Analysis: Intrusion of the East Australian Current
Over the last week or so, ocean temperatures have really heated up across the East Coast.
Most noticeably, the temperatures across Sydney and southern NSW have been about as warm as it gets, with inshore temperatures of 24 degrees. The MHL Sydney waverider buoy, positioned 10km off Long Reef, has been recording peak sea surface temperatures just over 25 degrees since last Friday.
The Mid North Coast has had some small scale upwelling which has suppressed temperatures, though they rebounded late last week and over the weekend while up off the North Coast and SE Qld, the Byron and Tweed Head buoys have registered temperatures of 27 degrees.
Around this time of year we usually see an increase in ocean temperatures across southern NSW from the East Australian Current (EAC). The EAC is a western boundary current - it forms the south-west border of the Pacific Ocean - that flows southward from the Coral Sea, down along the East Coast before usually splitting from the coast around Seal Rocks and heading eastward towards New Zealand, known as the Tasman Front.
Any remaining flow heading south of this is known as the EAC Extension and can reach as far south as Tasmania before meeting the Antarctic Circumpolar Current. The flow is strongest during the summer months and varies year on year regarding its intrusion south towards Tasmania.
Having a look at the current satellite imagery of the sea surface temperatures off the southern NSW coast above, you can clearly see the EAC pushing into Sydney and then further down, to a position south-east of Eden.
While it's normal for the EAC to travel this far south, what's not normal are the temperatures. 24-25 degree water is sitting very far south, and comparing this to the long-term climatic average, it's 3-4 degrees warmer than average.
While more pleasant for surfing, there are long term implications if the EAC regularly begins pushing this far south. For one, we'll see more tropical fish species setting up habitat further south, while for Tasmania, the incursion of sea urchins on the East Coast pose a real threat to the rocky reef systems and ecology.
Satellites are constantly measuring the sea surface temperatures from above, scattered throughout the world's oceans are small observational buoys called ARGO floats. These floats have been deployed by various international organisations and institutions with the data freely available for all to use.
The floats measure salinity and temperature throughout the ocean column while also being at the mercy of the world's currents, hence providing real-time feedback of the speed and direction of such currents.
You can view the location and history of the ARGO floats around the world here and luckily we've seen one such float travel southward with the East Australian Current the past few months.
Observations start through October when it was positioned well east of Byron Bay, meandering for the month before being swept south in the EAC from mid-November. Once travelling south it covered ground steadily and travelled the 350km from Seal Rocks to its current position east of Jervis Bay in 21 days. That's an average of 20cm/s or 700m/hr.
It's still currently sitting east of Jervis Bay and we can examine the temperature profile through the water column. On the surface temperature have maxed at 25.536 degrees, while temperatures are still 20 degrees some 100m below the surface (the pressure in decibars is approximately equal to the depth in metres).
Moving back up the coast off northern NSW and it's worth considering the minimum ocean temperatures needed for cyclone formation, that being approximately 26.5 degrees. The current temperatures off Byron are easily within this threshold. However, what we also need is the mixed layer below the surface to contain these temperatures, otherwise the cyclone will quickly weaken once using the energy from the surface water.
Regarding the swell potential from this increase in water temperature over the coming months, it all depends on whether we get the right synoptic setup and instability aloft. We'll have to watch it closely and cross those fingers for a more classic autumn of surf compared to recent years.