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Daily modes of the South Asian monsoon variability and their relation with SST

Deepthi Achuthavarier

Work done with V. Krishnamurthy

Acknowledgments

Ben Kirtman, Kathy Pegion, Bohua Huang, Jennifer Adams

COLA SAC Presentation

April 12, 2010

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1 June 30 Sep

Intraseasonal oscillations weaker correlation with SST;

mainly due to Internal dynamics?

Slowly varying climate signals present in daily dataor Persistent modes

Strong correlation with SST

Dai

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A conceptual model for monsoon variability Seasonal mean = persistent modes + Intraseasonal oscillations

Cartoon based on Krishnamurthy and Shukla 2000; 2007

Flood year

Drought year

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Role of Indian and Pacific SST in ENSO and ISO modes

• ISO modes

– What is the role of Indian Ocean SST and air-sea interaction?

– What happens to ISO modes without Pacific SST variability?

• Persistent modes (mainly ENSO mode)

– What is the role of Indian Ocean SST and air-sea interaction?

• What controls the monsoon variability in the absence of the

ENSO? Is there an inherent rainfall-SST variability in the IO?

• These investigations will help us understand the

predictability of the system

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Analysis methodologyMulti-channel singular spectrum analysis (MSSA)

• Dominant modes in the daily data are obtained using MSSA

– Captures spatial structure of maximum variance and its propagation in time

– Captures nonlinear oscillations, trends, interannual signals (or repeating

sequences in data in their order of variance)

– Captures unstable periodic orbits of the system

– Data adaptive filtering

• Intraseasonal scale

– Oscillations are obtained without pre-filtering the data

– Both the 45 and 28 day modes and their propagation are addressed

• Interannual scale; why daily data? – Isolates low-frequency climate signals from daily data; Climate signal that is

not influenced by ISO can be obtained

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MethodologyNumerical experiments using regional coupling strategy

AGCM

OGCMSST

Heat and momentum fluxes

Prescribed SST

• Atmosphere sees prescribed SST in a certain region; but OGCM

produces global SST fields at each time step• Impact of SST variability or air-sea interaction over a certain basin

can be isolated • Blending of prescribed and OGCM-produced SST at the boundaries

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Regionally coupled simulations using NCEP CFS

List of simulations (all 30 year long)

ControlFully coupled everywhere

NA

IO-V(C)SST

PO-CSST

Prescribed SST regions

• Atmospheric initial conditions taken from the control; but perturbations introduced

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Northeastward propagating modelatitude-phase and longitude-phase diagrams of the composites

IO-VSSTNo IO air -sea interaction

PO-CSSTPacific climatology, No ENSO

ControlFully coupled everywhere

• Northeastward propagating mode obtained even without IO

coupling, but NOT without SST variability in the IO

• Northeastward propagating mode obtained without ENSO

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Northeastward propagating mode in IO-CSSTLatitude-time and longitude-time diagrams for the 106-day mode

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Northwestward propagating mode latitude-phase and longitude-phase diagrams of the composites

IO-VSSTNo IO air -sea interaction

IO-CSSTIO forced with climatology

• Westward propagation is obtained without IO SST variability

• SST variability in the WP may have influence on this mode; PO-

CSST does not produce this mode

ControlFully coupled everywhere

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Seasonally persistent ENSO mode (EOF1 of the ENSO mode)

CTL

IO-CSST

IO-VSST

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ENSO mode in the control run lead/lag regression between monthly SST, precipitation and wind anomalies and

monthly mean of the PC1 of the ENSO mode

Precipitation SST and UV at 850hPa

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ENSO-monsoon relation Lead/lag correlation between the JJAS EIMR and monthly NINO3

Total seasonal anomalies of rainfall

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Summary and conclusions

• Intraseasonal variability

– Northeastward propagating mode depends on SST variability in the IO

– No Pacific SST dependence in this mode

– Northwestward propagating mode has no clear dependence on IO SST; but has

dependence on West Pacific SST

• Interannual variability

– ENSO-induced rainfall does not reach the Indian subcontinent due to an over-

active Indian Ocean. Proper air-sea interaction in IO is important.

– Without ENSO, no dominant slowly varying SST-driven rainfall modes were

obtained; the dominant (atmospheric) mode produces SST anomalies in the IO.

– A weak dipole mode is found in the rainfall even in the absence of the ENSO; its

contribution to the EIMR region precipitation is negligible.

• Results are model dependent

• Further analysis should explore the ocean dynamics in the IO

End

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Remaining questions and future work

• A dipole in SST and rainfall is developed in the absence of ENSO; what triggers it?

• What is the impact of western Pacific SST on monsoon variability?

• Why western Indian Ocean appears to cool down fast in the model?

– Examine subsurface variables, like thermocline depth

• Future experiments

– Observed SST in the Indian Ocean

– Observed SST in the Pacific

• Why is the period of the ISO mode ~106 days?

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Intraseasonal modes in the NCEP CFS Phase composites of the ISO modes

106-d oscillation 30-d oscillation

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Intraseasonal modes in the NCEP CFS Area-average of the 106-day mode and total daily anomalies

over monsoon region

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Seasonally persistent signal in the NCEP CFS

EOF1 of RC1 (ENSO mode) PC1 of ENSO mode (black) and -NINO3 (red)

Point correlation between PC1 of ENSO mode and daily SST

• No additional dipole mode is

obtained; but ENSO mode has

good correlation with DMI also

C = 0.53

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Contribution of the ISO and persistent modes to seasonal mean

JJAS seasonal mean of total anomalies and ISO and ENSO modes for EIMR and IOMR regions

Standard deviation of seasonal means of RCs as fraction of the total

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Northeastward propagating mode in Control runPhase composites of the winds, latent heat flux, shortwave radiation and

SST based on the phases of the precipitation

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Northwestward propagating mode latitude-phase and longitude-phase diagrams of the composites

SST composites for 106-day mode

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ENSO mode in the IO-VSST lead/lag regression between monthly SST, precipitation and wind anomalies and

monthly mean of the PC1 of the ENSO mode

Precipitation SST and UV at 850hPa

Dipole-like mode in the absence of the ENSO

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Intraseasonal modes in the experiments

• Is SST variability in IO crucial for the ISO modes?

• Is air-sea interaction in IO crucial for the ISO modes?

• What happens to ISO modes in the absence of the ENSO?

Run

ISO mode

IO-VSST IO-CSST PO-CSST

Northeastward propagation (106d)

Yes No Yes

Northwestward propagation (30d)

Yes Yes No

Background: Dominant daily modes in observed OLR

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Northeastward propagation 45-day ISO mode

Persistent, non-oscillatory modes ENSO and Dipole

ENSO

Dipole

Northwestward propagation 28-day ISO mode

Krishnamurthy and Shukla 2008

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Background: Daily modes in observed OLR and relation with SST

Seasonal anomalies of the oscillatory modes Seasonal anomalies of the persistent modes

Krishnamurthy and Shukla 2008; Krishnamurthy and Kirtman 2009

Contribution to the seasonal anomalies

45-d28-d

ENSODipole

Correlation with SST: Oscillatory modes Correlation with SST: Persistent modesRelative roles of the Indian and Pacific Ocean SST variability

45-d

28-d

ENSO

Dipole