Idealised hydrodynamic and Lagrangian modelling of flow past the Bremer Canyon system: implication for upwelling

Author: Nathan Teder

Teder, Nathan, 2020 Idealised hydrodynamic and Lagrangian modelling of flow past the Bremer Canyon system: implication for upwelling, Flinders University, College of Science and Engineering

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The western Great Australian Bight is home to a seasonal hotspot for orca aggregation (Orcinus orca, also known as killer whales), which occurs from November to March. The current reason why this aggregation occurs is unknown, however, Pattiaratchi (2007) linked this to a possible subsurface upwelling event. A potential cause of subsurface upwelling is through a submarine canyon being able to drive an upwelling conducive flow upwards, propelling either a source of nutrient rich water into the euphotic zone, or creating a pathway for marine organisms to congregate higher up in the water column. This research is focused around the Bremer, Hood and Whale Canyon due to the proximity to where orcas are aggregating, as well as the Bremer and Whale Canyon existing on the edge of the continental shelf.

To test if either the Hood, Bremer or Whale Canyon can support an upwelling event capable of sustaining an orca aggregation, a single layer hydrodynamic model based on the shallow-water equations was used. This model predicted flow velocity from a barotropic pressure gradient due to a sloping sea surface, and ignored wind, tidal forcing, and density stratification. The single-layer model was set to adjust to a forced change on the northern boundary to create that barotropic pressure gradient for six days before being treated as a constant. Neutrally buoyant particles were released into the model on the fifth day and allowed to run until the sixtieth day. Temperature and salinity data from either a CTD device, or ocean-gliders was used to test if the results from these models could realistically happen.

When the single-layer model was used to test if the formation of a topographic Rossby wave was possible, 71.76% of particles released were upwelled onto the continental shelf from the Whale Canyon, and 83.58% of particles were upwelled from the Bremer Canyon. This showed that both canyons were capable of supporting upwelling via a topographic Rossby wave and temperature and salinity profiles taken in the region during February 2019 showed a >2°C change of temperature in a 10 m band between a depth of 30 m to 70 m. This rapid decrease occurred regardless where the measurement was taken, it ruled out that upwelling was occurring in the upper 100 m during February 2019.

Using a single-layer model to test if a deeper upwelling was occurring, the results showed that 89.1% of particles were entering the area where orcas were aggregating (“hotspot”). This indicated that when deep flow interacted with the Hood and Bremer Canyon, it was able to upwell and provide a pathway for marine organisms to end up in the hotspot and support orca aggregation. This was supported by temperature and salinity profiles which showed similar conditions in the hotspot and the head of the Hood Canyon.

Keywords: Upwelling, Submarine Canyons, Ocean Modelling, Shelf Break Upwelling, Continential Slope Upwelling, Orcas

Subject: Meteorology and Oceanography thesis

Thesis type: Masters
Completed: 2020
School: College of Science and Engineering
Supervisor: Associate Professor Jochen Kämpf