Measuring internal tides in the Indonesian archipelago
The Indonesian seas provide a low-latitude pathway for the transfer of warm, low salinity Pacific waters into the Indian Ocean. It is essential that this so-called Indonesian Throughflow (ITF) be properly measured, as it will lead to a much improved understanding of the large-scale climate system (Song et al. 2007, Koch-Larrouy et al. 2009), specifically the climate affecting the Maritime Continents of the Indonesian region. In the recent years an international co-operative effort known as the International Nusantara Stratification and Transport (INSTANT) involving Indonesia, United States, Australia, France and the Netherlands took place. INSTANT consisted of approximately a three year deployment of current meter and T/S sensors moorings and shallow pressure gauges in both the inflow and outflow passages. This has led to a considerable improvement of our knowledge of the complex pathways of the Pacific waters in the ITF leading to the Indian Ocean (Admatipoera et al, 2009, Sprintall et al. 2009).
As these waters flow through the ITF their properties are strongly modified by turbulent motions especially in the thermocline (Ffield et al 1992, Hautala et Reid 1996, Koch-Larrouy et al, 2007, 2008). This intense turbulent mixing makes the ITF a region of particular importance. Indeed mixing has two major effects: first it enables a significant heat transfer into the deep ocean which is essential in maintaining the stratification; secondly it leads to an input of nutrients from the deep layer thus favouring primary production. The key process which drives this turbulence is the breaking of internal waves. In this area the main energy source for these waves are barotropic tides which generate internal tides when they interact with the topography. In fact the ITF is one of the most energetic sites of internal tide generation in the world and it is composed by several semi enclosed basin which prevent the internal tides to radiate away and force them to dissipate locally (Koch-Larrouy et al. 2007). This explains why turbulent mixing is especially intense there.
Physical processes involved in the generation, propagation and eventual dissipation through wavebreaking are subject of numerous studies (e.g. Egbert et al (2001), Ffield & Gordon (1996), Carter & Gregg (2006)). The main question is to quantify the energy fluxes coming into play during the different stages of evolution of internal tides, from their generation site to dissipation. Detailed studies that aim to quantify the amount of energy that is used for turbulent mixing as well as to specify where this mixing occurs are thus required. Indeed details of small-scale processes are also a key for the development of small-scale parameterization that will mimic these transformations in numerical models. Previous work by Koch-Larrouy et al (2007) proposed a physically based parameterization of internal tidal mixing which improved significantly the water masses, especially in Halmahera sea, where high level of energy (mean kz=16cm2/s) allow to reproduce the salinity erosion of pacific water as shown in the observations. However this parameterization was based upon the use of a linear model of internal tide generation along a few vertical sections and a more accurate description based on in-situ measurement using microstructure sensors is required.
However, to do so the location of internal tides has to be known. Indeed, one measurement was dedicated to internal tides in the Indonesian archipelago (Alford et al. 1999), but its location (Banda Sea) was not in an active region of internal tides. Numerical study (see Lyard et le Provost 2002, Hatayama et al. 2004, Koch-Larrouy et al. 2007, Robertson et al. 2008) and TS in-situ data (Atmadipoera et al. 2010, in prep) have refined evidence of the very complex horizontal location of these internal tides in the archipelago. The Halmahera, the Ombai strait and Dewakang strait are one of the key region for very strong internal tides generation and dissipation.
Schedule and objectives of the cruise:
The INDOMIX cruise will focus on one of the most energetic section for internal tides through Halmahera sea and Ombai strait. Classical fine-scale CTD/LADCP measurements will be performed together with microstructure measurements. Repeated profiles over 24 hours will be performed at five locations, three in Halmahera sea, one in Banda sea and one in Ombai strait (Fig.2). The aim is two-fold: first to characterize internal tides with CTD/LADCP profiles and secondly to quantify small-scale turbulence and mixing, its variability and its relationship with the larger scale internal tide signal. Also measurements of fluorescence and oxygen will enable to estimate biological activity and its relationship with the internal wave field and turbulence intensity. Eventually individual profiles will be collected along the section as indicated in Figure 2. These individual profiles will provide a snap shot of water masses and turbulence.