Boundary Current Observations with ALPS

Robert E. Todd, Daniel L. Rudnick, Luca R. Centurioni, Steven R. Jayne, and Craig M. Lee

Oceanic boundaries are where society interacts with the ocean through fisheries, transportation, oil and gas extraction, and recreation. These boundary regions are also where intense oceanic currents play a key role in the transport of mass, heat, salt, biogeochemical constituents, and plankton. In the large ocean basins, western boundary currents dominate the poleward transport of warm water or equatorward transport of cold water and are major drivers of climate variability. Eastern boundary currents are often upwelling systems that comprise some of the most biologically productive regions in the world. Boundary currents in marginal seas provide the major means of exchange with the open ocean and impact regional ecosystems. Finally, boundary currents that flow along the continental slopes mediate communication between the coast and open ocean, affecting ecosystems, flood levels, erosion, and commercial activity. Sustained observations of these highly dynamic boundary current regions are a necessary component of a global ocean observing system; over the past decade, autonomous platforms, particularly drifters, profiling floats, and gliders, have become key tools for collecting long-duration measurements in boundary currents.

Figure 1. Trajectories and near-surface velocity estimates from Global Drifter Program drifters in the western Pacific and marginal seas. Paths of various boundary currents are clearly visible, as is the rich eddy field to the west of the Kuroshio.

  Drifters have long been used to study boundary current systems (e.g., Fuglister, 1963; Davis, 1985a,b). By following the flow, either at the surface or at the depth of a drogue, networks of drifters can effectively map circulation patterns. Drifters drogued at 15 m depth, part of the Global Drifter Program (GDP; Niiler, 2001), reveal northwestern Pacific surface circulation (Figure 1), including a variety of boundary currents in both the open ocean and the marginal seas, as well as associated eddy fields. Several studies have investigated the kinematics and dynamics of boundary current systems and their interactions with marginal seas (e.g., Centurioni et al., 2004, 2009; Vélez-Belchí et al., 2013). GDP drifters are routinely equipped to measure temperature and sea level pressure (Centurioni et al., 2016) along their trajectories. A subset of GDP drifters also measures surface salinity, surface winds, subsurface temperature and pressure, and directional wave spectra; additional sensing capability may be anticipated as cost-effective sensors emerge.

The sustained, subsurface sampling provided by the network of Argo profiling floats has allowed for new insights into circulation along ocean boundaries. For example, the subthermocline circulation of the western boundary current system in the low-latitude western Pacific has been substantially revised in light of Argo observations (Qiu et al., 2015), and Argo observations have contributed to identifying the fate of the Deep Western Boundary Current in the Atlantic (Garzoli et al., 2015). Observations from Argo also capture spatial and temporal evolution along the flanks of boundary currents where there are recirculation gyres in which mode waters often form and spread (e.g., Wong, 2005; Qiu et al., 2006; Billheimer and Talley, 2013; Rainville et al., 2014) and where eddy fields are often particularly strong (e.g., Castelao, 2014).

Autonomous underwater gliders (Rudnick, 2016) have proven to be effective platforms for collecting sustained, high-resolution observations boundary currents. In typical use, gliders profile from the surface to 500–1,000 m, taking three to six hours to complete a cycle from the surface to depth and back. Deployments of three to six months are now routine, during which time a glider’s survey track extends well over 2,000 km. Crucially, because gliders can move through the water, they are able to measure the property gradients at scales relevant to boundary current regions. Velocity, averaged over the depth a glider profile, can be estimated by differencing displacement calculated from a hydrodynamic flight model (motion in still water) from observed displacement over the dive. Absolute geostrophic velocity then can be calculated by referencing geostrophic shear, derived from lateral density gradients quantified by gliders, to these depth-average velocities. Comparisons between velocities observed from mooring arrays and glider-derived absolute geostrophic currents (e.g., Lien et al., 2014) show excellent agreement, confirming that glider-based sections can successfully quantify boundary current transports.

Figure 2. Observations from the California Underwater Glider Network (CUGN). (a) Trajectories of CUGN gliders along CalCOFI Lines 66.7, 80.0, and 90.0 since 2006 (blue) with locations of Argo profiles during the same time period (red). (b) Temporal variability in the depth of the 26 kg m–3 isopycnal relative to its mean annual cycle along Line 80.0. (c–e) Mean cross-shore transects of (c) potential temperature, (d) salinity, and (e) alongshore geostrophic velocity along Line 80.0. Panels b–e are based on the CUGN climatology of Rudnick et al. (2017) and are available from https://doi.org/10.21238/S8SPRAY7292.

Gliders are routinely deployed in a variety of boundary current systems globally. The California Underwater Glider Network (CUGN; Figure 2), which consists of three cross-shore transects that have been continuously occupied for a decade (Rudnick et al., 2017), exemplifies sustained glider observations in an eastern boundary current system. CUGN observations fill a gap between the coast and Argo observations in the interior ocean (Figure 2a), and have allowed for examination of interannual variability (e.g., Figure 2b) and development of various climatologies (e.g., Figure 2c–e). Western boundary currents typically have depth-averaged currents that are significantly faster than a glider’s speed through the water, so gliders surveying western boundary currents generally cross those currents obliquely. Multiyear glider surveys of the Kuroshio and Gulf Stream (Figure 3) have demonstrated the feasibility of using gliders to routinely survey across western boundary currents. While the strong and variable currents lead to less well-repeated transects (Figure 3a,c), various methods have been used to combine observations from many glider missions in western boundary currents to produce both maps of the mean flow (e.g., Figure 3b,d) and mean vertical sections (e.g., Figure 3b; Todd et al., 2016; Schönau and Rudnick, 2017). Gliders capable of full-depth profiling (e.g., Deepglider) offer the possibility of occupying transects perpendicular to a western boundary current at the cost of spatial and temporal resolution.

Figure 3. Multiyear glider observations in two mid-latitude western boundary currents. (a) Trajectories from 20 Seaglider missions in the Kuroshio during 2011–2013. (b) Mean depth averaged currents in the upper 1,000 m from gliders (black) and a moored array (blue) and mean cross-track geostrophic currents along select transects (insets). (c) Trajectories of 12 Spray glider missions in the Gulf Stream from 2004–2017. (d) Averages of potential temperature at 200 m and depth averaged currents in 0.5° x 0.5° boxes. (Panels c–d are adapted from Todd, 2017)

The numerical modeling community has expressed a need for additional observations in boundary currents to constrain models; the sustained, high-resolution observations that can be provided by ALPS are ideal for constraining and validating numerical models and have been used in a variety of boundary current regions to date (e.g., Centurioni et al., 2008; Todd et al., 2011; Rudnick et al., 2015; Schönau et al., 2015; Todd and Locke-Wynn, 2017). Drifters, floats, and gliders return observations in near-real time, thus making those observations available for operational usage. Though observations from autonomous platforms are routinely assimilated into various numerical simulations and appear to provide useful constraints, quantitative assessment of observation impact in the models remains a challenge; for instance, the importance of subsurface observations from gliders relative to that of satellite remote sensing observations for constraining frontal positions should be determined.

Autonomous and Lagrangian platforms have the potential to form the backbone of a global boundary current observing system that connects the coast and boundary currents to the interior ocean. Such a system would complement the global coverage of the Argo and Global Drifter Programs and expand the footprint of the OceanSites moorings that provide high-frequency measurements of many variables at specific sites. Building on repeated ship-based surveys, some of which have endured for decades, a boundary current observing network built on autonomous and Lagrangian platforms would allow for observations in difficult locations and conditions while improving spatial and temporal resolution. At present, sustained boundary current measurements from gliders and drifters are largely comprised of physical (pressure, temperature, salinity, velocity) and a limited set of bio-optical or bio-acoustic properties (e.g., chlorophyll, chromophoric dissolved organic matter, acoustic backscatter, passive acoustics for mammals or fish). As additional sensors suitable for long-duration (or even expendable) deployment on autonomous and Lagrangian platforms become available (e.g., phosphate, silicate, species-level classification of plankton, biomass, or turbulence), a global boundary current observing network could become truly multidisciplinary.

Because boundary currents invariably reside within Exclusive Economic Zones (EEZs), their observation must depend upon regional efforts that are respectful of coastal countries. As such, a global boundary current observing system would consist of a coordinated set of regional observing networks. Efforts to coordinate boundary current observing at the international level are currently underway through the Global Ocean Observing System (GOOS) and related groups. For example, there is currently a growing effort to organize sustained boundary current measurements with gliders under the OceanGliders Boundary Ocean Observing Network initiative within GOOS. Included in this international coordination should be building financial support for sustained boundary current observations in coastal countries, establishment of (and support for) an Argo-like data distribution system for integrated boundary current observations, and defining protocols for public release of observations within EEZs.

 

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Authors

Robert E. Todd, Woods Hole Oceanographic Institution, Woods Hole, MA, USA, rtodd@whoi.edu

Daniel L. Rudnick, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA, drudnick@ucsd.edu

Luca R. Centurioni, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA, lcenturioni@ucsd.edu

Steven R. Jayne, Woods Hole Oceanographic Institution, Woods Hole, MA, USA, sjayne@whoi.edu

Craig M. Lee, Applied Physics Laboratory, University of Washington, Seattle, WA, USA, craig@apl.washington.edu