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Water mass transformations in the Indonesian Throughflow

Water mass transformations in the Indonesian Throughflow. Parameterization in an OGCM of the mixing in the ITF : Effect on Water masses. Ariane Koch-Larrouy, Gurvan Madec, Robert Molcard. Introduction. Only low latitude passage between two oceans => key region for circulation and climate

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Water mass transformations in the Indonesian Throughflow

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  1. Water mass transformations in the Indonesian Throughflow Parameterization in an OGCM of the mixing in the ITF : Effect on Water masses Ariane Koch-Larrouy, Gurvan Madec, Robert Molcard

  2. Introduction • Only low latitude passage between two oceans => key region for circulation and climate • fresh and cool water flow from Pacific Ocean to Indian Ocean In situ Observations (Gordon 2005)

  3. Cruise • Only low latitude passage between two oceans => key region for circulation and climate • Cruises to understand variability and characteristics of this flow : JADE, ARLINDO, INSTANT INSTANT (2004-2007), R. Molcard, A. Atmadipoera at the LOCEAN (+ 300 Indonesian CTD recovered)

  4. halmahera makassar ceram banda WODB 2001 data Water masses transformation Advection diffusion model -> Kz ~ 1-2 cm2/s Mixing is necessary but where ? Ffield & Gordon 92

  5. Objectives Tools • OPA-NEMO (OGCM) 1/4th degree open boundaries tke (wind & shear param) • 2d internal tides generation model • Results from tidal model • WHERE ? • WHY ? because of what phenomena is the ITF transformed

  6. Results

  7. What do we know ? Where ? Why ? Microstructure measurement Solve the explicit tides ? Schiller 2004 & Robertson 2006 Show that internal tides could and must be responsible for the mixing Did not look precisely at the effect on water masses Hard to link the breaking to the mixing 2D internal tides model 0.1 cm2/s 60 cm2/s Our strategy : Parameterization of the effects of the tides Hatayama 2004 Alford et al. Mixing may occur preferentially above rough topography

  8. Internal tides 2 sinks of the tidal energy Internal tide generated Internal wave drag Energy transfered to barotropic tides to baroclinic tides bottom friction

  9. Internal Tides Internal wave drag from tidal model Internal tide generated Internal wave drag bottom friction Le provost & Lyard 2002 This energy transfer is 20 times more concentrated in the ITF than over the global ocean Lyard & Le Provost

  10. Internal Tides ITF = unic region in the world - 20 times more concentrated than for global ocean - semi enclosed sea => all the energy is avalaible for dissipation Internal tide generated Internal wave drag 1.1 TW bottom friction 0.11 TW

  11. parameterization St Laurent 2002 How much is dissipated ? q Where on the horizontal ? ITF specificities E(x,y) Where on the vertical ? F(z)

  12. parameterization St Laurent 2002 q tidal dissipation efficiency Complex topography, series of semi enclosed sea. Once generated internal tides remain confined How much is dissipated ? q Where on the horizontal ? E(x,y) Where on the vertical ? F(z) All the energy available for mixing q = 1

  13. parameterization St Laurent 2002 E(x,y) drag coefficient from tidal model How much is dissipated ? q = 1 Where on the horizontal ? Highly heterogeneous Maximum of energy in Maluku and Halmahera Seas E(x,y) Where on the vertical ? F(z) 2 tests - E(x,y) averaged - E(x,y) apply locally

  14. parameterization St Laurent 2002 F(z) vertical structure of the energy to be dissipated How much is dissipated ? F(z)~N q = 1 F(z)~N2 Where on the horizontal ? E(x,y) Where on the vertical ? F(z) T. Gerkema & P. Bouruet Abertot Internal tidal model 2D Maximum of energy in the thermocline

  15. Results

  16. Results

  17. Conclusion parameterization • ITF strong internal tides, trapped in the different semi-enclosed seas • build a parameterization of 3d varying kz • average kz = 1.5 cm2/s, independently agrees with the estimates inferred from observations, suggesting that tides are a major phenomenon for the water massestransformation. • the parameterization improves the water masses characteristics in the different Indonesian seas, suggesting that the horizontal and vertical distributions of the mixing are adequately prescribed. • Role of Dewakang sill and Halmahera and Seram seas in mixing

  18. Conclusion parameterization • development of a new parameterization taking into account internal tides in an OGCM specific to Indonesian region that reproduce well the water masses and their transformations. • mixing due to internal tides is a major phenomenon explaining the strong transformation of water masses in the ITF • Inter-annual Variability G70 DRAKKAR, comparison with data (INSTANT, …) • Impact of the mixing on a coupled model. Does it modify the atmospheric convection ?

  19. Questions ?

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