Why is the East Australian Current Behaving so Badly?

18th October 2016

The East Australian Current (EAC) is a flow of water that is formed from the South Equatorial Current crossing the Coral Sea and reaching the eastern coast of Australia off Queensland. As the South Equatorial Current hits the Australian coast it divides forming the southward flow of the EAC. eacThe EAC is the largest ocean current close to the shores of Australia reaching a maximum velocity at about Coffs Harbour in NSW where its flow can reach a speed of 3 km per hour.

In the animated film Finding Nemo Marlin and Dory use the EAC as a superhighway travelling with fish and sea turtles to Sydney Harbour. The EAC is dominated by eddies which are circular currents of water that form whirlpools of up to 100km in diameter.

Their swirling motion is one of the forces that make nutrients found in cold, deep waters to come up to the surface of the ocean where phytoplankton (microscopic plants) feed on them.

Eddies do occur off the coast of Tasmania but a change in their behaviour over the last 24 years is concerning. These eddies are generated in the EAC and most of them do not go south of Bass Strait. When they do, they bring warm water with them and the bigger they are the more heat they can bring.

eke_fig_final

Scientists have noted a trend in eddies off Tasmania becoming larger, stronger and more frequent. Following the 1990s, eddy kinetic energy (EKE) increased gradually both north (red line in graph below) and south (blue line in graph below) of Bass Strait,  with a huge spike in eddy activity  off Tasmania (8 times the average EKE of the 1990s) in 2014 (see animation below). This trend is in agreement with climate modelling but there has been a dramatic increase over the last couple of years.

The presence of eddies south of Bass Strait is believed to be responsible for the atypically warm sea surface temperatures experienced off the east coast of Tasmania in 2015. If the eddy encountered off north east Tasmania in July this year is anything to go by (see image below), this heating trend is expected to continue into 2016.

A team of CSIRO scientists, AFMA observer and commercial fishermen from the South East Trawl Fishery came upon the eddy while conducting acoustic surveys of orange roughy off the north east coast of Tasmania in July 2016 as part of an ongoing monitoring program. eacIt was fast and hot and gave the team a bit of a hard time by making the deployment and retrieval of their sampling gear very challenging.

The eddy had current speeds of more than 2 knots and temperatures at its centre were more than 2 degrees warmer than the year round average between 100-400m depth and almost 1 degree warmer at 1200m depth. The eddy’s outer edge was close to the continental slope.

The implications of increased eddy activity on adult orange roughy spawning, orange roughy larvae, aquaculture, other fisheries and Tasmanian coastal waters in general are unknown. However, hot eddies warm up all the water around them and the bigger they are the deeper and wider their impact so if this trend continues there is no doubt that the Tasmanian offshore environment will change – the ‘how’ is anyone’s guess!

For up to date ocean information around Australia visit the IMOS Ocean Current website.