Argo Float Trajectories at Mid-Depth in the Southwest Pacific

Trajectories from Argo floats have been used for a preliminary estimate of the mean ocean circulation of the southwest Pacific at 1000 m and to study zonal jets in the South Equatorial Current (SEC) where it encounters island chains and ridges in that region. Since 2004, 240 Argo floats have passed through the southwest Pacific, producing over 9800 trajectories. Most floats stay submerged for 9 days before ascending to telemeter their data and record their position at the sea surface. The World Ocean Circulation Experiment (WOCE) also deployed floats in this region and data from these floats are being added to the dataset. Accuracy of the submerged velocities depends on knowing the position and time of the float sinking and rising to the surface again. An extrapolation method was used to estimate the rise and fall locations and times from the series of position fixes and the known residence time on the sea surface. Near Fiji and New Caledonia, zonal jets appear on the north side of these islands. The mean velocity of the jet at 1000 m around the north of Fiji is 5 cm/s with a maximum velocity of approximately 18cm/s. The mean velocity of the jet around the north of New Caledonia is 5cm/s with a maximum of near 15cm/s. At present, not many floats have passed near the southern side of these islands, and it is difficult to confirm whether zonal jets are present at these locations as well. Other island chains also show trajectories that are steered by the topography. A map of dynamic height from Argo profile data shows reasonable agreement of geostrophic velocity with the mean velocity vectors calculated from the trajectories in the SEC region. The present Argo dataset shows the main circulation features of the southwest Pacific, and a strong influence of topography on the intermediate-depth circulation, but there is not yet enough data to produce a detailed description of the flow field.