1. A frontal eddy intensively sampled at sea and overflown by SARAL David Griffin, Moninya Roughan, Shane Keating, Amandine Schaeffer, Iain Suthers CSIRO Oceans and Atmosphere, University of NSW

2. Nearly-maiden voyage of RV Investigator Iain Suthers of UNSW led a June 2015 voyage with the ambitious goal of finding and studying a sub-mesoscale ‘frontal eddy’. These often form on the landward side of the East Australian Current. Success hinged on an eddy forming, and being visible (from space). We did not count on it being sampled by an altimeter. But it was (by AltiKa)! David Griffin 2 |

3. David Griffin 3 |

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16. David Griffin 16 | V geo =1.5m/s V ADCP =1m/s

18. ADCP at 60m: Swirl velocity 0.5-1m/s at 15km radius Solid-body rotation T=1day. R o ≈1 Cannot infer velocity from SLA using geostrophy David Griffin 18 | 15km 1m/s

19. Sub-meso cyclonic, so observed SLA gradient>fv David Griffin 19 | ADCP v (fitted) fv v 2 /r AltiKa ∫v 2 /r ∫fv ∫(fv + v 2 /r)

20. David Griffin 20 |

21. David Griffin 21 |

23. Conclusion Sub-mesoscale eddies are often seen in imagery but rarely sampled so intensively. Their physical properties and ecological significance are the subject of speculation. Further analysis of the cruise data will shed some light on these questions. By amazing coincidence, AltiKa sampled the sea level anomaly through the centre of the eddy, allowing direct comparison with the in situ observations. Having a radius of 15km and rotation period of 1day, the centripetal acceleration accounts for about 1/3 of the SLA. This is not a calculation that can be done confidently in reverse with nadir altimetry. Possible with 2d (e.g. SWOT)? Hope so! David Griffin 23 |

24. DAVID.GRIFFIN@CSIRO.AUDAVID.GRIFFIN@CSIRO.AU WWW.MARINE.CSIRO.AU/~GRIFFIN Thank you