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Influence of internal tidal mixing on the water
mass transformation in the Indonesian
Throughflow Ariane Koch-Larrouy (1), Gurvan Madec
(1), Pascale Bouruet aubertot (1), Theo Gerkema
(2), Laurent Bessière (3), Agus Atmadipoera (1),
Robert Molcard (1) (1) Laboratoire
dOcéanographie Expérimentation et Analyse
Numérique (LOCEAN), (2) NIOZ, Laboratoire des
Ecoulement Géophysique et de lOcéanographie
Spatiale (LEGOS)
Contact Ariane.Koch-Larrouy_at_locean-ipsl.upmc.fr
4 - Routes
1 - Introduction
The total transport in the model is 16.6 Sv (1 Sv
106 m3/s) in good agreement with the Island Rule
calculation (Godfrey1999). The paths within the
Indonesian archipelago are well represented by
the model, even if the total transport is
overestimated. Indeed, the major route is going
through Makassar with a North Pacific origin.
This flow is divided into the 3 exits in a good
proportion compared with observations. Part of
the flow recirculates in west of Banda Sea before
exits through Timor passage. On the eastern
route, a deep flow enters the Indonesian seas
from the south pacific via Maluku and Lifamatola
Strait. Furthermore, in the density range of the
salinity maximum of the SPSW in the model (23.5 -
26.8 sigma0, that correspond to 100-450 m) the
net flow is northward (with a NP origin). This
brief validation gives us confidence in studying
the water mass transformation in the model.
6 - Influence of tidal mixing in the thermocline
Due to its strategic geographic position, the
Indonesian ThroughFlow (ITF), the only
low-latitude passage between major ocean basins,
has always been suspected to play an important
role in the ocean circulation and regulation. The
main route consist of a part of the Mindanao
current advecting North Pacific Water. The second
route divides South pacific waters consecutivly
through the Halmahera, Seram and Banda
sea (Gordon and Fine, 1996). Its upper part
represents only 10 with no penetration of the
saltier SPSW into the upper thermocline of Banda
(150 m) but the signature of this water mass is
visible in the lower thermocline (Gordon 1996,
2005). The Pacific salinity maximum of the
thermocline water disappears as it reaches Banda
Sea to form a unique characterized water mass
with homogeneous salinity bellow 20C (hautala
reid 1996, ) (34.58 psu, box 8, fig 1). It is
commonly accepted that Banda plays a key role in
the mixing. (Gordon 2005) In fact the
transformation are so intense that diffusion
advection model calculated an averaged vertical
diffusivity of 1 cm²/s to reproduce the observed
water mass modification from Pacific Ocean to the
central Banda Sea (Ffield and Gordon 1992 and
Hautala and Reid 1996). But this value is a
measure of vertical mixing integrated along the
flow path, therefore these models do not answer
the question whether the mixing occurs at sills
and boundaries, or in basin interiors. Many
studies suggest that internal tides are
responsible for this transformation (Schiller
(2004) and Simmons et al., 2004, hatayama 2004,
Robertson 2005). Indeed, the energy transferred
from barotropics to baroclinics tides that
generate internal tidal is highly concentrated in
this region (fig1) (15 the total global
transfer). The Indonesian archipelago forms a
unique place in the world that gather a strong
internal tides generation and no possibility to
radiate them away (semi closed basins). The issue
of where and how the transformations of the water
mass happened is still unanswered. In this study,
we aim at investigating water mass
transformation. To that end we use an OGCM with a
specific parameterization to mimic the internal
tides effect in this particular region.
Fig2 barotropic stream function of TIDES exp,
quasi identical to the one of NOTIDES exp.
Contours are in Sverdrup
Fig 4 Salinity in the upper and lower
thermocline for the TIDES and NOTIDES run
5 - Influence of tidal mixing
Fig 3 Central 2D maps of energy transfer
Logarithme from barotropic to baroclinic tides
from Le Provost Lyard 1994. Only red patterns
can generates a significant vertical diffusivity.
In the average boxes from 1 to 9 T,S diagram are
calculated for the observations (World Ocean Data
Base 2001 and Levitus 98, in black) and for the
model experiments (NOTIDES in blue, TIDES in
red). In these boxes are also calculated the
averaged vertical diffusity given in cm2/s.
Vertical scale is the depth from 0 to 2000 m.

• for TIDES exp no Penetration of SPSW in the
  upper thermocline and weak signature in the lower
  thermocline in Banda sea as observed (Gordon
  2005).
• for NOTIDES exp In the upper thermocline salty
  water from SPSW invade the entire Banda Sea. In
  the lower the salinity that penetrate is too
  salty.
• how can the internal tides prevent a salt
  invasion in the upper thermocline ?
• -gt the salinity in Seram sea is so reduced that
  the southward flow in the winter time (see fig4)
  export freshened water from SPSW.

2 - Model
We use the global NEMO/OPA ocean model Madec et
al.1998 with 0.25 horizontal resolution
(Barnier et al. 2006). The domain extends from 95
E to 145 E over 25 S to 25 N, with open
boundaries conditions from a global
climatological simulation. The model is forced by
a daily climatology derived from weekly ERS
10-year (1992-2001) wind stress. Surface heat
fluxes and evaporation are computed with
climatologies from NCEP/NCAR and observations
using bulk formulas. Surface flux used an
additional relaxation to the surface salinity of
Levitus et al.1998.
For the upper and lower thermocline the
penetration of salty water from SPSW is controled
by the mixing induced by internal tides in
Halmahera and Seram Sea
The observed SPSW salinity maximum is strongly
eroded from its entrance in Halmahera sea (box 5)
to locally vanish around Seram Sea (box 6-7).
Therefore contrary to the common acceptance that
the mixing occurs in Banda Sea these
observational results clearly show that this
mixing already happened before entering Banda. In
the NOTIDES experiment, the salinity maximum is
gradually eroded but its signature is still
visible in the Banda sea in contrast to what
observed. In the TIDES run, the vertical mixing
induced by internal tides improved the T,S
properties in all the different sub basins. In
particular, for the eastern route, the salinity
maximum is attenuated in all the sub basins as
the observations, suggesting that the
transformation happened in the right place all
along the route. In particular, the disappearance
of the SPSW signature at the exit of Seram Sea is
well reproduced by the model. The comparison
between the two experiments suggest that vertical
mixing induced by internal tides in Halmahera and
Seram Seas is responsible for the transformation
of the SPSW before entering Banda Sea. Indeed,
the parameterization of internal tides can
generates vertical diffusivity as high as 4 cm2/s
in the thermocline in average for the Seram and
Halmahera seas (boxes 5-6), with local maximum of
20 cm2/s. Furthermore the average vertical
diffusivity in the Banda sea is about 0.1cm2/s
with local maximum that does not exceed 0.5
cm2/s.
3 - Parameterization of vertical diffusivity
Because the Indonesian archipelago is the unique
place in the world that gather strong internal
tides generation and no possibility to radiate
them away (semi closed basins), a specific
parameterization is necessary to reproduce the
physic of the vertical mixing. On the horizontal
the distribution of energy is given by the tidal
model of Le Provost et al. (1994). Whereas the
vertical distribution is inferred from a 2D
linear model (Gerkema et al. 2004). The Jayne
St Laurent parameterization is used. It allows
the system to evolve with the stratification.
8 a) - Perspectives quantify route properties
Fig 6 stream function of lagrangian calculation
(ARIANE, B. Blanke) for western route (left
pannel) and eastern route (right panel)
Jayne, St Laurent 2001
Tab 1 transport, resident time, etc..calulated
with lagrangian method, for the surface water
(NPsurf), subtropical water (NPSW), central water
(NPCW) and intermediate water (NPIW, SPIW). In
blue values on pacific sections (north or south
depending of the water mass) in red values in the
Indian Ocean section
Where on the horizontal ?
Where on the vertical ?
E(x,y)
F(z)

• The parameterization improved the model
  reproducing the highly localized transformations
  all along the routes
• Vertical mixing du to internal tides of the SPSW
  occurs before entering Banda Sea
• Vertical diffusivity in Banda is not important
  compared to Flores, Seram and Timor

Maximum of energy in Molucca Halmahera Sea
Maximum of energy in the thermocline
NP surf NPSW NPCW NPIW SP surf SPIW
Transport (Sv) 2.4 3.3 1.6 3 0.2 2.6
Resident time (year) 0.5 0.5 0.7 1.2 0.75 10.5
Mean depth (m) 35 - 54 125 - 74 240 - 140 390 - 230 24 - 50 1000 - 1000
Mean temperature C 28.6 - 24.8 23.9 - 23.3 15.2 - 19 10.1 - 15 28.8 - 25.5 5.1 - 6.7
Mean salinity (psu) 34.29 - 34.31 34.8 - 34.3 34.68 - 34.5 34.51 - 34.57 34.4 - 34.3 34.57 - 34.61
Mean density 21.6 - 22.8 23.5 - 23.3 25.7 - 24.5 26.5 - 25.5 21.7 - 22.6 27.29 - 27.1
Le Provost Lyard 1994
Tidal model
T. Gerkema 2004
Internal tidal model 2D
Fig1 a Energy transfer logarithme from
barotropic to baroclinic tides
Fig1 b Energy of internal tides generated in
Timor Passage(left panel), energy profil in Searm
and Flores Sea (right panel)
7 - Banda Sea seasonal horizontal blender

• During Northwest Monsoon
• - Salt penetration in Banda due to a southward
  flux from november to february (not shown,
  represented by black arrows in left pannel fig3)
• - Water from Makassar stay south of 6S and
  goes through Ombai Sait and Timor Passage without
  recirculating in Banda Sea
• During Southeast Monsoon
• - water from makassar goes to Malukku sea and
  recirculate in the north western part of Banda
  Sea before exiting

Two 10 years experiments TIDES with
parameterizatio, NOTIDES without
Ref
Alford, M. H., M. C. Gregg, and M. Ilyas, 1999,
Diapycnal mixing in the Banda Sea Results of
the first microstructure measurements in the
Indonesian Throughflow, Geophys. Res. Lett.,
26(17), 27412744. Blanke, B., and S. Raynaud,
1997 Kinematics of the Pacific Equatorial
Undercurrent a Eulerian and Lagrangian approach
from GCM results. J. Phys. Oceanogr., 27,
1038-1053. Bessières L, Madec G, Lyard F, Le
Provost C (2006) Improved tidally driven mixing
in a numerical model of the ocean general
circulation. Ocean Modell, submitted for
publication. Egbert GB, Ray RD (2001) Estimates
of M2 tidal energy dissipation from
TOPEX/POSEIDON altimeter data. J Geophys Res
106 22475-22502 Ffield, A., and R. Robertson
(2005). Indonesian Seas finestructure
variability, Oceanography, vol 18, December,
108-111. Ffield, A. and A. L. Gordon, 1992
Vertical mixing in the Indonesian thermocline. J.
of Phys. Oceanogr., 22 (2), 184-195. Ffield, A.
and A. L. Gordon,1996, Tidal mixing signatures in
the Indonesian Seas, J. Phys. Oceanogr., 26,
1924-1937 Gordon, A.L.,(2005), Oceanography of
the Indonesian Seas and Their Throughflow.
Oceanography 18(4) December 14-27 Gerkema T.,
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Studies in Oceanography, Volume 51, Issues
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Pierre-Simon Laplace, N11, 91 pp.
(http//www.lodyc.jussieu.fr/opa/) Schiller A.,
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OGCM on the water-mass structure and
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region , Ocean Modelling, Volume 6, Issue 1,
Pages 31-49 Simmons H. L., Jayne S. R., St.
Laurent L.C. and Weaver A. J., 2004, Tidally
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Issues 3-4, Pages 245-263
Salinity maximum of the NPSW is erased and
replace by fresher water
Mean resident time is 6 months except for deep
eastern route of the SPIW at 1000 m that stays in
the ITF 10 years at minimum
NPCW and NPIW become warmer and lighter
Next ? describe the evolution of properties along
the route.
Banda Sea may have a seasonal role of an
horizontal blender that mix water from North
Pacific to South Pacific
8 b) - Perspectives Comparison with INSTANT data
Fig 5 Salinity at 325 m for December (left
panel) and August (right panel). Black arrow
represent the seasonal circulation