Water mass transformations in the Indonesian Throughflow

Ariane Koch-Larrouy, Gurvan Madec, Robert Molcard

Parameterization in an OGCM of the mixing in the ITF : Effect on Water masses

> Only low latitude passage between two oceans Only low latitude passage between two oceans => key region for circulation and climate=> key region for circulation and climate

> fresh and cool water flow from Pacific Ocean to Indian Oceanfresh and cool water flow from Pacific Ocean to Indian Ocean

1. Introduction

In situ Observations (Gordon 2005)

1. Cruise> Only low latitude passage between two oceans Only low latitude passage between two oceans => key region for circulation and climate=> key region for circulation and climate

> Cruises to understand variability and characteristics of this Cruises to understand variability and characteristics of this flow : JADE, ARLINDO, INSTANTflow : JADE, ARLINDO, INSTANT

INSTANT (2004-2007), R. Molcard, A. Atmadipoera at the LOCEAN (+ 300 Indonesian CTD recovered)

1. Water masses transformation

Mixing is necessary but where ?

Advection diffusion model -> Kz ~ 1-2 cm2/s Ffield & Gordon 92

WODB 2001 data

makassar

banda

ceram

halmahera

WHERE ? WHERE ?

WHY ? WHY ?

because of what because of what

phenomenaphenomena

is the ITF transformed is the ITF transformed

1. Objectives Tools

OPA-NEMO (OGCM) OPA-NEMO (OGCM)

1/4th degree 1/4th degree

open boundaries open boundaries

tke (wind & shear param)tke (wind & shear param)

2d internal tides generation model2d internal tides generation model

Results from tidal modelResults from tidal model

1. Results

Mixing may occur preferentially above rough topography

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

Our strategy :

Parameterization of the effects of the tides

1. What do we know ?

60 cm2/s 0.1 cm2/s

2D internal tides model

Microstructure measurement

Alford et al.Hatayama 2004

Where ? Why ?

bottom friction

Internal tide generated

1. Internal tides

Internal wave drag

2 sinks of the tidal energy

Energy transfered to barotropic tides to baroclinic tides

bottom friction

Internal tide generated

Internal wave drag from tidal model

Le provost & Lyard 2002

1. Internal Tides

Internal wave drag

This energy transfer is 20 times more concentrated in the ITF than over the global ocean

Lyard & Le Provost

bottom friction

Internal tide generated

1. Internal Tides

Internal wave drag

ITF = unic region in the world

- 20 times more concentrated than for global ocean - semi enclosed sea => all the energy is avalaible for dissipation

0.11 TW

1.1 TW

1. parameterization

St Laurent 2002

E(x,y)

F(z)

Where on the horizontal ?

Where on the vertical ?

q

How much is dissipated ?

ITF specificities

E(x,y)

F(z)

1. parameterization

St Laurent 2002

Where on the horizontal ?

Where on the vertical ?

q

How much is dissipated ?

q tidal dissipation efficiency

All the energy available for mixing q = 1

Complex topography, series of semi enclosed sea.Once generated internal tides remain confined

E(x,y)

F(z)

Highly heterogeneous Maximum of energy in Maluku and Halmahera Seas

1. parameterization

St Laurent 2002

Where on the horizontal ?

Where on the vertical ?

q = 1

How much is dissipated ?E(x,y) drag coefficient from tidal model

2 tests - E(x,y) averaged - E(x,y) apply locally

Maximum of energy in the thermocline

E(x,y)

F(z)

1. parameterization

St Laurent 2002

Where on the horizontal ?

Where on the vertical ?

q = 1

How much is dissipated ?

T. Gerkema &P. Bouruet AbertotInternal tidal model 2D

F(z)~N

F(z)~N2

F(z) vertical structure of the energy to be dissipated

1. Results

1. Results

- 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

1. Conclusion parameterization

4. 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 ?Questions ?