Research paper The density-driven circulation of the coastal hypersaline system of the Great Barrier Reef, Australia Gerry G. Salamena a, ⁎, Flávio Martins b , Peter V. Ridd c a College of Marine and Environmental Science, James Cook University, Townsville, Queensland 4811, Australia b CIMA-EST/UAlg., Campus da Penha, P8000-117, Faro, Portugal c Marine Geophysics Laboratory, College of Science Technology and Engineering, James Cook University, Townsville, Queensland 4811, Australia abstract article info Article history: Received 16 September 2015 Received in revised form 30 January 2016 Accepted 4 February 2016 Available online 12 February 2016 The coastal hypersaline system of the Great Barrier Reef (GBR) in the dry season, was investigated for the first time using a 3D baroclinic model. In the shallow coastal embayments, salinity increases to c.a. 1‰ above typical offshore salinity (~35.4‰). This salinity increase is due to high evaporation rates and negligible freshwater input. The hypersalinity drifts longshore north-westward due to south-easterly trade winds and may eventually pass capes or headlands, e.g. Cape Cleveland, where the water is considerably deeper (c.a. 15 m). Here, a pronounced thermohaline circulation is predicted to occur which flushes the hypersalinity offshore at velocities of up to 0.08 m/s. Flushing time of the coastal embayments is around 2–3 weeks. During the dry season early summer, the thermohaline circulation reduces and therefore, flushing times are predicted to be slight longer due to the reduced onshore-offshore density gradient compared to that in the dry season winter period. © 2016 Elsevier Ltd. All rights reserved. Keywords: Density-driven circulation Hypersaline waters Great Barrier Reef Flushing time 1. Introduction The existence of hypersaline waters in coastal zones of continental shelves is caused by the excess of evaporation over precipitation and river runoff (de Silva Samarasinghe and Lennon, 1987; Gräwe et al., 2009; Heggie and Skyring, 1999; Lavı́ n et al., 1998; Wolanski, 1986). Hy- persaline systems in continental shelves have been studied in numerous locations particularly around the dry continent of Australia, for example, in northern Australia (Wolanski, 1986), gulfs in the Southern Australia (de Silva Samarasinghe, 1989; de Silva Samarasinghe and Lennon, 1987; Nunes and Lennon, 1986, 1987), Hervey Bay, Australia (Gräwe et al., 2009) and coastal zones of Great Barrier Reef (GBR), Australia (Andutta et al., 2011; Wang et al., 2007; Wolanski, 1981). The extent of the hypersalinity depends upon the freshwater balance of the region which is highly associated with seasonal characteristics e.g. during the dry season summer (e.g. in Gulf of California, Lavı́n et al. (1998)) and the dry winter in the GBR (Walker, 1981b). One important hydrody- namical effect of the hypersaline waters is to produce a thermohaline circulation by which the saltier water masses will sink and be flushed out seaward along the sea-bed (Fig. 1)(Gräwe et al., 2009; Heggie and Skyring, 1999; Lennon et al., 1987b; Wolanski, 1986). As a result, this thermohaline circulation is an important oceanographic aspect in the hypersaline coastal waters of the continental shelf. The degree of hypersalinity on the continental shelf is not only influ- enced by the freshwater balance, but is also affected by the exchange of the coastal hypersaline waters with oceanic salinity from offshore (Wang et al., 2007). This exchange transport process is likely affected by turbulent diffusion, thermohaline circulation and large scale advection (Wang et al., 2007). Due to this exchange transport role, some authors have conducted studies connecting this transport process with the flushing time of the coastal hypersaline waters (de Silva Samarasinghe and Lennon, 1987; Hancock et al., 2006; Heggie and Skyring, 1999; Largier et al., 1997; Wang et al., 2007). The hypersaline system of the GBR shelf is different to most other hypersaline environments due to its aspect ratio. It is a continental shelf system 2000 km in the long-shelf direction and between 50 (further North) and 100 (further South) km across shelf. This contrasts with most other reported hypersaline systems which are bays, gulfs, or inverse estuaries and are smaller in the long-shelf direction than across shelf direction. Due to this morphological difference, along shore currents become important for the GBR (Andutta et al., 2011). In con- trast, for waters in narrow bays, it is the cross shelf tidal currents which predominate e.g. Gulf of St. Vincent (de Silva Samarasinghe and Lennon, 1987), Gulf of California (Lavı́ n et al., 1998), Shark Bay (Nahas et al., 2005) and San Diego Bay (Largier et al., 1997). Thus, along-shore currents of the GBR enables the hypersaline waters in the GBR to be transported alongshore from one coastal embayment to another (Andutta et al., 2011). There has been considerable works on residence or flushing times of the GBR waters due to the potential influence of residence time on pollutant build-up in the GBR lagoon (Andutta et al., 2013; Choukroun Marine Pollution Bulletin 105 (2016) 277–285 ⁎ Corresponding author at: LIPI's Centre for Deep Sea Research, Indonesian Institute of Sciences (LIPI), Jl. Y. Syaranamual, Guru-Guru, Poka, Kota Ambon, Provinsi Maluku 97233, Indonesia. E-mail address: gerry.salamena@my.jcu.edu.au (G.G. Salamena). http://dx.doi.org/10.1016/j.marpolbul.2016.02.015 0025-326X/© 2016 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul