Observed features of the Halmahera and Mindanao Eddies

The structure and variability of the Halmahera and Mindanao Eddies (HE and ME), which are thought to be quasi-stationary eddies, are described based on onboard and time-series observations in the gateway region of the Indonesian Throughflow (ITF). The HE was found to have a clear anticyclonic eddy structure during all cruises. It tilted northward and/or westward with increasing depth. The HE mixes waters from the northern and southern hemispheres and transfers this mixed water to the eastern route of the ITF. The HE shifted northwestward in the boreal summer and the cold phase of ENSO (El Nino and Southern Oscillation) and southeastward in the boreal winter and the warm phase of ENSO. In contrast, the ME, with its cyclonic eddy structure, was observed only once and seems not to be a quasi-stationary eddy. It appears in the mean field and is confined above the thermocline. The stationary northward undercurrent, the Mindanao Undercurrent, was also not found at 7 degrees N east of Mindanao, but the northern tip of the HE was found at this latitude.