By Jaci Brown, CSIRO
Occasional erratic bursts southward of the East Australian Current (EAC) are thought to have moderated the weather of south-east Australia this autumn and winter and they continue to introduce tropical and sub-tropical marine species to Tasmanian waters.
Ocean monitoring by Australia’s Integrated Marine Observing System is providing scientists with significant new insights into the changing structure of the EAC. Over the past 50 years sporadic warm bursts have become more common as the EAC moves further south. With global warming, the warm burst we’ve seen this year may also become the norm.
Had our little friend Nemo the clownfish been riding the EAC this year he might have found himself holidaying in Tasmania rather than admiring the Sydney Opera House. He wouldn’t have been on the trip alone, though. Sea nettles (Chrysaora) have headed from their usual home in Sydney to be found for the first time ever in Tasmania and the Gippsland Lakes.
Waters in the EAC travel southward along the east coast of Australia, with most of it splitting from the coast near Sydney and heading for New Zealand. A small part of the current, known as the EAC Extension, works its way southward past Victoria and Tasmania.
A typical signature in this region are the large eddies, around 200 kilometres across and hundreds of metres deep. Some of the warm water is trapped here along with marine life.
This year a larger proportion of the EAC was sent southward instead of breaking away to the east. Winter ocean temperatures off Bass Strait were around 19C, an increase of 4C. This impacted local fishing, beach conditions and the weather.
In the video (above) the animation on the left shows the actual sea surface temperature and speed of the ocean currents. The animation on the right shows the difference in the temperature from average conditions.
Through autumn and winter, you can see two interesting changes occur. A strong warm current heads down the coast from Sydney to the coast of Victoria. At the same time, warm water peels off from the EAC and swirls around in large eddies as it meanders toward Tasmania.
An unusual catch down south
One advantage of warm eddies is the refuge they provide for tuna. They congregate in the centre of the eddy where the waters are warm and dine at the nutrient-rich edges.
Local fishers in north-east Tasmania report a remarkable year that allowed them to fish longer than usual, providing game fishers with more opportunities to catch tuna.
Last summer’s (2013-2014) warmth provided an abundance of skipjack and striped marlin, while winter brought a run of bluefin tuna.
Redmap is a website where locals can report sightings of marine species that are unusual for a given area.
Last summer a manta ray, a tropical cartilaginous fish (in a group including rays and skates), was sighted off the north-eastern coast of Tasmania. Previously the southern-most sighting of a manta ray was just south of Sydney.
Its not just new species visiting Tassie either. Local jellyfish such as the Lion’s Mane (cyanea) – more commonly known as “snotty” – are usually quite elusive, but turned up in unprecedented numbers last summer in Tasmania.
But there’s a catch
This movement south of the EAC may have an impact on other systems, including our health. We rely on fish such as those from the Tasman Sea as a source of omega-3 fatty acids for our brain health. But the concentration of omega-3 fatty acids in the fish is likely to decrease with global warming.
The original source of fatty acids come from algal species. As our waters warm, we will see more of the algae from the tropics take up residence in the south-east.
But the algae from the tropics are much smaller, which means more steps in the food chain from the algae to the fish we eat. The more steps in the food chain, the more the omega-3 fatty acids in the fish are replaced by fatty acids that are less favourable to brain health.
The warmer coastal waters also contributed to the balmy autumn and winter in south-eastern Australia this year. Afternoon sea breezes cool coastal temperatures by drawing cool oceanic air onto the coast.
Sydney’s heat wave in May this year had 19 consecutive days of 22C or more – this is partly due to the sea breezes failing to bring in the usual cooling air.
What’s causing the EAC to move south?
Over the past 50 years the EAC Extension has stretched about 350km further south. This extension doesn’t happen smoothly but in erratic bursts.
The southward extent of the EAC is controlled by the collective behaviour of the winds between Australia and South America. Over that same 50-year period these winds changed their pattern due to a strengthening of a climate system known as the Southern Annular Mode.
The changes to this mode have been attributed to a combination of ozone depletion and increasing atmospheric CO2.
One of the most robust and consistent responses of the climate system to increasing CO2 is a further strengthening of the Southern Annular Mode.
So the result will likely be a further enhancement of the EAC extension southward and even warmer waters in the Tasman Sea.
Jaci Brown does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.
This article was originally published on The Conversation.
Read the original article.
28th October 2018 at 9:28 pm
Every new generation builds its own present, past and future, with or without the use or help of historical notes and anecdotes, which are always biased. So don’t gripe or be sour when your own experience tells you different from what you are told is happening or did or will. Go with the flow, the Egyptians did and so will the Martians.
24th November 2014 at 11:30 pm
I wonder did any body bother to destroy the old maps, its extremely interesting that the Oceans current maps of 1943 show that the Eastern Australian current was off Tasmania not Sydney. Anybody with half a brain and some research skills can access old images, maps, data from over the last 70 years. If I could up load the images of the oceans currents I would. Not only didn’t any one bother to check history and destroy that data as has been done before ,they didn’t destroy the old mapping and images either, instead they completely ignored it, what a mistake to just ignore it and push their agenda. This report is not telling the whole truth, sorry to burst your bubble, but world ocean currents have been mapped and studied for years, and the facts are you cant ignore that research and rewrite history to suit yourself. Well you can I suppose, but be sure to kill off all the old maps, images and research, all the data and records first. Map after map from 1943 to 1960 clearly show where the Eastern Australian current started and finished and the false imagery above ( or I should say misleading Imagery ) doesn’t represent the truth. As I said I have those ocean current maps, some of those maps are US army version- File: Ocean currents 1943. They clearly show all the worlds Ocean Currents at that time. So please stop with the doctored data… Not all of us are that dumb that we wont do our own research or didn’t do high school geography, history and multi strand sciences and after school studies ( higher learning: Tafe, Uni ). Not all of us are dumbed down or just plain ignorant or just in a fog because of the way of modern life. Shame Shame Shame, I wont throw out proven research with the bath water, just so some modern new ager scientist can state what ever to suit the current Global warming agenda. Oh and by the way my grand father used to speak of manta rays of Victoria and other rays back when he was a teenager and a young adult 50 years ago. So please just stop the bull dust. Not the first article CSIRO has published that’s misleading or ignores old proven scientific data…