Title: Monsoon Interannual Variability
1- Monsoon Interannual Variability
- Relationship to ENSO and Tropospheric Biennial
Oscillation (TBO) - Tim Li
- University of Hawaii
- Introduction - observations
- Monsoon-ENSO relationships
- Biennial and lower-frequency variability of
Indian monsoon - Asian-Australian monsoon phase relations
- Seasonal evolving TBO structures
- A monsoon-warm ocean interaction scenario for the
TBO
2Webster et al. 1998
3El Nino structure and time evolution
4Gilbert Walker 1924 Southern Oscillation
5El Nino-Southern Oscillation (ENSO)
J. Bjerknes, 1969
6Normal condition
Mechanism for El Nino growth and phase
transition A positive dynamic ocean-atmosphere
feedback Bjerknes feedback J. Bjerknes 1969
El Nino condition
7A delayed negative feedback leads to El Nino
phase transition
8Seasonal differential precipitation and winds
(JA-JF)
Top domains for three sub-monsoon systems IM
westerly, north-south T gradient EAM southerly,
east-west T gradient WNPM hemispheric asymmetric
SST gradient
EAM
IM
WNPM
WNPM
Bottom area-averaged rainfall evolution WNPM
strongest, with a peak phase lag to IM and EAM.
IM
JA-JF
EAM
Wang, Clemens and Liu 2003
9Wang and LinHo 2002
(Climatology 1979-2001)
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
10- How does the ENSO influence the monsoon?
- Indian monsoon ENSO relationship
- WNP monsoon ENSO relationship
- East Asian monsoon ENSO relationship
- What are other factors that influences the
monsoon in addition to ENSO? - TBO
- Indian Ocean SSTA/western Pacific SSTA
- Tropical-mid-latitude interaction
11Possible role of Indian Ocean IO SSTA warm prior
to a strong Indian Monsoon, but cold after a
strong monsoon. No significant simultaneous
correlation between Indian monsoon and IO SSTA
Lau and Yang 1996
12- How does the El Nino impact South Asian monsoon?
Why is the atmosphere response equatorially
asymmetric to a symmetric forcing? How the SSTA
off Sumatra is initiated? Why is the Indian
monsoon anomaly opposite to the WNP monsoon
anomaly?
12-El Nino composite JJA(0)
13Gill model
The governing equations
Two special cases I, The heating is symmetric
about the equator and has the form of
II, The heating is antisymmetric about the
equator with the form
The forcing will be assumed localized and to have
the form
For a steady state
where
14Gill model
Model results For symmetric forcing, there are
two parts of the response. 1,
2,
15Gill model
- Fig. 1 Solutions for heating symmetric about the
equator in the region xlt2 for decay factor
e0.1 - Contours of vertical velocity w ( solid contours
are 0, 0.3, 0.6, broken contour is -0.1)
superimposed on the velocity field for the lower
layer. The field is dominated by the upward
motion in the heating region where it has
approximately the same shape as the heating
function. Elsewhere there is subsidence with the
same pattern as the pressure field. - Contours of perturbation pressure p (contour
interval 0.3) which is everywhere negative. There
is a trough at the equator in the easterly regime
to the east of the forcing region. On the other
hand, the pressure in the westerlies to the west
of the forcing region, though depressed, is high
relative to its value off the equator. Two
cyclones are found on the north-west and
south-west flanks of the forcing region. - The meridionally integrated flow showing (i)
stream function contours, and (ii) perturbation
pressure. Note the rising motion in the heating
region (where there is a trough) and subsidence
elsewhere. The circulation in the right-hand
(Walker) cell is five times that in each of the
Hadley cells shown in (c).
16Gill model
For asymmetric forcing,
- Fig. 2 Solutions for heating antisymmetric about
the equator in the region xlt2 for decay factor
e0.1 - Contours of vertical velocity w (contours
interval 0.3) super-imposed on the velocity field
for the lower layer. The field is dominated by
the motion in the heating region where it is
approximately the same shape as the heating
function (positive in the northern hemisphere).
Outside the forcing region, the pattern is the
same as for the pressure field with subsidence in
the northern hemisphere and weak upward motion in
the southern hemisphere. There is no motion for
xgt2. - Contours of perturbation pressure p (contour
interval 0.3) which is positive in the south
(where there is an anticyclone) and negative
north of the equator (where there is a cyclone).
The flow has the expected sense of rotation
around the pressure centers and flows down the
pressure gradient where it crosses the equator.
All fields are zero for xgt2.
17Gill model
Further solutions
Fig.3 Solution obtained by adding the solutions
shown in the two previous figures, corresponding
to heating which is confined to the region xlt2
and is mainly concentrated to the north of the
equator. (a) contours of vertical velocity w
(contour interval 0.3) showing the dominance of
the heating north of the equator. The flow to the
east of the forcing region is the same as in
Fig.1, this being provided entirely by the
symmetric part of the heating. West of the
forcing, the westerly inflow is concentrated
between the equator and y2. An easterly flow is
found south of the equator. (b) Contours of
perturbation pressure p (contour interval 0.3).
The pattern is dominated by a low on the western
flank of the heating region and by the
equatorial trough. A high is found in the
southern hemisphere.
18How does the El Nino remotely impact the Asian
monsoon? Large-scale east-west overturning
? Equatorial asymmetric response to a symmetric
El Nino forcing, why?
Anomaly dry AGCM with specified 3D summer mean
flow and diabatic heating
Wang, Wu and Li 2003, J. Climate
19East Asian Monsoon (EAM)-ENSO Relationship Observ
ational fact EAM meiyu-rainfall significantly
increases 6 months after the peak of ENSO.
By the summer after peak El Nino, the eastern
Pacific SST becomes normal. How can ENSO has a
delayed impacts on EAM?
El Nino developing year
El Nino decay year
20Leading mode of Monsoon Interannual Variation
850 hPa winds and local SST anomalies (19572001)
- Biennial tendency associated with ENSO turnabout
- Distinct evolutions of anticyclonic anomalies
over SIO and WNP
Wang, Wu, Li 2003, J. Climate
21Wang and Zhang 2002, JC
22A positive thermodynamic air-sea feedback
mechanism Wang et al. 2000
El Nino heating ? atmospheric Rossby wave
response ? cold SSTA in WNP ? anomalous AC ? EAM
23(significance level 95 shaded)
Strong ENSO composite
eastward displacement of the Low-level
anticyclone
Another possible process that leads to formation
of anomalous Philippine Sea anticyclone
24Chen, Li and Shih, 2007, J. Climate
divergence
Dry, cold
A
Subsidence
Wet, warm
dry
The zonal asymmetry of divergence, vertical
motion, and precipitable water leads to the
eastward propagation of the low-level anticyclone.
25Quasi-biennial (2-3-yr) and lower-frequency (LF,
3-7-yr) variabilities of the Indian monsoon
rainfall
Monsoon prediction issue
Q What processes are responsible for the QB and
LF interannual variabilities of the Indian
monsoon rainfall?
Li et al. 2001, GRL
261000hPa moisture flux convergence (2-3 yr)
27- Large-scale east-west circulation
- Land-ocean thermal contrast
- Synoptic wave activity/monsoon trough in WP
28Li and Zhang 2002, J. Meteor. Soc. Japan
Local processes (IO SSTA moisture transport)
prior to the monsoon season
Large-scale overturning circulation
Convective activity at WP monsoon trough
Monsoon LFV
Land-ocean thermal contrast prior to the monsoon
onset
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31Indian and Australian Monsoon in-Phase
Relationship
- Why does a strong (weak) Australian monsoon
often follow a strong (weak) Indian monsoon? - Hypothesis El Nino control (Yanai and Liu 2001)
32Velocity potential difference fields
33Zhang and Li 2007
The result suggests that Indian Ocean SSTA may
play a active role in bridging the Indian and
Australian summer monsoons.
34Monsoon TBO
Yasunari 1990
35What is the TBO? The tendency of the monsoon
rainfall anomaly to flip-flop in successive
years is referred to as the tropospheric biennial
oscillation (TBO, Meehl 1994, 1997).
Observational studies have shown that the TBO is
manifested over various monsoon regions such as
Indonesia/northern Australia (Nicholls 1978,
Yasunari and Suppiah 1988), East Asia (Lau and
Sheu 1988, Tian and Yasunari 1992, Shen and Lau
1995, Chang et al. 2000ab, Wang and Li 2004) and
India (Mooley and Parthasarathy 1984 Meehl 1987
Rasmusson et al. 1990 Yasunari 1990, 1991).
36Lagged correlation between ISM and SST in EP and
WP
Yasunari 1990
37Shen and Lau 1995
38A review on TBO theories
- TBO theories
- Local air-sea interactions (Nicholls 1978 Meehl
1987 Clarke et al 1998) - Inter-basin ocean-atmosphere interactions in the
tropics (Chang and Li 2000, Li et al. 2001) - Tropical-midlatitude teleconnection (Meehl
1994,1997 Tomita and Yasunari 1996 Ogasawara et
al 1999, among others)
39Local air-sea interaction theory
Nicholls 1978 MWR
40Meehl 1987 Monsoon-SST local negative feedback
mechanism
41Meehl 1987, MWR
42Chang and Li 2000, J. Atmos. Sci.
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44Meehl 1987
45Li et al. 2006, JC
TBO activity centers
Time filter
46A seasonal-sequence EOF analysis using NCAR/NCEP
reanalysis data (1950-1999) Shading
rainfall Vector 925mb wind 200mb wind first
baroclinic mode structure Data seasonal mean
(DJF, MAM, JJA, SON) fields of      Â
Precipitation, SLP, SST Â Â Â Â Â Â 925mb U and
V Â Â Â Â Â Â 200mb U and V Â Â Â Â Â Â 850mb
geopotential height
47TAS feedback in western North Pacific (Wang et
al. 2000)
48A Season-dependent Air-Sea Feedback in the
Southeast Indian Ocean
Warm
EQ
Sumatra
Cold
Anticyclonic Circulation
Mean southeasterly in boreal summer
? A primary mechanism that leads to the phase
locking of the Indian Ocean Dipole (Li et al.
2003, JAS)
49What is the origin of the TBO ? Hypothesis
1 TBO is forced by remote forcing from the
eastern equatorial Pacific. Hypothesis 2 TBO is
an air-sea coupled mode in the monsoon region.
? This implies that the QB component of ENSO may
result from the inter-basin teleconnection
between the monsoon/warm ocean and eastern
Pacific.
50A Monsoon-Warm Ocean Interaction Scenario for the
TroposphericBiennial Oscillation
Li, T., P. Liu, X. Fu, B. Wang, and G. A. Meehl,
2006 Tempo-Spatial Structures and Mechanisms of
the Tropospheric Biennial Oscillation in the
Indo-Pacific Warm Ocean Regions. J. Climate, Vol.
19, No. 13, pages 30703087.
51TBO mechanism
?
?
QB Monsoon
QB ENSO
LF ENSO
LF Monsoon
Delayed oscillator
52- IPRC Hybrid coupled atmosphere-ocean GCM (Fu et
al. 2003, 2004) - ECHAM4 AGCM
- UH 2.5-layer intermediate ocean model (Wang, Li
and Chang 1995 Fu and Wang 1999)
50-yr integration
53Hybrid coupled GCM simulations Summer vs. Winter
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55Coupled ECHAM4-ocean model 50-yr simulation
56The hybrid coupled GCM experiments suggest that
the TBO originates from the monsoon-warm ocean
interaction. ?Analogy to the PNA pattern
(internal atmospheric dynamics vs. external
forcing) The TBO is an inherent monsoon mode,
while the El Nino forcing may magnify the signal.
Question Through what processes does the air-sea
interaction in the warm ocean lead to a TBO?
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58 Top TRMM precipitation (mm/day) (annual mean
removed)
Bottom ECHAM4 simulated precipitation (mm/day)
59Box average (10-15N, 140-160E)
A Total SST tendency B Net heat flux term C
Sum of ocean dynamic (3D advection) terms
60Simulations from an intermediate coupled model
that suppresses the delayed oscillator
mode (i.e., no El Nino dynamics)
61A monsoon-warm ocean interaction scenario for the
TBO
62 Conclusions
- During the El Nino developing summer, El Nino
tends to decrease rainfall over India but enhance
rainfall over WNP. During the El Nino decaying
phase, a WNP anticyclone anomaly persists from
northern winter to subsequent summer, leading to
enhanced Meiyu rainfall over Central China. - Indian summer monsoon has two significant
spectrum peaks on quasi-biennial (2-3-yr) and
lower-frequency timescales. Physical mechanisms
determining the two timescale variabilities are
different. The former is primarily attributed to
local processes over the tropical Indian Ocean,
while the latter is attributed to remote forcing
from the Pacific. - Asian-Australian monsoon in-phase relationship
in addition to remote ENSO forcing, eastward
propagation/expansion of Indian Ocean SSTA may
contribute to the in-phase relationship.
63 Conclusions
- TBO structure and evolution The major convective
activity centers associated with the TBO appear
in SEIO and WNP, accompanied with anticyclonic
(or cyclonic) circulation patterns with
distinctive life cycles. - TBO originates from air-sea interactions in the
warm ocean. Key processes involved include the
variability of the WNP monsoon and associated
cross-equatorial flows, convective activity over
Southeast Asia/maritime continent and anomalous
Walker circulation, and ocean dynamic responses
to anomalous wind stress curl in the western
Pacific.