Observations of the shelf circulation dynamics along Ningaloo Reef, Western Australia during the austral spring and summer

The dynamics of the three-dimensional circulation on the shelf off Ningaloo Reef in Western Australia were investigated using observations from two field experiments (Nov 2009-Jan 2010 and Sep 2010-Nov 2010). The observations revealed an onshore geostrophic flow of ~0.015 m s−1 along the relatively straight Ningaloo Peninsula, coinciding with a substantial poleward increase of the along-shelf flow along this short (∼60 km) section of coast. The cross-shelf transport analysis distinguished between two cross-shelf advection scenarios prevalent along this coast: 1) a transient coastal upwelling scenario when the offshore surface Ekman transport was replenished by an onshore transport, supplied mainly from the interior of the water column due to the combined effects of the stratification and steep continental slope; and 2) a downwelling scenario when offshore bottom boundary layer Ekman transport occurred, associated with a deeper pressure-gradient driven flow. A detailed along-shelf momentum balance indicated that there was a southward increase in the along-shelf pressure gradient from the northern limit of the Ningaloo Peninsula (the North West Cape), which explains the increase in the along-shelf flow down the coast. The results indicate that the Ningaloo shelf can be considered as one of the important formation regions of the poleward Leeuwin Current which, while very weak on Australia North West Shelf, is the dominant current feature south of Ningaloo and along the remaining southwestern Australia coast.