Having collected another set of data from our familiar C2 site, we are now steaming to a location a bit south of the A1 mooring. Once there we will begin another CTD/LADCP profile until the weather chases us away again.
This time a storm is coming up from the south, and so we will be running north to stay just out of its reach. I’m told by some of the locals on board that our weather during this cruise seems unseasonably cool and windy. Yet underneath this cool exterior, warmer waters are slowing pushing into the Tasman Sea. And that is where weather and climate do not always agree – until they do.
The East Australian Current (EAC) runs southward along the coast of eastern Australia, much like the Gulf Stream in the U.S. If the EAC sounds familiar, you can thank a little clownfish named Nemo. Known as western boundary currents (because they occur on the western sides of their respective oceans), their effect is to move warm equatorial waters poleward into temperate regions. These are naturally occurring currents generated by the large ocean gyres that dominate oceanic regions, generally driven by the circulation of the earth (and referred to as geostrophic flow). What makes the EAC different from other western boundary currents is that its average flow is weaker and eddies are much more predominant. Mesoscale (mid-size and -duration) eddies are constantly forming and reforming along the EAC pathway from north Queensland down to Tasmania.
Australian scientists (including Pete Strutton and Randall Lee on this cruise) have observed a general intensification of EAC flow in its southern regions and weakening in its northern regions over the past 60 years. More flow means warmer, saltier water, and possibly more eddies in the Tasman Sea. The biological effects are already being seen. Marine organisms ranging from plankton, to rock lobster, to southern Bluefin tuna are moving further to the south. Large scale impacts are also apparent – sea surface temperatures are rising in the south, as is sea level. But some of the mid-scale effects, especially the eddies, are still not well documented or understood.
The role of eddies
As we’ve mentioned before, eddies are an important element in understanding how the energy of the internal tide beam may be dissipated as it moves across the Tasman Sea, or as it is reflected off the Tasman shelf. Circulating in either clockwise or counterclockwise directions, the eddies create shear that can disrupt the low mode internal wave into higher mode waves (with smaller amplitudes), which don’t propagate as far. So, just as the internal tide can affect climate models by contributing to the movement of ocean energy around the globe, so might climate change impact how the internal tide moves that energy by gradually increasing eddy formation in the Tasman Sea. In their efforts to understand how the internal tide system works under current ocean conditions, T-Beam scientists are planning to collect more information on how eddies interact with the internal tide. Perhaps these data may eventually help them also predict how this system could be affected by ongoing changes in the EAC and regional climate.