Lau Basin Float Experiment (LAUB-FLEX)

[Get the float data here.]

Rationale

Before this experiment was begun in 2004, little was known about the ocean circulation in the Lau Basin, one of the RIDGE 2000 hydrothermally active Integrated Study Sites, where coordinated interdisciplinary experiments are being carried out between 2004 and 2010. Knowledge of the oceanic circulation is required in order to answer important questions of hydrothermal research. The near-vent-field velocities are required, for example, if fluxes of heat and chemicals associated with the hydrothermal plumes are to be estimated. On a larger scale, the regional flow field severely constrains the dispersal pathways of the larvae of hydrothermal animals and, as a consequence, the biogeography of vent organisms. Because of these considerations, determination of the regional circulation in the Lau Basin has been made a priority in the Lau Basin implementation plan.

After a preliminary analysis of (sparse) hydrographic data was carried out, we (Andreas Thurnherr and Kevin Speer) submitted a proposal to the National Science Foundation (NSF) to study the deep flow field in the Lau Basin with autonomous floats autonomous floats] that are programmed to drift at the depth of the hydrothermal plumes (approximately 1800m). The floats periodically return to the surface in order to obtain their positions and to transmit their data back to shore using a network of satellites. In 2003, NSF funded the proposal and, as a result, 5 APEX and 10 PROVOR floats were deployed during three scientific expeditions to the region in 2004 and 2005. Out of the 15 floats, 3 Metocean and 1 APEX failed during deployment. Two of the Metocean failures were later diagnosed as ballasting errors - the replacement floats are currently awaiting deployment. One additional Metocean float failed after completing 6 3-week cycles. The remaining 10 floats are fully functional. The expected life time of the floats (several years) implies that there will be near-real-time velocity measurements available during the entire duration of the planned RIDGE 2000 experiments in the Lau Basin.

In addition to providing background data that are required to interpret hydrothermal-plume dispersal observations and biogeographic patterns, the float trajectories will also provide the first measurements of the deep circulation in the Lau Basin, yield new insights into processes acting in confined basins and near rough topography, and they are expected to be useful for improving numerical circulation models.

Results

Because typical velocities in the ocean are similar to the speed of a garden snail the first set of floats was programmed to return to the surface once every 4 weeks. At the surface the expected velocities are 10-100 times larger, especially during storms. Therefore, it is important that the floats spend much more time at depth than at the surface, where they have to remain for a minimum of about 10 hours in order to be able to transmit all their data. Because of the unexpectedly strong 4-week averaged flow observed by the first batch of floats, the cycling interval was reduced to 3 weeks for the remaining instruments.

The animation on the right shows the float trajectories. (Click here for a static image and here for the data files. Some web browsers have problems displaying large animated gifs - it may be useful to download and save the animation and display it with a better gif-animation viewer, such as the QuickTime player on MacOSX.) The floats are programmed to drift along a surface of constant pressure of 1750dbar, which corresponds to a depth of approximately 1730m. This depth was chosen to be as close as possible to the depth of the hydrothermal vent fields known at the time of deployment (1730-1850m), while being shallow enough so that the floats would not run aground very often. (While the floats are designed to be able to run aground without being damaged we nevertheless try to avoid that.)

After more than three years of float measurements some aspects of a mean circulation are only beginning to emerge. Initially, all the floats from the first batch deployed in the eastern Lau Basin were heading north, consistent with the hydrographic analysis. Later, however, the situation has become much less clear. A few observations stand out:

• the floats "cluster" in the western Lau Basin, primarily between between ~18° and 21°S;

• there are indications of a strong northward boundary current along the western rim of the basin;

• several floats have been drifting toward the deep passages in the NE and NW of the Lau Basin;

• even though the flow along the ELSC is predominantly northward, a float deployed on the ELSC near 20°S has managed to drift to the southern end of the spreading center after two years before turning northward;

The cycle-averaged drift velocities recorded so far peak at just under 9 cm/s. Even the more typical speeds between 1 and 2 cm/s are significantly stronger than expected from the hydrographic analysis. The reason for this discrepancy is not yet known. It is possible that the hydrographic data were collected during a lull in the deep flow.

Regardless of the floats' future behavior we already know a lot more about the deep circulation in the Lau Basin than we knew before the first float was deployed. As is usually the case in observational physical oceanography, the ocean turns out to be more complex than previously imagined. Therefore, the experiment can already be called a success!

If you are interested in knowing what the deep flow in the Lau Basin is doing, visit this site regularly as new data will be uploaded whenever it becomes available.

[The material found on this page is based upon work supported by the National Science Foundation under Grant No. 0241785. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.]