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Internal tides in the Indonesian Archipelago

Par ecola — Dernière modification 04/03/2014 10:14

Scientific Context

 

bathy

 


Parameterisation of the internal tidal mixing in the indonesian region

 

The Indonesian archipelago is characterized by strong internal tides, which are trapped in the different semi- enclosed seas of the archipelago. Using tidal model results a parameterization of the associated 3d tidal mixing is developed. The resulting average vertical diffusivity is 1.5 cm2/s, which independently agrees with the estimates inferred from observations. Introduced in a regional OGCM, the parameterization improves the water mass characteristics in the different Indonesian seas, suggesting that the horizontal and vertical distributions of the mixing are adequately prescribed. In particular, the salinity maximum of the inflow water is reduced along the main route, mainly in the Dewakang sill area. But also it is erased in the Halmahera and Seram seas, the entrance of the eastern route, so that salty waters doesn't penetrate the Banda Sea. As a result the simulated Indonesian Throughflow Water are in good agreement with observations.

We have constructed a new parametrisation based on the St Laurent et al. 2002 parameterisation.

 

 

 

Picture_GRL

 

This results are more detailed in the following publication :

Koch-Larrouy, A., G. Madec, P. Bouruet-Aubertot, T. Gerkema, L. Bessieres, R. Molcard (2007) On the transformation of Pacific Water into Indonesian Throughflow Water by internal tidal mixing. Geophys. Res. Lett., 34, L04604, DOI :10.1029/2006GL028405

 

Quantification of the water mass transformation

 

---> I. Eulerian view

We study this ocean model, with the previous parameterization of tidal mixing in order to diagnose the mechanisms and magnitude of the water mass transformations using a thermodynamical methodology.

The thermodynamical methodology is based upon Walin's theory (1982) For any tracer :

 

equation_1

 

The diapycnal transformation is :

 

equation_2

 

And can be decomposed as follow :

 

equation_3

 

transformation_totale_trendslegende_trends

A combination of air/sea forcing and mixing is found to significantly change the character of the Indonesian Throughflow (ITF). Around 6 Sv (approximately 1/3 the model net ITF transport) of the flow leaves the Indonesian Seas with reduced density. Mixing transforms both the intermediate depth waters (transforming 4.3 Sv to lighter density) and the surface waters (made denser despite the buoyancy input by air/sea exchange, net transformation=2 Sv). The intermediate transformation to lighter waters suggests that the Indonesian transformation contributes significantly to the upwelling of cold water in the global conveyor belt. The mixing induced by the wind is not driving the transformation. In contrast, the baroclinic tides have a major role in this transformation. In particular, they are the only source of energy acting on the thermocline and are responsible for creating the homostad thermocline water, a characteristic of the Indonesian outflow water. Furthermore, they cool the sea surface temperature by between 0.6 and 1.5°C, and thus allow the ocean to absorb more heat from the atmosphere. The additional heat imprints its characteristics into the thermocline. The Indonesian Seas cannot only be seen as a region of water mass transformation (in the sense of only transforming water masses in its interior) but also as a region of water mass formation (as it modifies the heat flux and induced more buoyancy flux). This results are more detailed in the following publication :

Koch-Larrouy A., G. Madec, D. Iudicone, R. Molcard, A. Atmadipoera, (2008a) Physical processes contributing in the water mass transformation of the Indonesian ThroughFlow , Ocean Dynamics, DOI 10.1007/s10236-008-0154-5

 

 

---> II. Lagrangian view

 

finale_lagrangian

 

The oceanic pathways connecting the Pacific Ocean to the Indian Ocean are described using a quantitative Lagrangian method applied to Eulerian fields from an ocean general circulation model simulation of the Indonesian seas. The main routes diagnosed are in good agreement with those inferred from observations. The secondary routes and the Pacific recirculation are also quantified.

Trajectories are obtained which link the water masses at the entrance and at the exit of the Indonesian throughflow (ITF), and the mixing along each trajectory is quantified. A recipe to form Indonesian water masses is proposed. We present three major features of the circulation that revisit the classical picture of the ITF and its associated water mass transformation, while still being in agreement with observations.

- Firstly, the homohaline layer is not a result of pure isopycnal mixing of the North Pacific Intermediate Water and South Pacific Subtropical Water (SPSW) within the Banda Sea, as previously thought. Instead, the observed homohaline layer is reproduced by the model, but it is caused by both isopycnal mixing with the SPSW and a dominant vertical mixing before the Banda Sea with the NPSW. This new mechanism could be real since the model reproduces the SPSW penetration as observed.

- Secondly, the model explains why the Banda Sea thermocline water is so fresh compared to the SPSW. Until now, the only explanation was a recirculation of the freshwater from the western route. The model does not reproduce this recirculation but instead shows strong mixing of the SPSW within the Halmahera and Seram Seas, which erodes the salinity maximum so that its signature is not longer perceptible.

- Finally, this work highlights the key role of the Java Sea freshwater. Even though its annual net mass contribution is small, its fresh salinity contribution is highly significant and represents the main reason why the Pacific salinity maxima are eroded.

 

salt_transformation

 

This results are more detailed in the following publication :

Koch-Larrouy A., G. Madec, B. Blanke, R. Molcard, (2008b) Quantification of the water paths and exchanges in the indonesian archipelago Ocean Dynamics, DOI 10.1007/s10236-008-0155-4

 

 

 

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