Dynamics of the indian ocean response to boreal summer atmospheric intraseasonal oscillations
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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

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Dynamics of the Indian Ocean response to boreal summer atmospheric intraseasonal oscillations

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Dynamics of the indian ocean response to boreal summer atmospheric intraseasonal oscillations

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


Overview

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


Boreal summer atmospheric intraseasonal oscillation

Boreal summer atmospheric intraseasonal oscillation

Chatterjee and Goswami, 2004

BoB rainfall

Time scales between 10 and 90 days

* 10 – 20 day oscillation

* 30 – 60 day oscillation

Oscillations are related to monsoon active/break conditions

Associated with significant fluctuations of surface winds, cloudiness and precipitation

C.IO zonal winds

30 – 60 day mode is characterized by a northward propagation

Lawrence and Webster, 2002


Intraseasonal oscillation evidence in the indian ocean

Intraseasonal oscillation evidence in the Indian Ocean

TOPEX/Poseidon power spectrum SLA along equator (1993-2002)

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]


Air sea interaction

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

Role played by SST on determining the amplitude and phase propagation ?

Sengupta et al 2001


Indian ocean response

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

A better understanding of how the Indian Ocean responds to the atmospheric intraseasonal oscillation is necessary.


Model description

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


Dynamics of the indian ocean response to boreal summer atmospheric intraseasonal oscillations

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


Experiments

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


Composite

Composite

  • Hablar con carlos


Roms simulation vs soda

ROMS simulation vs SODA

Vertical salinity structure in the Arabian Sea

Vertical salinity structure in the Bay of Bengal


Ocean response currents

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


Ocean response currents1

70 ºE

Ocean response: currents

Primary peak occurs at 90 days

The magnitude is maximum at the equator

80 ºE

90 ºE


Ocean response sea level anomalies

Ocean response: Sea Level Anomalies

Power spectra of SLA along 70E

From observations: TOPEX/Poseidon

Power spectra of SLA along 90E


Ocean response sea level anomalies1

Ocean response: Sea Level Anomalies

Power spectra of SLA along 90E

Power spectra of SLA along the Equator


Ocean response sea level anomalies2

Ocean response: Sea Level Anomalies

Power spectra of SLA along 90E

Power spectra of SLA along the Equator


Ocean response sea level anomalies3

Time longitude plot along the equator

Time longitude plot along the 5N

Ocean response: Sea Level Anomalies

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


Ocean response sea level anomalies4

Ocean response: Sea Level Anomalies


Wave patterns in sea level

Upwelling Kelvin Wave

Downwelling Kelvin Wave

Wave patterns in sea level


Dynamics of the indian ocean response to boreal summer atmospheric intraseasonal oscillations

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.


Eddy variability in the bay of bengal

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


Summary

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 40-60 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


Summary1

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.


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