Water mass variation in the upper layer of the Halmahera eddy region observed from a TRITON buoy

Time change of water properties in the upper layer associated with development of the Halmahera eddy, and water mass front between the northern and southern hemispheres water in their confluence region are investigated using the TRITON buoy data and hydrographic survey in the western equatorial Pacific conducted by JAMSTEC and jointly with BPPT Indonesia. Observation period of TRITON buoy in the Halmahera eddy region (2°N, 130°E) is from October 2001 to November 2002, while two hydrographic surveys data carried out by the R/V MIRAI in October 2001 and July 2002 are used to complement the TRITON data. It is shown that the Southern Pacific Tropical Water (SPTW) with salinity above 35.0 psu and occupied between 22.5 and 26.0 sigma-theta was clearly observed almost during the observation period. However, an anomalous intrusion of the North Pacific Tropical Water (NPTW) with salinity less than 34.85 psu and centered at 23.5 sigma-theta into the SPTW´s dominated region was found during October 2001. It is considered that during this period part of strong southward flowing of the Mindanao Current which supplied the NPTW was observed further to south of 2°N, as confirmed by hydrographic survey data. It is concluded that during this observation period the front between the southern and northern hemispheres water was taken place around 2°N. However, in summer monsoon (July 2002) when the northwestward flowing of the New Guinea Coastal Current and Undercurrent which supplied high salinity water of the SPTW were fully developed, the front was shifted to near 5°N. The variability of surface current, dynamic height, and 20°C isotherm depth from the TRITON buoy indicated the dominant oscillation around 35-65 days period which may link to the Halmahera eddy activities because the time series of salinity on the surface of 24.5 sigma-theta changed at the similar time period.