Detailed investigation of the three-dimensional structure of a mesoscale cold eddy in the Kuroshio extension region

The Kuroshio Extension Region (KER) has the most active oceanic mesoscale eddies within the North Pacific Ocean, nevertheless, there is a lack of observations on the three-dimensional (3D) fine structure of these eddies. Satellite altimeter data and underway observations were used in combination to conduct a fine-scale characterisation of the 3D thermohaline structure and circulation of a mesoscale cyclonic eddy within the KER during June, 2014. The results showed that isotherms in the six in situ sections consistently exhibited enhanced upward bending and negative temperature anomalies were distinct. At locations closer to the centre of the eddy, the negative temperature anomalies became more significant and the intensity of the cold core became more pronounced. The isohalines also trended upwards, with the upper and lower levels having negative and positive anomalies, respectively. The isohalines at the various layers similarly exhibited a clear eddy-like structure from the subsurface down to 1,000 m. Both the actual flow field measured by the Acoustic Doppler Current Profiler (ADCP) and the geostrophic flow field based on thermohaline data indicated a significant asymmetric cyclonic circulation structure. By comparing with the in situ data, the HYbrid Coordinate Ocean Model (HYCOM) is capable of depicting the mesoscale eddy while the magnitude and eddy location deviate from in situ observations.