Much of the warming can be attributed to the unusual presence of EAC eddies south of Bass Strait. For example (right) the eddy off NE Tasmania encountered by the Orange Roughy survey team during July this year. Eddies have been tracked travelling down the coast of Tasmania before - what is unusual is the dramatic increase in the size and frequency of these eddies over the last few years.
An estimate of the spatially-averaged eddy kinetic energy¹ (EKE) in the waters off eastern Tasmania shows how much the eddy climate has varied over the last 24 years. Throughout the 1990s, EKE south of Bass Strait (blue line) is much lower than that in the EAC extension region just north of Bass Strait (red line). Prior to the 1990s few eddies got past Bass Strait. After the 1990s EKE increased gradually both north and south of Bass Strait. In the summer of 2014, Tasmanian waters saw a huge spike in eddy activity (8 times the average EKE of the 1990s) and since then it has peaked a number of times to levels much higher than those seen before 2014. In that time, EKE in the EAC extension has also increased, consistent with an increase in the strength of the EAC.
The influence of these eddies goes well beyond the sea surface temperature. Argo floats sampled the eddy pictured both at its centre and at its outer edge near the continental slope. Temperatures at the centre of the eddy were more than 2°C warmer than the year round average between 100 and 400 m depth and almost 1°C warmer down to 1200 m depth. How much the eddy properties impinge on the shelf is highly dependent on the size and path of each eddy but with their greater frequency and size, these eddies will inevitably impact coastal waters.
In July of both 2015 and 2016 there were large eddies off the NE of Tasmania impacting on the eastern Orange Roughy spawning ground during their spawning time. The eddy velocities of over 2 knots (Rudy Kloser, pers comm) persisted for much of the survey² in July this year. How the spawning ground preference and larvae are impacted by the increased temperatures and velocities due to the eddies is unknown at this stage but given that it has occurred now two years in a row we may well find out in the future. Transport within the EAC extension has increased between 1948 and 2014 and is predicted to increase further with climate change. The observed trend in eddy activity over the last 24 years, particularly off Tasmania, is in agreement with the direction of the trend predicted with climate change. The sudden pulses of eddies, which we have seen over the last few years, may be temporary but at this point we cannot be sure. Impacts on the state’s aquaculture and fisheries are similarly unclear at this point but it does appear that the risk of a continuation of recent trends should be taken seriously.
¹ Eddy kinetic energy was estimated using geostrophic currents from the IMOS OceanCurrent gridded sea level anomaly (DM00)
² The Orange Roughy survey is part of the SETFIA/AFMA/CSIRO ongoing monitoring program.
This article originally appeared in IMOS OceanCurrent.