Physical oceanography of the present day Indonesian Throughflow

The Indonesian Throughflow (ITF) transfers c. 15 Sv (1 Sv 1⁄4 10 ms) of relatively cool, fresh water from the tropical Pacific Ocean to the tropical Indian Ocean. Additionally, the ITF is a key interocean component of the global ocean warm water route, which returns water from the Pacific Ocean to the Atlantic Ocean to close the loop of the thermohaline overturning circulation associated with North Atlantic Deep Water. That flow consequently freshens the Indian Ocean and transports heat between basins. The ITF can also be described by the island rule, which relates the winds over the entire South Pacific Ocean to the magnitude of the ITF. El Nino-Southern Oscillation (ENSO) dominates the regional variability in the Pacific Ocean and exerts a strong control over the variability of ITF transport. The Indian Ocean responds to the ENSO signal as well, but is also influenced by the Indian Ocean Dipole, a climate phenomenon that may act independently of ENSO to affect the ITF. On a local scale, the surface layer of the ITF is controlled by local winds, which are primarily monsoonal, but at depth the ITF responds to the pressure gradient between the Pacific Ocean and the Indian Ocean. Newly available observational data from within the major straits of the Indonesian Seas allow for an improved resolution of the total ITF and its variability. Makassar Strait, which is the main inflow channel of the ITF, transported an average of 11.8 Sv of North Pacific water. Lifamatola passage, a route for deep South Pacific water, transported 2.7 Sv below 1250 m and returned 0.9–1.3 Sv to the north. The Karimata Strait has not been monitored observationally, but models estimate a contribution of 1.4 Sv, which is likely important for its role in the transport of freshwater. Transport through the outflow passages was 2.6 Sv through Lombok Strait, 4.9 Sv through Ombai Strait, and 7.5 Sv through Timor Passage. Temporal variability within the straits is seen on timescales from the interannual ENSO signal to semi-diurnal tidal signals. The Indonesian Throughflow (ITF) transports c. 15 Sv (1 Sv 1⁄4 10 m s) from the Pacific to the Indian Ocean. The structure and magnitude of the ITF varies on temporal scales from days to decades and even over geological time. As the water is transported, its hydrological characteristics are altered by heat and freshwater inputs from the Indonesian Seas and by strong vertical mixing. The physical oceanography of the present day ITF therefore cannot be characterized by a mean transport alone and requires an understanding of the surrounding current systems and climatic conditions. The ITF can be understood as both a local phenomenon, affecting the Indonesian Seas and the edges of the western Pacific and eastern Indian basins, and as a global phenomenon, providing a path for water to circumnavigate the world ocean. The objective of this paper is to understand the significance of the ITF and its variability in both a global and a local sense. The first section places the ITF in the context of global ocean circulation, showing its importance to thermohaline circulation, estimates of volume, heat, and salt transport, and the framework of large-scale wind-driven circulation. The next section narrows in focus from a global to a regional scale, providing the physical oceanography and prevailing climate phenomena that interact with the ITF. The ITF is considered on a local scale in the following two sections. The third section develops the theoretical frameworks for understanding the existence of the ITF, while the fourth provides observational data for understanding the mean flow and temporal variability within individual straits. The study concludes with a discussion of the implications of the ITF and directions for future work. Global ocean circulation Thermohaline circulation The circulation of the world ocean is often compared to a conveyor belt. In the North Atlantic, cold, salty surface water sinks and spreads southward through the Atlantic Ocean as North Atlantic Deep Water (NADW). That NADW is then carried by the Antarctic Circumpolar Current and deep western boundary currents to the Pacific and Indian Oceans. In order for this transport to occur, there must be a return flow of comparatively warm water balancing the mass transport. Gordon (1986) proposed that this exchange must happen in the From: Hall, R., Cottam, M. A. & Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 267–281. DOI: 10.1144/SP355.13 0305-8719/11/$15.00 # The Geological Society of London 2011. oceans’ thermocline, the layer of seawater between the surface mixed layer and the cooler deepwater in which the water temperature changes rapidly with depth. This occurs via the ITF and the subsequent export of those waters from the Indian Ocean into the South Atlantic through the Agulhas Leakage. This concept, now well accepted in principle, requires that the ITF balance the transport of mass, heat, and salt from the Pacific to the Indian Ocean. The metaphor of the ‘conveyor belt’ originated with Broecker & Peng (1982) and referred to a conceptual model of global circulation (for a discussion of the history of the ‘conveyor belt,’ see Richardson 2008). The concept was popularized through a magazine article (Broecker 1987) that showed a simplified version of the circulation described in Gordon 1986 (Fig. 1). This version has been reproduced many times, although it fails to show the true path of the ITF through the Indonesian Straits and neglects the formation of Antarctic Bottom Water A more sophisticated conveyor belt, developed by Gordon (1991) and updated by Schmitz (1996) and Lumpkin & Speer (2007) attempts to show a more three-dimensional view of global ocean circulation (Fig. 2). The structure of these later diagrams emphasize the fact the ITF is the only low latitude connection between ocean basins, and the largest interocean flow outside of the Southern Ocean. As the conceptual framework of a conveyor belt expands to include more details about ocean circulation, it builds in complexity and becomes more difficult to show schematically. However, a more crucial concern is that it gives the impression of an ocean governed by a strong mean flow, with little, if any, variability. Such a deterministic view of ocean circulation is contradicted by observational evidence, including evidence that the ITF is highly variable in its transport not only of volume, but of heat and salt as well. Transport of volume, heat and salt Whether ocean circulation is best described by the mean flow or by temporally variant eddies and currents, transport of mass, heat and salt must balance over long timescales. The World Ocean Circulation Experiment (WOCE) project attempted to quantify these fluxes using observational data. These data, along with satellite and other data, are then incorporated into general circulation models which can provide an estimate for both the mean state and the variability of these fluxes (see Wunsch et al. 2009 for a full discussion of these methods). Volume transport is the rate of volume flow across a unit area, calculated as the product of the integrated velocity through the area and the cross section of that area. Given that the mass of seawater is not variable, conversions between volume and mass transport are simply a matter of units and the two terms are often used interchangeably. The Fig. 1. The ocean conveyor belt as proposed by Broecker (1987). D. TILLINGER 268