drivers of inter-decadal variability of the asian summer ...€¦ · drivers of inter-decadal...
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Drivers of Inter-decadal Variability of the Asian Summer Monsoon
H. Annamalai IPRC/SOEST, University of Hawaii
Contributions from: K. Lakshmi and V. Krishnamurthy (COLA) M. Rajeevan (MoES, India) R. Krishnan (IITM, Pune, India)
• Interannual – decadal variability (analogy to intraseasonal - interannual variability)
• Observations (current and past), and models (current)
• Decadal ENSO-monsoon association (observations and models)
• AMO/PDO linkages from observations (land-sea thermal contrast ?)
• Is there a breakdown in this decadal-multidecadal variability?
• Issues
Talk Outline
Summer (JJAS) all-India rainfall anomalies (mm) and Nino3.4 SST anomalies (C)
“decadal epochs” – alternating decadal clustering of strong and weak monsoons – economic conditions – green revolution (since 1960s) – education in 1950s “relationship between interannual and decadal variability?” – analogy to ISV - IAV “intensity of ENSO – no direct relationship with intensity of monsoon rainfall” – other factors?
10% change in last 50-60 years!
Decadal-multidecadal variations in monsoon rainfall over India
Decadal variability and trend?
amplitude – observational constraints (Turner and Annamalai 2012)
Proxy indications of multi-decadal monsoon rainfall variations
M. Berkelhammer et al. / Earth and Planetary Science Letters 290 (2010) 166–172
M. Berkelhammer et al. / Earth and Planetary Science Letters 290 (2010) 166–172
Decadal-multidecadal rainfall variations since 600 AD
0
10
20
30
40 19
01
1905
19
09
1913
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17
1921
19
25
1929
19
33
1937
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41
1945
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49
1953
19
57
1961
19
65
1969
19
73
1977
19
81
1985
19
89
1993
19
97
2001
Observed very heavy rainfall events (R> 150 mm/day) over central India (red line) and its smoothed variation (black line) for the period 1901–2004.
Rajeevan et al. (2009) -
All-India rainfall
Smoothed variation of frequency of very heavy rainfall events over central India and SST anomalies over the Equatorial Indian Ocean. The smoothing has been done to remove the sub-decadal fluctuations using a 13-point filter [IPCC, 2007].
0
5
10
15
20
25
30
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1901
19
04
1907
19
10
1913
19
16
1919
19
22
1925
19
28
1931
19
34
1937
19
40
1943
19
46
1949
19
52
1955
19
58
1961
19
64
1967
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70
1973
19
76
1979
19
82
1985
19
88
1991
19
94
1997
20
00
RAIN
FALL FRQ
SS
T AN
OM
ALIE
S
Y E A R
INDIAN OCEAN SST ANOMALIES AND HEAVY RAINFALL
FREQUENCY OVER CENTRAL INDIA
IO SST ANOMALY
RAINFALL FREQUENCY
Decadal variability of the Indian monsoon •Low-frequency (11-yr running mean) IMR and Nino-3 vary coherently with negative correlation and follow a common interdecadal variation
•The regressions of SST on low-frequency IMR and Nino-3 show similar ENSO-like patterns
•Hypothesized to be a low-frequency tropical ocean-atmosphere mode (Krishnamurthy & Goswami 2000)
“need for ensemble”
(a) MRI
(b) CCSM4
“decadal modulation tied to decadal ENSO variance”
Decadal Variability of Indian Monsoon Rainfall
Field: Rainfall (5-year running mean) Domain: Indian monsoon region Data: IMD Period: 1901-2004
RC (1,2) - EOF RC (5,6) - EOF RC (7,8) - EOF
Result from MSSA: 3 statistical oscillatory modes
(All MSSA modes are significant at 95% confidence level based on MSSA Monte Carlo test)
K. Lakshmi (COLA) South of the mean monsoon trough
Relation between decadal modes in IMR and SST
RC(1,2)
RC(5,6)
RC(7,8)
Regression of SST on rainfall RC index
Dotted regions - 90% confidence level
Mode Period Relation to SST
RC (1,2) 52 AMO
RC (5,6) 21 PDO
RC (7,8) 13 Tripole
“statistical oscillatory mode”
Shifts in PDO index – not seen in monsoon rainfall –
Observations GFDL_CM2.1
AMO has better relationship to monsoon rainfall variations at multi-decadal time scales? Does the relationship break down in recent decades?
Zhang and Delworth (2006)
“time-scale”
“dominated by Sahel rainfall”
In-phase
Is there a break down in decadal monsoon variability?
10% change in last 50-60 years!
JJAS – Precipitation and SST Climatology
I II
III
• Multiple regional heat sources - • EIO and SPCZ – still experience high precipitation (thermal equator at 20oN) • Central India rainfall – dynamical effects; Rain-shadow regions • absolute ascent over a large domain
CM2.1 Diagnostics
Rainfall over tropical west Pacific in CM2.1
•Salinity observations since 1955 support this increase in rainfall (Cravatte et al. 2009) • Decadal-multidecadal SST variability over west Pacific (Guan and Nigam 2008)
JJAS SST – Observations (trend /50 year)
No significant warming over the equatorial Pacific (PDO? No AMO signature?)
AMO…? PDO…?
SLP and 850 hPa Vertical velocity 400 hPa
Day 6
Day 9
Day 3
Figure 7: LBM solutions of perturbation SLP and wind at 850 hPa (left) and vertical velocity at 400 hPa (right) to SST anomalies over tropical west Pacific for Day 3 (top), Day 6 (middle), and Day 9 (bottom)
Summary….
Existing ideas on the role of AMO/PDO on the monsoon – land/sea contrast What about the variability over tropical western Pacific? Thermodynamics – moist static energy principle – (i) Pre-industrial control runs with CMIP3/5 models (ii) Coordinated coupled model experiments by AAMP?
Summer (JJAS) all-India rainfall anomalies (mm) and Nino3.4 SST anomalies (C)
“decadal – multi-decadal epochs” – alternating decadal clustering of strong and weak monsoons “data manipulation or a natural mode of variability?”
Turner and Annamalai (2012, Nature CC)
Coupled ocean-atmosphere-land (also by fixed orography)
• Dynamics and thermodynamics – very different from simple land-breeze • Thermodynamics forcing during summer – rainfall poleward of Equator Yet, sufficient rainfall occurs along EIO “did not review the decadal variability”
CCSM4
“decadal modulation tied to decadal ENSO variance”
CM2.1 diagnostics
What governs the decadal-multidecadal modulation of ENSO ? Wittenberg (2006)-
SLP and wind at 850hPa
“how to choose sliding window”
Moisture Budget (AM2.1 – SST trend over warm pool)
Precipitation Moisture convergence
Moisture advection Evaporation
Annamalai et al. (2012)