Intrusive upwelling in the Central Great Barrier Reef

Abstract

In the Central Great Barrier Reef, the outer continental shelf has an open reef matrix that facilitates the exchange of waters with the Coral Sea. During austral summer, cool water intrudes onto the shelf along the seafloor. Temperature observations reveal cool, bottom intrusions during a 6 year period from the Queensland Integrated Marine Observing System's Palm Passage mooring. A metric is used to identify 64 intrusion events. These intrusions predominantly occur from October to March including the wet season. During an event, the outer‐shelf's near‐bottom temperature decreases by 1–3°C typically over 1 week. The near‐bottom salinity tends to increase, while near‐surface changes do not reflect these tendencies. Intrusion events occur predominantly with either weakening equatorward winds or poleward wind bursts. A regional hydrodynamic model for the Great Barrier Reef captures the timing and amplitude of these intrusions. During intrusion events, isotherms tend to uplift over the continental slope and onto the shelf and the East Australian Current intensifies poleward. Over the shelf, a bottom‐intensified onshore current coincides with bottom cooling. For numerous events, the model diagnostics reveal that the cross‐shelf flow is dominated by the geostrophic contribution. A vertical circulation tilts the isopycnals upward on the southern side of the passage, causing an along‐shelf density gradient and geostrophic onshore flow with depth. While wind fluctuations play a major role in controlling the along‐shelf currents, model results indicate that a concurrent topographically induced circulation can assist the onshore spread of cool water.

Organic matter bioavailability in tropical coastal waters: The Great Barrier Reef

“…1 and 2 could potentially have been impacted by upwelling events that occur mostly from October to March (summer), when the south‐easterly trade winds relax and monsoon winds are active. However, a recent study (Benthuysen et al ), which characterized upwelling events in the GBR over the period 2010–2015, did not detect any events within the weeks of our sampling times and an impact on our results therefore seems unlikely.…”

Extreme Marine Warming Across Tropical Australia During Austral Summer 2015–2016

“…Furthermore, near‐surface warming increases the stratification, reducing vertical mixing with deeper oceanic waters, and potentially changing the depth of upwelling waters and nutrient sources (García‐Reyes et al, ). For 2016, in the Central GBR, typical upwelling events appeared to have been suppressed during the peak of the MHW, compared with past years during the same month (Benthuysen et al, ). The Slocum gliders revealed that the warm waters were found from midshelf to the shelf break over hundreds of kilometers and even in the reef passages known for intrusive upwelling (Figures c and d).…”

“…As they traversed the continental shelf and through reef passages, the glider temperatures revealed that the warm waters extended throughout the water column (Figures c and d). In the reef passages off Townsville, the typical March outer‐shelf temperatures can be cooler by up to several degrees along the bottom due to intrusive upwelling or well‐mixed with the transition into April with the seasonal breakdown of the stratification (Benthuysen et al, ). Intrusive upwelling has been associated with weakening or a reversal in the southeasterly winds during the monsoon season (Benthuysen et al, ).…”

“…In the reef passages off Townsville, the typical March outer‐shelf temperatures can be cooler by up to several degrees along the bottom due to intrusive upwelling or well‐mixed with the transition into April with the seasonal breakdown of the stratification (Benthuysen et al, ). Intrusive upwelling has been associated with weakening or a reversal in the southeasterly winds during the monsoon season (Benthuysen et al, ). For March 2016, this region's surface waters had average SST anomalies of +1 – 1.5°C, and given the weak stratification present, the temperature anomalies were likely greater near the bottom.…”

Subsurface intensification of marine heatwaves off southeastern Australia: The role of stratification and local winds

“…Future projections along the SE coast of Australia suggest a decrease of downwelling favorable winds [Sen Gupta et al, 2016, Figure S5]. This might lead to shallower MHWs, while local persistent upwelling [Lourenço et al, 2016] or intrusive bottom uplift [Benthuysen et al, 2016] could provide respite from ocean warming and new habitat for some species.…”