Palm Passage mooring surface buoy. Image credit: Craig Steinberg, AIMS.


Intrusive upwelling in the central Great Barrier Reef

The central Great Barrier Reef (GBR) has an open reef matrix along the outer-shelf compared to the north and south which presents more as a true barrier.

Reef passages allow the exchange of water between the continental shelf and the Coral Sea. During the summer months, cool water is uplifted over the slope and flows onto the shelf along the seafloor. These periods of bottom cooling are termed “intrusions” as they are not reflected in the surface temperature.

These intrusions are important as they influence the thermal structure of waters over the shelf. The near-bottom cooling can potentially provide relief for the shelf’s benthic marine ecosystems from the impacts of strong summertime warming and marine heat waves. Intrusive upwelling induces cross-shelf changes in nutrient supply for reef communities from the shelf-edge to the lagoon. The upwelled water entrains nutrients providing an important source for the Central GBR continental shelf and can enhance biological productivity.

Research by a team of scientists at the Australian Institute of Marine Science led by Dr Jessica Benthuysen was published recently in the Journal of Geophysical Research: Oceans, and examined cool, salty bottom intrusions in the Central GBR. The study aimed to characterise their seasonal occurrence, the intensity of bottom cooling and duration, and to determine the physical mechanisms causing them.

The research identified the intrusions using six years of mooring observations from the IMOS mooring in the Palm Passage and linked them to fluctuations in the winds and source waters over the slope. A complementary analysis using a regional hydrodynamic eReefs model for the Great Barrier Reef further revealed the roles of winds and density field in forcing the shelf circulation during intrusion events.

The research identified 64 intrusion events, which predominantly occurred from October to March including the wet season. During an event the outer-shelf’s near-bottom temperature decreased by 1-3°C typically over 1 week. The near-bottom salinity tended to increase, while near-surface changes did not reflect these tendencies. The hydrodynamic model revealed widespread cooling along the seafloor concentrated through the reef passages. During intrusion events, isotherms tended to uplift over the continental slope and onto the shelf and the East Australian Current intensified poleward. While wind fluctuations played a major role in controlling the along-shelf currents, the model results indicated that a concurrent topographically induced circulation can assist the onshore spread of cool water.

The full paper can be accessed here.

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Palm Passage bottom frame awaiting redeployment from the back deck of the RV Cape Ferguson. Image credit: Craig Steinberg, AIMS.