Direct observations of three-dimensional submesoscale trajectories using underwater gliders and neutrally buoyant floats
Abstract
Vigorous three-dimensional circulations exist in the submesoscale regime with especially strong vertical velocities. The submesoscale is defined by a Rossby number of order one, which is typically found at horizontal length scales <10 km. To quantify the three-dimensional trajectories in subsurface, submesoscale circulations we will deploy an acoustically-tracked, isopycnal-following array (IFA) of glider and floats in conjunction with other investigators. We anticipate an array of a few kilometers in extent, which drifts for about a week, but emphasize that how the IFA disperses is actually a research question.Observing these three-dimensional trajectories depends on solving two technical problems: (a) designing a neutrally buoyant platform that can follow a parcel of seawaterand (b) acoustic tracking of the IFA in the horizontal. To obtain neutral buoyancy, we will add a mini trimming system (MTS) to buoyancy-driven profilers, which permits fine control of volume via a single stroke pump. The MTS will have a total displacement of about 100 mL. We will make this solution available to RIOT. To obtain horizontal localization, two well-established acoustic optionsare available: (i) simple long baseline transponders/interrogators to obtain slant ranges and (ii) the WHOI MicroModem which adds two-way communications for issuing commands and transferring data. With option (i), assets must surface to obtain new commands and transfer data. Both options are made available by the APL-UW group. Such systems will operate at 10 kHz and have ranges of about 10 km in the subtropics. Vertical location is easily determined with pressure.Since all assets are equipped with the same acoustic tracking hardware and MTS, their roles are interchangeable. Some assets will profile, while other assets form the IFA. The profiling assets obtain accurate positions at surface intervals, which are used to reference the relative positions from acoustic tracking. Depending on the acoustic range and location of the IFA, the number of profiling assets can be adjusted to either join the IFA or obtain sections to characterize the environment at scales larger than the IFA.While this autonomous fleet may be deployed anywhere, we prefer targeting strong submesoscale features that are predictable and locked to topography, such as island wake eddies. We suggest two Intensive Observing Periods (IOP) at two locations, where wake eddies with Rossby numbers ~ 1 and scales < 10 km are identified from previous work. IOP 1 will take place at the end of Year 2 in the Southern California Bight, where flow past the Channel Islandsmakes many suitable eddies. The easy logistics and proximity to our labs allow RIOT the best chance to deal with inevitable problems that arise with the new technology of the IFA. IOP 2 will take place near Saipan, where vigorous island wakes are seen and logistics are known. In both cases, there is the opportunity to study both cyclonic and anticyclonic wakes in close proximity. Our technical achievement will be an adaptable, autonomous fleet of floats and gliders for direct measurement of three-dimensional submesocaletrajectories. The scientific outcome is an unprecedented resolution of subsurface, submesocale trajectories. Approved for public release
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Nov 09, 2024
- Source ID
- N000142412745
Entities
People
- Daniel L. Rudnick
Organizations
- Office of Naval Research
- United States Navy
- University of California, San Diego