Characterization and causes of variability of sea level and thermocline depth in the tropical South

Indian Ocean

Laurie TrenaryUniversity of Colorado

Seasonal cycle: winds and thermocline depth

Schott et al. 2009

Open Ocean Upwelling and Weather/Climate

Tropical Cyclone1851-2006

NASA-Earth Observatory

Saji et al. 2006

SSTXie et al. 2002

Tropical Cyclones Formation

Strong intraseaonal SST variabilityInteractions with the MJO?

Interannually: Intrinsic part of the IOD with significant climatic consequences

What are the possible mechanisms?

Rossby waves?

Local Ekman Pumping?

Remote Influence from the Pacific

Wijffels and Meyers (2004)

Interannual:5-10% Energy flux from the Pacific(Clarke 1991; Spall and Pedlosky 2005)

Annual:80% energy flux from the Pacific-- -only 10% is found off shore (Potemra 2001)

Present Study What drives sea level/thermocline depth

variability on multiple timescales and what is the relative importance of ?

1. Local: Direct Ekman pumping

2. Regionally Remote: Regionally forced large scale Ekman pumping and Rossby wave propagation

3. Remote: Transmission from the Pacific

2. Models and ExperimentsModelsHYbrid Coordinate Ocean Model (HYCOM):

Domain: Indian-Pacific basin 55oS-55oN; 30 °E to 290 °E Resolution: 0.33ox0.33o resolution; 20 vertical layers Forcing: ERA40: 3-day-mean winds, specific humidity, air temperature,

precipitation, net shortwave and longwave radiation (1958-2001)Linear Ocean Model (LOM)

Domain: Indian-Pacific basin 45oS-45oN; 30 °E to 290 °E. Damping is applied in a 5o band extending from the boundaries

Resolution: 0.33ox0.33o resolution; 15 vertical mode Continuously stratified using Levitus temperature and salinity (Levitus and

Boyer 1994; Levitus et al. 1994)

Reanalysis and Observations SODA-POP: D20A Aviso: SSHA INSTANT : ITF Transport and Water Mass Properties

2. Methods: Experiment Design

INDOPAC = Pacific +Indian Ocean forcingIND= Indian Ocean forcing

Experiment Design Model bathometry

3. Results: Model/Data ComparisonStandard Deviation of Seasonal to Interannual SSHA and D20

MODEL OBSERVATIONS

REANALYSIS

SSHA

D20A

3. Results: Model/Data ComparisonSeasonal to Interannual SSHA and D20A

Good agreement with observations!!!

SSHA

D20A-SSHA highly correlated

3. Results: Model/Data ComparisonStandard Deviation of Intraseasonal SSHA

MODEL OBSERVATIONS

3. Results: Model/Data ComparisonObserved and modeled ITF

Decent agreement of ITF transport

Model captures variability of hydrodynamic properties

Interannual Variability

SSHA SSHA: INDIAN

D20A

D20A: INDIAN

3. Interannual: Standard Deviation Maps

INDOPAC IND DIFF

LOM:SSHA

var( ) var( ) var( ) *x y x y r x y

Interpreting our results

2

2

h c h

t f x f

3. InterannualRegion 1 Region 1

INDOPAC (total) Local Forcing : IND (Indian Ocean)INDOPAC-IND (Pacific forcing) Remote Forcing IO :

fτ

ew

1, curl ,

R R

xx t x t dx

C Ch

3. InterannualRegion 2

Region 2

D20A: IO

3. Interannual: Positive CompositeINDOPAC IND DIFF

Seasonal Variability

3. SeasonalRegion 1

Region 2

Region 2

20_

Dw w wtotal local IO Rossby t

wlocal

τf

Region 1

D20A: IO

3. Seasonal Evolution: Hovmöller INDOPAC IND DIFF

D20A: PACIFIC+IO D20A: IO D20A: PACIFIC

Intraseasonal Variability

3. Intraseasonal : Standard Deviation Maps

INDOPAC IND DIFF

Controls of ITF

Conclusions• On seasonal-to-interannual timescales sea level/thermocline depth

variability is driven by winds acting on Indian Ocean

• Interannual– SSHA/D20A is associated with Rossby wave propagation forced by

windstress curl in the eastern IO

– Pacific influence is greatest south of 10oS and transmission strongly modifies ITF

• Seasonal:– SSHA/D20A forcing varies based on location: combination of local Ekman

pumping and Rossby wave propagation

– Indian Ocean determines phase of the ITF, the Pacific damps the transport

• Intraseasonal:– In the ridge region, sea level variability is relatively weak, and it results from

IO wind forcing

– Forcing over the IO is the major cause for intraseasonal variability of the ITF

• Decadal:– Pacific appears to contribute to the subsurface temperature variability of the

SIO

Thank You!

Decadal Variability

3. Long term trend

3. Subsurface temperature variability

INDOPAC IND DIFF

INDOPAC IND DIFF

INDOPAC IND DIFF

INDOPAC IND DIFF

3. Pacific Tropical winds

c = phase speed = frequencyR

Earth = Earth radius

Turning latitude

Earth

c R

c

2tan

3. Interannual: Negative Composite

D20A: HYCOM-MR D20A: HYCOM-EXP D20A: MR-EXP

Composite events: 73-74;74-75;75-76;80-81;81-82;84-85;92-93;98-99

Thermocline Ridge of the Indian Ocean

Vialard et al. 2009

3. Transmission and the ITF