It is also possibile that 50-day oscillations are induced by eddy activity in this region. Numerical results by Inoue and Welsh [1992] and Qiu and Lukas [1996] suggest that variability associated with ocean eddies is dominant in the Philippine Sea. Lukas et al. [1991] show that the NECC meanders with space scale of the ocean eddies.
Snap shots every 10 days from JEGCM are given in Plate 1. These pictures suggest that the Mindanao and Halmahera Eddies repeatedly grow and decay. In particular, the Mindanao eddy cuts off from the Mindanao Current at the southern tip of the Mindanao, moves westward and a new eddy appears with period of 40 days. Although its period is a little different from that in observation, it is possible that variability induced by this phenomenon is appeared in current time series.
Qiu et al. [in press] also shows similar result and suggest that this phenomenon is resonance of the gravest Rossby wave in the Celebes Sea. In order to confirm their result, more observation, in particular, moored observations are needed in and around the Celebes Sea.
4. Summary
We analyzed the results from mooring and hydrographic observations in the southernmost Philippine Sea conducted by JAMSTEC and BPPT from 1992 to 1996 in order to investigate the ocean variability in the southernmost Philippine Sea. The numerical results from the JAMSTEC Eddy-resolving General Circulation Model (JEGCM) were also analyzed to understand the phenomenon observed in the above observations. These results are summarized as follows:
a) Strong northwestward flow was observed below subsurface (300m-800m) from November 1994 to March 1995 near the Talaud Island, Indonesia. Because of matching with monsoon variability, this phenomenon seems to be one of seasonal signal in this region. Results from Hydrographic observations in the southernmost Philippine Sea and JEGCM suggest that this is occurred due to southward shift of the Halmahera Eddy associated with an enlargement of the Mindanao Dome.
b) Intraseasonal variability (50-day oscillations) was dominant at the southwestern boundary of the Philippine Sea. This variability might be related to local wind variability with period of 40-50 days period: Maddan Julian Oscillation. Results from JEGCM also suggest that eddy activity in this region is dominant with similar period and the oscillations are related with this activity.
Acknowledgments
The cruises in the southernmost Philippine Sea were conducted as a joint project by Japan Marine Science and Technology Center (JAMSTEC), Japan, and Badan Pengkajian Dan Penerapan Teknologi (BPPT), Indonesia. We thank our colleagues in JAMSTEC and BPPT for their efforts in conducting these cruises. During the cruises, crew of R/V Kaiyo; scientists of JAMSTEC, BPPT and Central Research Institute Electric Power Industry; and technicians of Nihon Marine Enterprise Co. Ltd., Sanyo Techno Marine Inc., and Kansai Environmental Engineering Center Co. Ltd. participated in these cruises and helpfully supported us.
Part of this research was performed under the Japan WOCE project funded by the Special Coordination Funds of the Science and Technology Agency of the Japanese Government.
