CGCM2 Monsoon

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Modeling of present-day monsoon and ENSO Akio
KITOH Meteorological Research Institute, Japan
Meteorological Agency
1 Climate model 2 Simulation of monsoon 3
Simulation of ENSO
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• Definition of Monsoon
• Originated from ancient Arabian word mausim
  which means a season. It was first used by Arab
  sailors to describe the seasonal winds that blow
  across the Arabian Sea.
• Original definition based on Ramage (1972) is
  most often applied to the seasonal reversals of
  the wind direction.

• The zone of maximum rain shifts from around 10S
  in winter to north of 10N in summer. The maximum
  northward shift in rainbelt is over the
  Asia-Pacific .
• The red box denotes monsoonal regions as defined
  by Ramage.
• The rainbelt over this region is in fact a part
  of the planetary scale TCZ and occurs due to its
  variation with the season.

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Low pressure over the Asian continent forcing
large-scale convergence of moist air across
surrounding oceans.
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Introduction Monsoon
As monsoons have come to be better understood,
the definition has been broadened to include
almost all of the phenomena associated with the
annual weather cycle over the affected regions
Seasonality in wind and precipitation.

• Regions experiencing a seasonal surface wind
  shift of at least 120 with a frequency of
  prevailing octant gt40

Asian Summer Monsoon
South Asian Monsoon
Southeast Asian Monsoon

• Asian summer monsoon region A part of the
  monsoon region including adjacent regions with
  predominant rainy season occurring during boreal
  summer.

• Two major components of Asian monsoon South
  Asian and Southeast Asian monsoon (1) Indo-China
  region (between), (2) equatorial Indian Ocean and
  (3) western North Pacific (adjacent).

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Seasonal Mean Major features of Indian Summer
Monsoon
Mean observed Rainfall and OLR (Proxy for deep
convection over tropics) during the peak monsoon
months of July-August.
It has been widely studied for the last few
decades and found to have multiple preferred
locations for the seasonal TCZ to occur.

• I the monsoon rainbelt stretching from the
  head Bay of Bengal westwards across central India
  (corresponds to seasonal mean position of monsoon
  trough). Variation in the intensity and location
  of this rainbelt gives rise to fluctuations in
  ISMR ,i.e., active and weak spells (eventually
  results in dry and wet seasons). A realistic
  simulation of rainbelt I is a prerequisite for a
  model to be useful in predicting Indian monsoon
  its variability.
• II secondary rainbelt over the equatorial
  Indian Ocean
• III IV due to the orography of the Himalayas
  and Western Ghats (west coast).

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Climate Model
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MRI CGCM2

• AGCM
• MRI/JMA98
• T42 (2.8x2.8), L30 (top at 0.4 hPa)
• Longwave radiation - Shibata and Aoki (1989)
• Shortwave radiation - Shibata and Uchiyama (1992)
• Cumulus - Prognostic Arakawa-Schubert type
• PBL - Mellor and Yamada level 2 (1974)
• Land Surface - L3SiB or MRI/JMA_SiB
• OGCM
• Resolution 2.5x(0.5-2.0), 23layers
• Eddy mixing Isopycnal mixing, GM
• Seaice Mellor and Kantha (1989)
• Coupling
• Time interval 24hours
• Flux adjustment without in this experiment

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Observed SST, Precip, Wind (JJA)
Simulation
MRI
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Simulation
MRI
Observed SST, Precip, Wind (DJF)
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???
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Koppen climate
Koppen climate
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Wet-Day Frequency
precipitation gt 1 mm/day
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Simulation of Monsoon
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• JJAS Mean Rainfall 850 hPa Wind Field

• Realistic Features
• Rainbelts over parts of India, Indo-China, South
  China Sea through Philippine Sea
• Zonal rainbelt from Indonesian region to
  mid-Pacific
• The rainbelt associated with Meiyu-Baiu front.
• Deficiency
• Southward shift of rainbelt over Bay of Bengal
• Deficit of rainfall over the equator

MRI/JMA
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Seasonal Variation of pentad mean precipitation
over India

• Onset to around 10N in late May/early June.
• The equatorial rainbelt is active throughout the
  year and the continental rainbelt is centered
  around 15N and extends beyond 25N.

• Onset occurs around late May as a quick
  transition
• Persistent heavy rainfall throughout the season
  ? longer duration of monsoon
• The oceanic rainbelt is weaker, the northward
  extent of continental rainbelt is limited within
  20N
• The two rainbelts are connected by very weak
  intraseasonal northward propagations.

MRI/JMA
CRD
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• Over 120E-140E
• Observation shows disappearance of dry region
  over the Philippine Sea in early May and a
  northward shift of rainbelt from 20N in May to
  35N in July.
• This northward shift and late June Baiu rainfall
  peak are well reproduced by the model.
• The model simulates revival of rainbelt in late
  August/September with a slightly early peak.

MRI/JMA
CRD
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Mean Evolution of Monsoon Strength of monsoon
extent of domain
Monsoon annual range? difference between maximum
May-Sep. pentad mean rainfall Jan. mean
rainfall ("maximum of relative climatological
pentad mean rainfall").

• Contour 4 mm/day delineates monsoon domain.
• Simulation is close to observation.

CRD
MRI/JMA
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Kang et al. (2002)
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The Monsoon Domains observed and simulated by 10
AGCMs. (Kang et al., 2002, CD)
CLIVAR/GCM Monsoon Intercomparison Project COLA,
DNM, GEOS, GFDL, IAP, IITM, MRI, NCAR, SNU,
SUNY/GLA
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Smoothed annual cycle defined as the sum of
annual mean plus first 12 harmonics of the
climatological pentad mean rainfall.
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Onset of Monsoon on Indian Landmass
Mean Onset Date
After Rao (1976)

• Gradual northwestward progression of monsoon
  from Bay of Bengal.
• Monsoon establishes over the subcontinent by
  the middle of July.

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Onset Pentad the Julian pentad in which the
relative climatological pentad mean rainfall rate
exceeds 4 mm/day.

• Indian region
• Northeastward progression over AS and the
  northwestward progression over the Bay of Bengal
  are well reproduced.
• East Asia
• The model simulates earlier monsoon onset over
  southeast Asia.
• Onset over Indochina in early May, the mid-May
  onset over the SCS later northward progression
  due to Meiyu/Baiu rainband are all simulated,
  although the precise timings differ slightly.
• In northern China, onset is earlier and
  precipitation is heavier.

CRD
MRI/JMA
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Mean Evolution of Monsoon Peak Rainfall Pentad

• The peak pentad is realistic over most of the
  Indian region, except for the late peak rainfall
  over northwest India.
• Over most of the west Pacific region model peak
  rainfall timing realistic

MRI/JMA
CRD
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Mean Evolution of Monsoon Monsoon withdrawal
Withdrawal Pentad the transitional pentad in
which rainfall drops below 4 mm/day.

• Observation shows
• southward retreat of monsoon over India,
  southeast Asia and Western north Pacific
• northward retreat over East Asia.

Simulation close to observation.
CRD
MRI/JMA
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• The Role of Air-Sea Interaction Local Coupled
  Feedbacks

MRI/JMA
CRD
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The role of air-sea interaction Local coupled
feedbacks
LCC between anomalies of area averaged SST and
precipitation (mm/day), surface shortwave,
longwave, sensible heat and evaporative fluxes
(Wm-2) and magnitude of surface wind stress
(Nm-2). Correlation significant at 95 level is
0.36.
Highest SST leads maximum Convection (5 days)
SST Warming Phase (negative lags) SST
SST Cooling Phase (positive lags) SST
Low surface wind stress (6 days lead) Low
Evaporation (9 days lead) increased stability
Enhanced wind stress (5-6 days lag) Increased
Evaporation (4-5 days lag) Reduced stability
Increased convection Enhanced cloudiness
Reduced Convection Reduced cloudiness
High SWsfc (4-5 days lead) Reduced Evaporative
Cooling
Reduced SWsfc (6 days lag) Increased evaporative
cooling
Cooler SST
Warmer SST

• During SST warming phase, reduced evaporation,
  weak surface wind stress and increased net
  surface shortwave flux are associated with
  reduced convection.
• Once the convection is established due to
  increased instability of the lower atmosphere
  associated with SST warming, the enhanced surface
  wind convergence results in further enhancement
  of convection. Increased cloudiness by enhanced
  convection results in reduction of surface
  shortwave flux. Increased surface winds and
  decreased insolation in turn lead to the cooling
  of SSTs.
• Coupling in the model produces strong SST
  convection feedback with significant correlation
  between the fluxes and SST implying the dominance
  of the SST-wind-evaporation- feedback.
• This is consistent with proposed MJO mechanism
  but with closer lags between peaks of SST and
  precipitation/surface fluxes in the model.

MRI/JMA
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SST-Wind-Evaporation Feedback
Enhanced Convection Enhanced evaporative cooling
(Increased wind driven mixing Reduced SWsfc
due to cloudiness)
Cool Dry Air
Evaporation
Surface Wind
Warm SST
Cold SST
SST Gradient
Cool Dry Air
Higher Evaporation
Stronger Surface Wind
Larger SST Gradient
Warmer SST
Cold SST

• The increased advection of cool dry air towards
  a warmer SST region along the initial SST
  gradient produces stronger winds and higher
  evaporation, higher moisture convergence.
  Enhanced atmospheric instability leads to
  increased convection which in turn further
  increases the surface convergence, winds and
  evaporation.
• This results in increased evaporative cooling of
  the SST. Increased wind driven mixing and
  reduction in surface shortwave flux due to the
  impact of increased cloudiness also contribute to
  cooling the SST .

MRI/JMA
CRD
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ENSO and ENSO-monsoon relationship
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ENSO and its effect on climate
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ENSO 1997/98
NOAA/PMEL
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Rainfall anomalies during November 1997-April 1998
BAMS (1999)
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from M.Latif
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from M.Latif
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Monsoon - ENSO Connections
The three components of the monsoon system are
tied to the heated land, the tropical warm pools
and the cold winter ocean and land areas. All of
these produce the strongest heating gradients on
the globe. There are three major circulation
associated with the boreal and austral summer
monsoons. These form a simplified but integrated
view of the Monsoon ENSO atmospheric circulations
in the Indo-Pacific Region.
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SST-EOF1 with vs without flux adjustment
with flux adjustment
without flux adjustment
SST-EOF1
Surface air temperature
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Control run global SST EOF1 and regressions
SST-EOF1 and its regressions (MRI-CGCM2 control)
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spectrum of SST-NINO3 (Global Wavelet)
Model tends to shift towards higher frequency
than observed
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ENSO - Monsoon Relationship
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The role of air-sea interaction Remote SST
Impact on ISMR
Significant remote SST impact on Indian monsoon
is the inverse relationship between ISMR and ENSO
Standardized indices of model ISMR, DMI (a
measure of the monsoon strength in terms of the
zonal wind shear between 850 and 200 hPa over
40E-110E, 5N-20N) and NINO3 SST (over
150W-90W,5S-5N) . The correlation between ISMR
and DMI is 0.69 because DMI is a measure of the
large-scale monsoon circulation and does not
necessarily correspond to the regional rainfall
variation represented by ISMR. The correlation
between ISMR and NINO3 SST is -0.73 much larger
than observed value of -0.46 (which suggests
NINO3 SST is only one of the modulators of
monsoon). The correlation between DMI and NINO3
SST is also high -0.86. Both correlations are
negative consistent with the tendency for below
(above) normal monsoon during ENSO years but
imply a much stronger impact of NINO3 SST on ISMR
in the model.
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Remote SST Impact on ISMR

• LCC from -2 years to 2 years of equatorial
  monthly SSTs against JJAS mean NINO34
  (160E-90W,7.5S-7.5N) SST.
• Observation
• High positive correlations for about one and a
  half years beginning from around Jan (0) to
  around July (1) in the central and eastern
  equatorial Pacific.
• Simultaneously, there are negative correlations
  over the West Pacific and positive correlations
  over the Indian and Atlantic Oceans.
• Model
• A small amplitude QBO appears to be dominant in
  the model.
• Strong positive correlations (0.75) extending
  over the whole of Indian Ocean during JJAS season
  at lag (0). indicates the strong connection
  between Pacific SST and equatorial Indian Ocean
  SST in the model.

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Remote SST Impact on ISMR

• LCC from -2 years to 2 years of equatorial
  monthly 850 hPa zonal wind against JJAS mean
  NINO34 (160E-90W,7.5S-7.5N) SST.
• Observation
• Zonal wind anomalies propagates eastward from IO
  to the WP
• Model
• Wind anomalies also propagate eastwards but with
  a slightly faster speed. This difference in phase
  speed corresponds well with the simulated QBO
  time scale. (In the equatorial Pacific, simulated
  SST and the wind anomalies extended further
  westward with slightly narrower meridional
  extend. Consequently, the travel time of wind
  forced oceanic Rossby waves to reach the western
  boundary is shorter compared to observation. This
  acts as an essential factor for the shorter
  time-scale of model ENSO).
• During JJAS season at lag (0), the much higher
  negative correlations over the IO extends over
  the whole domain establishing the enhanced impact
  of Pacific SST in the model.
• The dominance of QBO time scale of Pacific
  Warming and its stronger correlation with Indian
  monsoon can play a major role in modifying the
  mean monsoon pattern during warm years.

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The role of air-sea interaction Remote SST
Impact on ISMR
Lag (0) regression of model wind speed (at 10m),
evaporation, precipitation and surface air
temperature over India against NINO3 SST.
Reduced Evaporation over equatorial IO India
Reduced moisture convergence towards Indian
monsoon region
Reduced Convection Reduced Precipitation
Weaker equatorial IO Zonal Wind
Tendency for Enhanced land temperature over India
Weaker Indian Summer Monsoon
Positive equatorial East Pacific SST Anomaly

• The weakening of zonal wind component (e.g., due
  to remote impact of Pacific SST) leads to
  reduction in evaporation over the IO which in
  turn result in reduced monsoon precipitation over
  India through reduced moisture convergence.
  Associated with this reduced evaporation and
  precipitation, the land temperature tends to
  increase over India.
• In summary, positive equatorial East Pacific SST
  anomalies have a negative impact on the large
  scale Indian summer monsoon (mostly on a biennial
  time scale in the model through the zonal
  shifting of the regions of maximum tropical
  precipitation and diabatic heating from
  convective precipitation processes).

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IMR and NINO3.4 SST from 1000-year run
MRI-CGCM2 1000-yr run
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IMR and NINO3.4 SST
Note a large interdecadal variability of
ENSO-monsoon relationship
MRI-CGCM2 1000-yr run
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IMR and NINO3.4 SST
A weakening of ENSO-monsoon relationship is
associated with warmer Eurasian continent
MRI-CGCM2 1000-yr run