Dynamics of the summer shelf circulation and transient upwelling off Ningaloo Reef, Western Australia

Abstract

[1] The shelf circulation off Ningaloo Reef near the North West Cape of Western Australia is driven by complex interactions between the southward flowing Leeuwin Current and wind-driven currents that episodically reverse the coastal flow toward the north. The presence of these northward (equatorward) wind-driven currents is thought to make this section of coast one of the few locations along Western Australia to experience periodic coastal upwelling. We used a combination of field observations and numerical modeling to investigate the summer circulation and upwelling dynamics along Ningaloo Reef. We analyzed current and temperature profiles from moorings at four sites across the shelf and used two Regional Ocean Modeling System (ROMS) sub-models: (1) a coarser model of northwestern Australia forced by a global ocean model and (2) a nested fine-scale model of the Ningaloo region. This nesting significantly improved model skill as it included the offshore mesoscale dynamics that strongly influenced the shelf circulation off Ningaloo. The field observations revealed several northward flow reversals, accompanied by cooling of the coastal waters adjacent to Ningaloo, which were associated with strong northward wind events. Analysis of the coastal heat budget revealed that cooling events were primarily driven by upwelling, whereas warming of coastal waters during relaxation events resulted mostly from along-shelf advection of warm water from the north. Due to the combined effects of its relatively steep (~1/50 slope) shelf and strong summer stratification, upwelled water was sourced from the interior of the water column, likely influencing the sources and fluxes of nutrients to Ningaloo Reef.

Surface circulation and upwelling patterns around Sri Lanka

“…The model formulation and numerical algorithms are described in detail in Shchepetkin and McWilliams (2005), and have been used to simulate the circulation and upwelling processes in a range of ocean basins (e.g. Di Lorenzo et al, 2007;Dong et al, 2009;Haidvogel et al, 2008;Marchesiello et al, 2003;Xu et al, 2013).…”

Is the East Australian Current causing a marine ecological hot-spot and an important fisheries near Fraser Island, Australia?

“…Studies by Hansen et al (2005), Pearce et al (2006), Rennie et al (2009), Rossi et al (2013) and Xu et al (2013) Currents. Pearce et al (2006) expanded on this and provided insight from the first long-term (27 month) field study of physical ocean processes including upwelling off Western Australia.…”

“…Rennie et al (2009) studied upwelling near Perth, Western Australia, using the Regional Ocean Model System (ROMS) and found that local sporadic upwelling was driven by a combination of physical processes including wind, mean current flow and eddy activity. A similar approach was used by Xu et al (2013) to study episodes of upwelling near Ningaloo Reef, while Rossi et al (2013) used 15 years of satellite data to quantify upwelling along sections of the west Australian coast. Favourable conditions ranged from a maximum of 140 days near Ningaloo to just a few days per year for locations in the south.…”

The combined effect of transient wind‐driven upwelling and eddies on vertical nutrient fluxes and phytoplankton dynamics along Ningaloo Reef, Western Australia

“…Our results display similar performance to Xu et al . [] and generally captured the dominant hydrodynamic processes operating on the shelf.…”

“…An additional characteristic that distinguishes the Ningaloo shelf from most shelves underlying other ECBS is the generally much steeper shelf slope (∼0.02 versus <0.01) [ Lentz and Chapman , ]. The steep slope, combined with seasonally strong stratification, causes upwelled waters to be mainly sourced from the interior water column rather than from the near‐bed region [ Lowe et al ., ; Xu et al ., ], consistent with the theoretical response to the locally high‐slope Burger numbers [see Lentz and Chapman , ]. As a consequence, the shallower source depth of the upwelled water, combined with the oligotrophic composition of the Leeuwin Current source waters, substantially limit the degree to which wind‐driven upwelling events cause temperature depressions and elevated nutrients in coastal waters along the Ningaloo Peninsula [ Koslow et al ., ].…”