitation in Reanalyses
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Monsoon Domain in reanalysis datasets
A common problem is in capturing the monsoon
regime realistically in the Southeast
Asia-Philippine Sea and southeast North
America-Caribbean Sea, where the east-west
land-ocean thermal contrast and meridional
hemispheric thermal contrast coexist.
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Performance on Mean States and its Linkage with
Seasonal Prediction
Pattern Correlation over Global Tropics 30S
30N
The seasonal prediction skills are positively
correlated with their performances on both the
annual mean and annual cycle in the coupled
climate models.
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Observed long-term Changes of Global Monsoon
Precipitation (1950-2004)
Wang and Ding, 2006,GRL
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Precipitation Datasets

• Land rain gauge datasets (1948-2004)
• NCEP/CPC, Precipitation REConstruction data over
  Land (PREC/L) (1948-2004)
• Global Precipitation Climatology Centre
  /Variability Analysis of Surface Climate
  Observations (VASCO) (1951-2000)
• Climatic Research Unit (CRU) (1948-2002)
• Delaware University (Delaware) (1950-1999)
• Global ocean/land datasets (1979-2004)
• Global Precipitation Climatology Project (GPCP)
  (1979-2004)
• CMAP compiled at NCEP (1979-2004)

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Measure GM strength change

• Global averaged Indices
• NHMI NH-averaged JJA monsoon precipitation
• SHMI SH-averaged DJF monsoon precipitation
• GMI The sum of NHMI and SHMI
• Spatial pattern of the variation and trend
• The leading EOF pattern of yearly annual range
  (AR) and the corresponding PC (ARI).
• MK statistical significance of the AR trend for
  each grid point within the monsoon domain.

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Significance test

• Trend to noise ratio
• Yt a bt et, where a is the intercept, b
  the slope, and et the residual, which is
  independent and normally distributed with zero
  mean and variance s2.
• Mann-Kendall (MK) test
• t4SMi/n(n-1)-1 where Mi represents the
  number of values that are greater than the ith
  value subsequent to its position in a raw series
  of n values.
• Resampling
• Randomly re-sampling data to create new
  samples, from which the distribution of the null
  hypothesis can be estimated.

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Global Land Monsoon Rainfall Indices
NHMI
SHMI
GMI
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Global land monsoon rain domain
Overall weakening of the global land monsoon
precipitation
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Global Monsoon Precipitation Domain
Definition based on summer-winter contrast
(Annual range greater than 150 mm (JJA minus DJF
in NH) and concentration of rain in summer (Local
summer (JJA in NH) exceeds 35 of the annual
rainfall)
Wang and Ding 2006 GRL
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Land and Ocean GM precipitation
(GPCP)
In the last 25 years, Oceanic monsoon rainfall
increases while land monsoon unchanged
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Summary

• An overall weakening of the global land monsoon
  precipitation during 1948-2004 is primarily due
  to weakening of the summer monsoon rainfall in
  the Northern Hemisphere. SH has no trend.
• Since 1980, the global land monsoon rainfall has
  seen no significant trend but the oceanic
  monsoon precipitation shows a significant
  increasing trend.
• The results provide a rigorous test for
  reanalysis and climate models that will be used
  in future climate-change assessment. 
• The MPI can be used to quantify global as well as
  regional monsoon variations on all time scales
  longer than a year.

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Zhou et al. 2007)
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Kim et al. 2007
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Centennial variations of the global monsoon
precipitation in the past millennium Results
from ECHO-G model
Jian Liu et al. 2007
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Time series of the 7-year running mean monsoon
indices (NHMI, SHMI, GMI) for CTL and ERIK runs
Free run
Forced run

• 30-year running means are added to highlight
  centennial variations

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Spectrum of the GMPI
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Conclusion

• Weak GM precipitation during the LIA (1450-1850)
  with three minima around the Spörer Minimum
  (1460), Maunder Minimum (1685), and Dalton
  Minimum (1800) periods of solar activity.
• Strong GM was simulated during the MWP (ca.
  1030-1240).
• Strength of the GM precipitation in the forced
  run exhibits a significant quasi-bi-centennial
  oscillation.
• Before the industrial period, the natural
  variations in the total amount of effective solar
  radiative forcing reinforce the thermal contrasts
  both between the ocean and continent resulting in
  the millennium-scale variation and the
  quasi-bi-centennial oscillation in the global
  monsoon.
• The prominent upward trend in GM precipitation
  occurring in the last 30 years (1961-1990) appear
  unprecedented and owed possibly in part to the
  increase of atmospheric carbon dioxide
  concentration.

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Rainy season of the Asian Monsoon
Understanding physical processes determining
the differences between IM and EAM in their
Annual cycle
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Annual Variation

• Why compare the annual variation?
• Indian and East Asian (EA) monsoon subsystems are
  driven by different lower-boundary thermal
  forcing associated with land-ocean configuration
  and topography.
• Examination of the different characteristics of
  the annual variability of the two subsystems may
  provide useful insight to understand how tectonic
  forcing and solar orbital forcing affect monsoon
  circulation.

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Asian-Australian Monsoon System
JA-JF 925 hPa winds and precipitation rate
(mm/day)
EA-WNP sector
Indian sector
Circulation systems differ between Indian and EA
sectors
Fig. 1
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Seasonal Distribution of rainfall
WNPM
IM
EAM
An eastward shift of convection centers from
Indian (in June-July) to the WNP (in August)
during boreal summer . Peak and retreat dates
differ. WNP is the largest heat source during NH
summer.
Wang, Clemens and Liu 2003
Fig. 2
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(Climatology 1979-2001)
Rainy Season
7/11
9/15
7/01
6/21
6/11
7/11
6/01
7/01
6/11
5/21
6/21
6/01
8/10
7/20
5/21
5/11
6/01
5/01
6/15
5/21
4/21
5/11
Wang and LinHo 2002
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Indo-China 100-110E
Seasonal March of ITCZ (SA Monsoon trough)
and EA Monsoon front
East Asia 110-145E
Indian monsoon 70-95E
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How important is land-sea contrast and orography
in Controlling monsoon AC?
Chang et al. 2006

• Marked cross-equatorial flows in the South China
  Sea and Celebes. Annual cycle of the Australian
  monsoon has a firmer link to the EA monsoon than
  to the Indian monsoon.
• Active convection and rainfall region shifts from
  Indian sector in boreal summer to the EA sector
  in austral summer

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Equinoctial asymmetry
In spring transition, EA sector has a
well-defined extratropical precipitation band
associated with the East Asian monsoon front.
In fall transition, the Indian monsoon rain
retreats to the south of the equator, whereas the
rain in the EA sector remains in the Northern
Hemisphere.
April
October
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Differences in the annual cycle

• Meridional extent and circulation systems
  tropical system vs. coupled tropical and
  subtropical system (EA)
• Seasonal march of major heat sources BOB and WNP
  heat sources behave differently.
• Rainy season onset and peak
• Strong EA winter monsoon more closely coupled to
  Australian summer monsoon
• Equinoctial asymmetry.
• The differences in the annual cycle are
  attributed to the effects of differing land-ocean
  configuration on atmospheric response to the
  annual solar forcing, which resembles the effects
  of the external (tectonic and orbital) forcing on
  paleo-monsoon variability.

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Conclusions

• The factors that control monsoon intensity may be
  classified as two groups The forcing external to
  the coupled atmosphere-ocean-land system
  (tectonic forcing and solar orbital forcing) and
  the forcing internal to the coupled climate
  system, such as (remote) El Nino/La Nino, local
  monsoon-ocean interaction, land-atmosphere
  interaction and extratropical influences (ice or
  snow cover).
• The mechanisms operating on the annual and
  interannual time scales are dominated,
  respectively, by the external and internal
  forcing.
• The differences between the Indian and East Asian
  monsoon is essentially determine by the relative
  strengths of the external versus internal
  forcings.

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Conclusion (Cont.)

• The robust coupling between the East Asian and
  Australian monsoon on both the annual and
  interannual time scales is essentially
  established by tectonic forcing. Thus, the
  increase in solar procession could enhance both
  the Indian summer monsoon and the East Asian
  winter-Australian summer monsoons.
• El Niño has little influence on the Arabian Sea
  summer monsoon, but considerable impacts on the
  South China Sea monsoon (about 10 on average and
  40 in strong events), suggesting that drastic
  changes in the Pacific thermal conditions could
  remarkably alter the East Asian-Australian
  monsoon intensity.