Autonomous Multi-Platform Observations During the Salinity Processes in the Upper-ocean Regional Study

Abstract

The Salinity Processes in the Upper-ocean Regional Study (SPURS) aims to understand the patterns and variability of sea surface salinity. In order to capture the wide range of spatial and temporal scales associated with processes controlling salinity in the upper ocean, research vessels delivered autonomous instruments to remote sites, one in the North Atlantic and one in the Eastern Pacific. Instruments sampled for one complete annual cycle at each of these two sites, which are subject to contrasting atmospheric forcing. The SPURS field programs coordinated sampling from many different platforms, using a mix of Lagrangian and Eulerian approaches. This article discusses the motivations, implementation, and first results of the SPURS-1 and SPURS-2 programs.

FIGURE 1.

Schematic illustration of (left) SPURS-2 Eulerian and (right) Lagrangian components. The former is “anchored” to a mooring site, while the latter follows the advection of the surface water mass tracked by a Lagrangian float and SVP drifters. Wave Gliders and Seagliders navigate pre-defined patterns in both the fixed and advected frames of reference.

FIGURE 2.

(left) Overview of the tracks of SPURS-1 ALPS, September 13, 2012–October 1, 2013, with (right) a close-up showing the details of the Focused Eulerian component. The tracks of SVP drifters and APEX floats are omitted from the latter plot for clarity.

FIGURE 3.

(top) Overview of the tracks of SPURS-2 ALPS, August 26, 2016–June 7, 2017, with (bottom) a close-up showing the details of the Focused Eulerian and Lagrangian components. The tracks of SVP drifters, Eulerian Wave Gliders, and APEX floats are omitted from the latter plot for clarity. Progression of the Coordinated Drift is indicated by date marks; a 10-day segment of the drift (September 13–23), shown in Figure 6 and discussed in the text, is marked by a thicker orange line. The inset shows relative trajectories of the Wave Glider (green) and the Seaglider (purple) in the drifting frame of reference defined by the Lagrangian float. In this frame of reference, the Lagrangian float (orange) is always at the center; the frame is rotated so that the x-axis is aligned with the geographic direction of the float’s drift. Intended Wave Glider navigation pattern, a 20 km × 20 km “butterfly” is shown by black dashed line.

FIGURE 4.

(right) The schooner Lady Amber is shown recovering a Wave Glider in the Southern Ocean before the beginning of SPURS-2. (above) Tracks of the first three Lady Amber cruises to the SPURS-2 site (star): Cruise 1, June 9–July 5, 2016; Cruise 2, August 29–October 25, 2016; and Cruise 3, December 1, 2016–January 16. 2017. Photo by Capt. Peter Flanagan

FIGURE 5.

Comparison of Eulerian and Lagrangian records of near-surface salinity evolution during SPURS-2. The Eulerian frame of reference is represented by near-surface salinity observations at the central mooring (red). The Wave Glider (green) followed the Coordinated Lagrangian Drift until December 11, 2016, and then relocated back to the vicinity of the central mooring site. Blue lines show Lagrangian evolution of near-surface salinity measured by SVP-S and SVP-S2 drifters that were released alongside the Lagrangian cluster, but later diverged from it.

FIGURE 6.

An example of joint upper-ocean observations during the SPURS-2 Coordinated Multi-Platform Lagrangian Drift (10 out of 100+ days shown). September 2016 Wave Glider observations of (a) wind speed, (b) salinity, and (d) temperature at 0.2 (red) and 6 m (blue). Lagrangian float observations of (c) salinity, (e) temperature, and (f) shear. Blue shading in (a) shows NASA Global Precipitation Measurement/Integrated Multi-satellitE Retrievals for GPM (GPM/IMERG) satellite precipitation interpolated to the Lagrangian array location. Circled numerals mark the salient features of the record, discussed in the text.