On sampling the ocean using underwater gliders

Abstract

[1] The sampling characteristics of an underwater glider are addressed through comparison with contemporaneous measurements from a ship survey using a towed vehicle. The comparison uses the underwater glider Spray and the towed vehicle SeaSoar north of Hawaii along 158°W between 22.75°N and 34.5°N. A Spray dive from the surface to 1000 m and back took 5.6 h and covered 5.3 km, resulting in a horizontal speed of 0.26 m s −1 . SeaSoar undulated between the surface and 400 m, completing a cycle in 11 min while covering 2.6 km, for a speed of 3.9 m s −1 . Adjacent profiles of temperature and salinity are compared between the two platforms to prove that each is accurate. Spray and SeaSoar data are compared through sections, isopycnal spatial series, and wave number spectra. The relative slowness of the glider results in the projection of highfrequency oceanic variability, such as internal waves, onto spatial structure. The projection is caused by Doppler smearing because of finite speed and aliasing due to discrete sampling. The projected variability is apparent in properties measured on depth surfaces or in isopycnal depth. No projected variability is seen in observations of properties on constant density surfaces because internal waves are intrinsically filtered. Wave number spectra suggest that projected variability affects properties at constant depth at wavelengths shorter than 30 km. These results imply that isobaric quantities, like geostrophic shear, are valid at wavelengths longer than 30 km, while isopycnal quantities, like spice, may be analyzed for scales as small as a glider measures.

Sustaining observations of the unsteady ocean circulation

“…The depth coverage of Argo supplements the spatial and temporal resolution of altimetry. Eulerian timeseries from moorings complement the slowly profiling but high vertical resolution of glider data [49], whereas the spatial evolution from satellite data compensates for the slow translational speeds of gliders [46,62]. In a hybrid between point measurements and spatial coverage, a recent experiment used gliders to analyse the ocean from a Lagrangian frame of reference.…”

“…The slow speed (about 0.5 knots or 0.2 m s −1 ) is dictated by power requirements and necessary to extend their duration. Variations associated with internal tides or waves can be aliased into individual profiles of data [46], whereas seasonal changes can be aliased into spatial variations [47]. Combining glider datasets with other data, such as from satellites, may ameliorate some of these problems ( figure 11).…”

Active Singularities for Multivehicle Motion Planning in an N-Vortex System

“…In particular, choosing (19) from becoming singular as long as |z α −z β |>1 for all pairs α, β due to the log(·) terms in ∂H c /∂γ. If is γ bounded, which remains to be shown, the invariance condition implies that H c converges to H d c , but it enforces nothing else about the configuration.…”

“…Significant hardware and sensor improvements [19] as well as algorithmic performance guarantees [9], [10] have further encouraged interest. However, there are open challenges about how mobile sensor platforms can most effectively sample and interact with strong, circulating flows [3], [10], [11].…”

Assimilating en-route Lagrangian observations

“…Lagrangian instruments can be fitted with a variety of sensors that collect auxiliary subsurface observations (Rudnick et al, 2004;Chyba, 2009;Eriksen and Perry, 2009;Rudnick and Cole, 2011). In the case of Argo floats, subsurface observations are taken both at the parking depth where latitude and longitude are unknown and during ascent where latitude and longitude measurements become available upon surfacing (Gilson and John, 2013;Ollitrault and Rannou, 2013).…”