Ocean Radar News

Ocean radar antennas at Coffs Harbour. Image credit: Dan Atwater, JCU.


Radar observations prove to be a useful tool for examining frontal eddies along the East Australian Current

A recent paper has used more than a year of high-resolution (1.5 km, hourly) surface velocity measurements from the IMOS HF radar at Coffs Harbour to quantify the propagation of frontal eddies and meanders along the eastern coast of Australia.

Ocean currents are usually characterized by instabilities, meandering and eddy shedding at different spatial and temporal scales. The East Australian Current (EAC) is a southern hemisphere western boundary current, closing the subtropical gyre in the South Pacific. Its temporal variability is associated with eddy-scale, seasonal, interannual, decadal and climate scales, however little is known about time scales on the order of days. Resolving these fine-scale structures in time and space requires high-resolution observations, as they are often too small to be captured by altimetry (see Figure at right).

The study led by Amandine Schaeffer of the University of New South Wales used the surface current measurements from the ocean radars, in conjunction with data from IMOS moorings, and satellite observations. It was the first time, to the authors knowledge, that the characteristics and motion of frontal eddies in a western boundary current have been systematically observed at high resolution.

The results of the study show that cyclonic eddies occur frequently along the EAC on average every seven days over the year. Cyclonic frontal eddies that are associated with the EAC meanders during weak wind stress propagate downstream with speeds of 0.3-0.4 m s-1. While frontal eddies propagate through the radar domain independently of wind stress, upfront wind can influence their stalling and growth, and can also generate large cold core eddies through intense shear.

The cyclonic frontal eddies are a major mechanism for the transport and entrainment of nutrient rich coastal or deep waters, influencing physical and biological dynamics, and connectivity over large distances.

To read the full paper:

Schaeffer, A., A. Gramoulle, M. Roughan, and A. Mantovanelli (2017), Characterizing frontal eddies along the East Australian Current from HF radar observations, J. Geophys. Res. Oceans, 122, doi:10.1002/2016JC012171.

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Top view of MODIS SST (top section, left colour bar) and ocean color (bottom, right colour bar) remote-sensed images on 29 September 2013 showing the signature of a frontal eddy. Velocity vectors show the surface currents measured by HF radars on the 29 September 2013 08:00 (plot every sixth grid point, top, black) and geostrophic current from altimetry (bottom, grey). Black contours overlaid on the chlorophyll-a concentrations (bottom) show positive surface divergence calculated from the HF radar velocities (contours of 0.2|f|, 0.3|f|, 0.4|f|, 0.5|f|, increasing to maximum in the center). Blue dots indicate the location of the two HF Radar systems. Figure from the paper published in J.Geophys. Res. Oceans.