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19.06.2015 04:20 Age: 2 yrs
Category: Home Slider, BlueWater, NSW-IMOS, Q-IMOS, SEA-IMOS

Investigator finds a 'Freddy' (a frontal eddy) in East Australian Current

Frontal eddies ('freddies') are small, short-lived, rapidly-rotating cyclonic (clockwise) eddies that form inshore of the main flow of the East Australian Current (EAC).

A large cold-core eddy off Brisbane at the start of the Investigator's recent voyage (source: OceanCurrent)

Current map shows formation of a 'freddy' off NSW (source: OceanCurrent)

The principal objective of the recent RV Investigator voyage, led by Professor Iain Suthers of the University of New South Wales, was an ambitious one  to locate then study the ecological role of one or more freddies. 

Larger mesoscale eddies associated with the meandering of the EAC itself were also the subject of the voyage, and since an extraordinary example of a large cold-core eddy happened to be off Brisbane at the start of the voyage, Investigator sampled it from 3 June to 6 June, as shown in the image on this page. Note the northward flow where the EAC normally flows southward along the continental margin.

On 6 June Prof Suthers and his team were excited to see evidence in IMOS SST imagery of the formation of a ~30km-diameter freddy at 32° 20'S, 153° 30'E (50km off Forster) and by 9 June several transects of it had been completed, delivering a wealth of information from the ship's suite of sensors and nets. As anticipated, the ship recorded elevated levels of fluorescence in the freddy, indicative of higher concentrations of chlorophyll-a. See OceanCurrent's technical news item to explore the voyage in more detail.

By massive coincidence, the French-Indian AltiKa satellite (one of the altimeter missions on which OceanCurrent depends for sea level information) overflew the centre of the freddy on 9 June, so the matchup of ship and satellite observations will help to interpret the combined dataset in the light of the historical archive of altimetry. The freddy is too small to be resolved in 2D gridded maps but the central depression of 15cm is clear in the raw along-track data, consistent with ship observations of ~1m/s rotational speed.