Cool Streaks In South Oz
Cool Streaks In South Oz
It was around this time last year stunning satellite imagery from the South Australian coast came in, depicting a beautiful, fluorescent phytoplankton bloom emanating off the Bonney Coast. That being the region immediately north-west of Tasmania, from Portland through to Robe in South Australia's South East.
It's a yearly event during summer under strong, persistent south-southeast blasts and this year we've got another great satellite signature of the upwelling, spreading with the associated currents to the west, towards Kangaroo Island.
There's no trickery being played with the visuals, this is the true colour imagery seen from space and what it depicts is one of the most significant upwelling systems in the world and the largest predictable one in south-eastern Australia.
Like the East Coast under strong north-east winds, strong south-east winds along the western Victorian and South Australian coast direct surface water to the left (opposite in the Northern Hemisphere), known as Ekman transport.
This transport of surface water to the west, away from the coast results in a drop of the sea level locally on the coast, which with all things being equal needs to be balanced. This is done by drawing up water from the depths, which is cooler and nutrient rich.
The unique characteristic regarding the Bonney Coast is the close proximity of the shelf break to the coast, only some 30km or so at its closest point.
This and the various canyons lining the shelf provide the perfect setup to pump cold, sub-surface water up from beyond the shelf.
Once the cool, nutrient rich water reaches the photic zone, where sunlight enters the water column, photosynthesis takes place, the fundamental building blocks for production in the ocean.
Current observations have water temperatures between 3-4°C below normal off the South Australian Coast, spreading westward though with less strength into the Bight.
This and a similar upwelling signal off the Pilbara Coast are the only areas that aren't warmer than normal Australia wide.
The upwelling zone is a key ecological feature and an area of high primary production and aggregation of marine life, tiny to large.
During summer, persistent south-southeast winds are prevalent across the Victorian and South Australian coasts, but the past month we've seen these winds coming in much stronger than normal (as most surfers can attest to).
With wind speeds being 3.5m/s (6kt) stronger than normal, we've seen a significant upwelling event with the resultant phytoplankton bloom being visually striking.
The associated surface currents are also pushing the bloom to the west with localised eddies clearly visible.
The bioluminescent properties depend on whether the bloom contains dinoflagellates which appear brown in colour, with possible signs showing in the western arm of the bloom.