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 fresh and cool water flow from Pacific Ocean to Indian Ocean In situ Observations (Gordon 2005)

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)

Water masses transformation WODB 2001 data makassar banda ceram halmahera the flow is going mostly by the western route with 9 Sv in Makassar and then exits in 3 different passages after recirculates in a banda sea. From the south pacific there is a deep flow that concerns 1 Sv this the dashed arrow. The halmahera eddy retroflecs and partly enters the halmahera sea But we don’t know wether or not there is flux at that depth than can enters the indonesian seas as it has been no measurement of it. But when we look at the temperature salinity diagram we can see that we start with a maximum of salinity that is the signature of the SPSW at about 35.5 psu and in Banda sea and just at the exit of ceram sea the salinity profil homogeneous profil of salinity above 20°C. So the question is to understand why there is a such homogeneous profil of salinity above 20°C. Some adection diffusion models calculated that a kz of 1 cm2/s is necessary between the entrance to the banda sea. But this is an integrated value that not tells us if the mixing occurs at the sills or in bassin interiors. Advection diffusion model -> Kz ~ 1-2 cm2/s Mixing is necessary but where ? Ffield & Gordon 92

Objectives Tools WHERE ? OPA-NEMO (OGCM) 1/4th degree WHY ? 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 The questions I want to answer are where the mixing occurs and where is the more efficient ? And because of what For that I wil use a configuration at 1/4th degree resolution with open boundaries conditions

Results .

What do we know ? Where ? Why ? Solve the explicit tides ? 2D internal tides model 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 0.1 cm2/s 60 cm2/s Hatayama 2004 Alford et al. Our strategy : Parameterization of the effects of the tides Mixing may occur preferentially above rough topography

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

Internal Tides Internal wave drag from tidal model Lyard & Le Provost 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

Internal Tides 1.1 TW 0.11 TW 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

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

parameterization All the energy available for mixing q = 1 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

parameterization How much is dissipated ? q = 1 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

parameterization How much is dissipated ? F(z)~N q = 1 F(z)~N2 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

Results Max eroded as in the observation Improved in all the subassin

Results Max eroded as in the observation Improved in all the subassin

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 masses transformation. 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 Un peu trop de bla bla

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 ?

Questions ?