Characterization and causes of variability of sea level and thermocline depth in the tropical South Indian Ocean Laurie Trenary University of Colorado

Seasonal cycle: winds and thermocline depth Schott et al. 2009

Open Ocean Upwelling and Weather/Climate Tropical Cyclone NASA-Earth Observatory Saji et al SST Xie et al Tropical Cyclones Formation Strong intraseaonal SST variability Interactions with the MJO? Interannually: Intrinsic part of the IOD with significant climatic consequences

What are the possible mechanisms? Rossby waves? Local Ekman Pumping? Time

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 Experiments Models HYbrid Coordinate Ocean Model (HYCOM):  Domain: Indian-Pacific basin 55 o S-55 o N; 30 °E to 290 °E  Resolution: 0.33 o x0.33 o resolution; 20 vertical layers  Forcing: ERA40: 3-day-mean winds, specific humidity, air temperature, precipitation, net shortwave and longwave radiation ( ) Linear Ocean Model (LOM)  Domain: Indian-Pacific basin 45 o S-45 o N; 30 °E to 290 °E. Damping is applied in a 5 o band extending from the boundaries  Resolution: 0.33 o x0.33 o 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 forcing IND= Indian Ocean forcing Experiment DesignModel bathometry

3. Results: Model/Data Comparison Standard Deviation of Seasonal to Interannual SSHA and D20 MODELOBSERVATIONS REANALYSIS SSHA D20A

3. Results: Model/Data Comparison Seasonal to Interannual SSHA and D20A Good agreement with observations!!! SSHA D20A-SSHA highly correlated

3. Results: Model/Data Comparison Standard Deviation of Intraseasonal SSHA MODELOBSERVATIONS

3. Results: Model/Data Comparison Observed 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 INDDIFF LOM:SSHA

Interpreting our results

3. Interannual Region 1 INDOPAC (total) Local Forcing : IND (Indian Ocean) INDOPAC-IND (Pacific forcing) Remote Forcing IO : upwelling downwelling

3. Interannual Region 2 upwelling downwelling

D20A: IO

3. Interannual: Positive Composite INDOPACINDDIFF

Seasonal Variability

3. Seasonal Region 1 Region 2 Region 1

D20A: IO 3. Seasonal Evolution: Hovmöller INDOPACIND DIFF

D20A: PACIFIC+IOD20A: IOD20A: PACIFIC

Intraseasonal Variability

3. Intraseasonal : Standard Deviation Maps INDOPACINDDIFF

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 10 o S 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

INDOPACINDDIFF INDOPACIND DIFF

INDOPACIND DIFF

INDOPACIND DIFF

3. Pacific Tropical winds warm cold

c = phase speed  = frequency R Earth = Earth radius Turning latitude

3. Interannual: Negative Composite D20A: HYCOM-MRD20A: HYCOM-EXPD20A: 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