JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS, VOL. 118, 6123–6144, doi:10.1002/2013JC009196, 2013 A model for quantifying oceanic transport and mesoscale variability in the Coral Triangle of the Indonesian/Philippines Archipelago Frederic S. Castruccio,1 Enrique N. Curchitser,2,3 and Joan A. Kleypas1 Received 12 June 2013; revised 11 September 2013; accepted 7 October 2013; published 19 November 2013. [1] The Indonesian Throughflow region (ITF) continues to pose significant research challenges with respect to its role in the global ocean circulation, the climate system, and the ecosystem sustainability in this region of maximum marine biodiversity. Complex geography and circulation features imply difficulties in both observational and numerical studies. In this work, results are presented from a newly developed high-resolution model for the Coral Triangle (CT) of the Indonesian/Philippines Archipelago specifically designed to address regional physical and ecological questions. Here, the model is used to quantify the transport through the various passages, sea surface temperature and mesoscale variability in the CT. Beyond extensive skill assessment exhibiting the model ability to represent many conspicuous features of the ITF, the high-resolution simulation is used to describe the mesoscale and submesoscale circulation through the application of Finite Size Lyapunov Exponents (FSLEs). The distribution of FSLEs is used to quantify the spatiotemporal variability in the regional mixing characteristics. The modeled seasonal and interannual variability of mixing suggests a link to large-scale climate signals such as ENSO and the Asian-Australian monsoon system. Citation: Castruccio, F. S., E. N. Curchitser, and J. A. Kleypas (2013), A model for quantifying oceanic transport and mesoscale variability in the Coral Triangle of the Indonesian/Philippines Archipelago, J. Geophys. Res. Oceans, 118, 6123–6144, doi:10.1002/2013JC009196. 1. Introduction [3] The oceanographic complexity of this region (Figure 1) presents major challenges to both field oceanog- [2] The Coral Triangle (CT) is a marine region that raphers and numerical modelers [Gordon and Kamenko- spans parts of Indonesia, Malaysia, Papua New Guinea, vich, 2010]. Within the CT, the Indonesian Archipelago the Philippines, the Solomon Islands, and Timor-Leste 2 (IA) represents a complex array of passages linking inter- (Figure 1). This region covers nearly 6 million km , which connected shelves, deep basins, shallow and deep sills, and is roughly three-quarters the land area of Australia and submerged ridges, that collectively provide a sea link encompasses portions of two biogeographic regions: the between two oceans [Gordon et al., 2003]. Known as the Indonesian-Philippines Region and the Far Southwestern Indonesian Throughflow (ITF), it is recognized as a key Pacific Region. Often referred to as the maritime continent, component of the global thermohaline circulation [Gordon this region is located at the confluence of tropical waters and Fine, 1996; Hirst and Godfrey, 1993; Wajsowicz and from the North and South Pacific and within the pathways Schneider, 2001]. It serves as the main return flow of upper of the inter-ocean exchange between the Pacific and Indian ocean warm waters from the tropical Pacific Ocean to the oceans. The maritime continent is recognized both as a key tropical Indian Ocean that balances the spreading of deep driver of atmospheric circulation due to its enormous abil- waters that form at high latitudes. Since water in the west- ity to transfer heat from the ocean to the atmosphere [Neale ern tropical Pacific is warmer and fresher than in the Indian and Slingo, 2003] and as a key checkpoint for the global Ocean, the ITF transport impacts the temperature and salin- thermohaline circulation [Gordon, 2005]. ity in the Pacific Ocean, Indian Ocean, and Indonesian seas and also affects the air-sea heat exchange patterns strongly influencing the Indo-Pacific climate [Song et al., 2007]. Observation-based estimates of the ITF transport are 15 Sv 6 3 21 1National Center for Atmospheric Research, Climate and Global (1 Sv 5 10 m s ). As the water is transported, its hydro- Dynamics Division, Boulder, Colorado, USA. logical characteristics are altered by heat and freshwater 2IMCS, Rutgers University, New Brunswick, New Jersey, USA. 3 inputs from the Indonesian seas and by strong vertical mix- DES, Rutgers University, New Brunswick, New Jersey, USA. ing. On a local scale, tides and winds, which are primarily Corresponding author: F. S. Castruccio, National Center for Atmos- monsoonal, are the dominant forcings but the large-scale pheric Research, Climate and Global Dynamics Division, P.O. Box 3000, pressure gradient between the Pacific and Indian oceans is Boulder, CO 80307, USA. ([email protected]) the main force driving the flow of Pacific water through the VC 2013. American Geophysical Union. All Rights Reserved. Indonesian Archipelago into the Indian Ocean. As a result, 2169-9275/13/10.1002/2013JC009196 the structure and magnitude of the ITF varies on timescales 6123 CASTRUCCIO ET AL.: MESOSCALE MODELING IN THE CORAL TRIANGLE Figure 1. Schematic of ocean circulation in the Coral Triangle region. The dashed orange line delineates the Coral Triangle following Veron et al. [2009]. Numbered passages are: (1) Makassar Strait, (2) Lifama- tola Strait, (3) Lombok Strait, (4) Ombai Strait, (5) Timor Passage, (6) Luzon Strait, (7) Karimata Strait, (8) Mindoro Strait, (9) Sibutu Strait, and (10) Torres Strait. Abbreviations are: NEC, North Equatorial Cur- rent; NECC, North Equatorial Countercurrent; SEC, South Equatorial Current; SECC, South Equatorial Countercurrent; ME, Mindinao Eddy; HE, Halmahera Eddy; and NGCC, New Guinea Coastal Current. from the interannual El Nino-Southern~ Oscillation (ENSO) oceanographic conditions are likely to vary spatially in signal to the semidiurnal tidal signals. response to climate change. Based on AVHRR Pathfinder [4] The CT region is also strongly influenced by the Sea Surface Temperature (SST) for 1985–2006, Penaflor~ South China Sea Throughflow (SCSTF). A recent study by et al. [2009] found that SST in the CT has increased an Qu et al. [2009] utilizing existing observations and results average of 0.2C per decade but with considerable variabil- from ocean GCMs showed that the SCSTF is a heat and ity across the region. freshwater conveyor, which may have an important influ- [6] The oceanographic complexity and large areal extent ence on the South China Sea (SCS) heat content, the path- of the CT, however, present challenges for understanding way and vertical structure of the ITF, and the heat and the roots of this spatial variability. Oceanographic models freshwater transport from the Pacific into the Indian Ocean. must consider the complex interactions between topogra- The interplay of the monsoon and the SCSTF and the phy, large-scale oceanic currents, surface heat fluxes, tidal resulting effect on the strength of the ITF are key to under- mixing, and wind-forced variations in thermocline depth of standing the regional climate variability and its implica- both the Indian and Pacific oceans (as reviewed by Qu tions on a global scale. et al. [2005]). In addition to the need to resolve the narrow [5] In addition to its importance in the global ocean and passages between the numerous islands of the CT, the climate variability, the CT region is also widely considered major factors that should be addressed to accurately simu- the apex of marine biodiversity for several major taxo- late ocean conditions in the CT are the wind field [Godfrey, nomic groups [Tittensor et al., 2010], and particularly for 1996], the tides [Ffield and Gordon, 1996; Koch-Larrouy zooxanthellate corals [Veron et al., 2009]. Over 120 million et al., 2007], and a proper treatment of boundary conditions people live in the CT and rely on its fisheries and coral that respects the mean flow currents from the Pacific to the reefs for food, income, and protection from storms. Conser- Indian Ocean [Sprintall et al., 2009]. vation in the CT has thus become a top priority of state [7] Several high-resolution modeling studies have been governments and international conservation efforts, with conducted in this region, but most have targeted particular the six Coral Triangle countries establishing the Coral Tri- subregions and/or specific processes. Robertson and Ffield angle Initiative (CTI) [Coral Triangle Secretariat, 2009] in [2008] used a regional high-resolution ocean model to sim- 2007. Conservation efforts recognize that because of the ulate the barotropic and baroclinic tides in the Indonesia CT’s oceanographic complexity, changes in SST and other seas and examine tide-induced mixing processes at the 6124 CASTRUCCIO ET AL.: MESOSCALE MODELING IN THE CORAL TRIANGLE INSTANT mooring locations [Robertson, 2010] and the the ROMS computational kernel. ROMS makes use of very interaction and transfer of energy among tidal constituents accurate and efficient physical and numerical algorithms. In [Robertson, 2011]. Metzger et al. [2010] analyzed the path- particular, it utilizes consistent temporal averaging of the way of ITF by using a global high-resolution model driven barotropic mode to guarantee both exact conservation and by atmospheric forcing. Both of these high-resolution stud- constancy preservation properties for tracers and yields ies described the Indonesian seas using a single forcing, more accurate resolved barotropic processes while prevent- either tidal or atmospheric. Kartadikaria et al. [2011] ing