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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
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