The tropical and subtropical Pacific Ocean are suggested to connect via a shallow meridional circulation cell by Gu and Philander (1997). This subtropical cell (STC) has been hypothesized to the El Nino-Southern Oscillation phenomena in the tropics, which then, in turn, can affect the subtropical ocean via rapid atmospheric forcing. But no direct measurements exist and indirect measurements and modeling support the hypothesis.
JAMSTEC and APL (Applied Physics Laboratory, University of Washington, USA) started a joint program to directly measure part of the circulation cell using acoustic tomography.
The observational centerpiece is tomography systems located in the Central Equatorial Pacific Region (Fig. 1). JAMSTEC began the fieldwork by deploying five acoustic transceivers in the observational region and this work will continue till Jan. 2001 (two year plan). APL will augment the five-element array with an additional transceiver mooring in the end of 1999, and this time JAMSTEC will deploy more three transceivers. This increasing the amount of data. APL will continue acoustic measurements with three transceivers until the end of 2003. The result will be a 5 year data set. Much of the data will be available in near real time, permitting timely data analysis.
2. Summary of deployment cruise
The following measurements were completed; five deployments of tomography transceiver, topography of the bottom of the sea of the deployment points by SEABEAM. 24 CTD (Conductivity-Temperature-Depth profiler) casts, 34 XCTD (expendable CTD) casts, 18 XBT (expendable bathythermographs) and continuous ADCP (Acoustic Doppler Current Profiler) measurements.
Fig. 2 Sound speed profile in this area from NODC data and CTDs at this cruise.
Fig. 3 EOFs of sound speed profile. This show 1 to 4 EOFs.
