1. Dynamics of the Indian Ocean response to boreal summer atmospheric intraseasonal oscillations
Sara Vieira
Committee members:
Dr. Peter Webster
Dr. Emanuele Di Lorenzo
Dr. Judith Curry
Dr. George Chimonas
Dr. Greg Huey
School of Earth and Atmospheric Sciences
Georgia Institute of Technology

2. Overview Evidence of intraseasonal oscillation
Rainfall, winds, sea level, SST
Models used to study Indian Ocean response
Reveal response on intraseasonal time scales
Modeling Experiments: ROMS
Isolate the response to atmospheric forcing on intraseasonal time scales
Results show dynamic response in currents and sea level
Spectra of rainfall anomalies

3. Boreal summer atmospheric intraseasonal oscillation
Chatterjee and Goswami, 2004
Time scales between 10 and 90 days
* 10 – 20 day oscillation
* 30 – 60 day oscillation
BoB rainfall
Oscillations are related to monsoon active/break conditions
Associated with significant fluctuations of surface winds, cloudiness and precipitation
C.IO zonal winds
Spectra of rainfall anomalies
30 – 60 day mode is characterized by a northward propagation
Lawrence and Webster, 2002

4. Intraseasonal oscillation evidence in the Indian Ocean
TOPEX/Poseidon power spectrum SLA along equator ( )
Daily sea level data at Gan Island
(0ºS – 73ºE)
Han, 2005
Sea level observation show a primary spectral peak at 90 days and secondary peaks at 30 – 60 days
Earlier studies show evidence of intraseasonal oscillations in time series of ocean currents [Knox, 1976; Mc Phaden, 1982, Reppin et al 1999]

5. Wave patterns in sea level Air-sea interaction
Atmospheric intraseasonal oscillation force intraseasonal oscillations in SST
Observations show intraseasonal fluctuations of SST and net heat flux
Intraseasonal SST oscillations propagate northward coherent with OLR, surface wind speed and net heat flux
In the eastern basin intraseasonal SST anomalies are mainly forced by Q fluctuations
terminar
Role played by SST on determining the amplitude and phase propagation ?
Sengupta et al 2001

6. Indian Ocean response
Indian Ocean broad scale response to intraseasonal atmospheric forcing has been done mainly in models [Qui et al, 1999; Han et al 1999; Kindle and Thompson, 1989]
Coupled experiments improve simulation of amplitude and northward propagation of the intraseasonal oscillation in GCMs
The ocean plays an important role in defining the characteristics of the intraseasonal oscillation
Add influence colera chlorophyll
A better understanding of how the Indian Ocean responds to the atmospheric intraseasonal oscillation is necessary.

7. Model Description ROMS: Regional Ocean Model System
Primitive equations with potential temperature, salinity
Hydrostatic approximation
Orthogonal curvilinear coordinates
S terrain following coordinates
Domain: 30ºN to 30ºS, 28ºE to 120ºE
Horizontal grid: 1/5º
Vertical: 20 layers

8. Data Simple Ocean Data Assimilation (SODA) reanalysis
Zonal and meridional velocity, sea surface height, temperature, salinity.
Initial conditions, climatology, boundary conditions
NCEP/NCAR reanalysis
Surface forcing: wind stress
Composite of winds stress associated with the atmospheric intraseasonal oscillation
TOPEX/POSEIDON
10 day sea level anomalies
Aclarar claculo de fluxes

9. Experiments Experiment 1 (CR)
Climatological forcing run: model is forced with climatological daily wind stress
Forcing at the equator
Climatology
Experiment 2 (IR)
Intraseasonal oscillation run: model is forced with climatological daily wind stress plus the composite of wind stress anomaly associated with the IO.
Climatology + ISO

10. Composite
Hablar con carlos
Agregar composite carlos..preguntar

11. ROMS simulation vs SODA
Vertical salinity structure in the Arabian Sea
Vertical salinity structure in the Bay of Bengal

12. Ocean response: currents
Power spectra of surface band-passed zonal current from IR at 80ºE and 90ºE
Spectra of zonal surface model currents along the equator

13. Ocean response: currents
Primary peak occurs at 90 days
The magnitude is maximum at the equator
80 ºE
90 ºE

14. Ocean response: Sea Level Anomalies
From observations: TOPEX/Poseidon
Power spectra of SLA along 70E
Power spectra of SLA along 90E

15. Ocean response: Sea Level Anomalies
Power spectra of SLA along 90E
Power spectra of SLA along the Equator

16. Ocean response: Sea Level Anomalies
Power spectra of SLA along 90E
Power spectra of SLA along the Equator

17. Ocean response: Sea Level Anomalies
Time longitude plot along the equator
Time longitude plot along the 5N
Velocidad de la kelvin
Signature of oceanic Kelvin and Rossby waves associated with intraseasonal forcing
Ekman Pumping forces additional intraseasonal variability North of the Equator by exciting open ocean Rossby Waves

18. Ocean response: Sea Level Anomaliesmo

19. Wave patterns in sea level
Upwelling Kelvin Wave
Downwelling Kelvin Wave

20. Wave patterns in sea level
Dissipation of Coastally trapped waves
Upwelling and downwelling Kelvin waves along the equator.
Coastal Kelvin waves propagate along the coast in the eastern boundary
Equatorial Kelvin waves reflect at the eastern boundary and become Rossby waves.

21. Eddy variability in the Bay of Bengal
It is possible that part of the wave energy along the coast in the Bay of Bengal is transferred into the mesoscale eddy field
Seasonal cycle of eddy variance is very non-unifrom in space in the bay of bengal
Understanding what drives the eddy variability and the role of the mesoscale eddy in the intraseasonal response needs further exploration

22. Summary
Modeled zonal surface currents from the IR in the central and eastern basin exhibit a strong peak at 90 days and secondary peak between days
Period offset between ocean response and forcing can be attributed to the excitation of Kelvin and Rossby waves
Patterns of equatorial Kelvin waves, Rossby waves and coastal Kelvin waves are observed in sea level anomaly fields.
The 90-day peak in zonal currents suggest air-sea interaction on similar time scales. It is necessary a better understanding of how the SST feedbacks to the atmosphere

23. Summary
The role of the mesoscale eddy in the intraseasonal response is yet to be fully explored
Studying the intraseasonal oscillation influence on ocean biology, can lead to the prediction of ocean productivity in short time scales.