Observing the Ice‐Covered Weddell Gyre With Profiling Floats: Position Uncertainties and Correlation Statistics

Abstract

Argo‐type profiling floats do not receive satellite positioning while under sea ice. Common practice is to approximate unknown positions by linearly interpolating latitude‐longitude between known positions before and after ice cover, although it has been suggested that some improvement may be obtained by interpolating along contours of planetary‐geostrophic potential vorticity. Profiles with linearly interpolated positions represent 16% of the Southern Ocean Argo data set; consequences arising from this approximation have not been quantified. Using three distinct data sets from the Weddell Gyre—10‐day satellite‐tracked Argo floats, daily‐tracked RAFOS‐enabled floats, and a particle release simulation in the Southern Ocean State Estimate—we perform a data withholding experiment to assess position uncertainty in latitude‐longitude and potential vorticity coordinates as a function of time since last fix. A spatial correlation analysis using the float data provides temperature and salinity uncertainty estimates as a function of distance error. Combining the spatial correlation scales and the position uncertainty, we estimate uncertainty in temperature and salinity as a function of duration of position loss. Maximum position uncertainty for interpolation during 8 months without position data is 116 ± 148 km for latitude‐longitude and 92 ± 121 km for potential vorticity coordinates. The estimated maximum uncertainty in local temperature and salinity over the entire 2,000‐m profiles during 8 months without position data is 0.66 ∘C and 0.15 psu in the upper 300 m and 0.16 ∘C and 0.01 psu below 300 m.

The Weddell Gyre, Southern Ocean: Present Knowledge and Future Challenges

“…These floats are also beginning to carry sensors that measure nitrate, oxygen, pH, and fluorescence, and several have already been deployed in the WG (Briggs et al, ; Talley et al, ). Adding the ability to acoustically track the location of these floats when they are under the seasonal ice will improve their utility (Chamberlain et al, ).…”

Southern Ocean Biogeochemical Float Deployment Strategy, With Example From the Greenwich Meridian Line (GO‐SHIP A12)

“…SOCCOM seeks to increase the number of Argo floats in the seasonal sea ice zone, where sea ice detection software protects the floats from surfacing under ice, with all under‐ice profiles reported when the float surfaces in ice‐free conditions (Chamberlain et al, ; Klatt et al, ; Wong & Riser, ). We used summer ice edges, the mean circulation, previous Argo trajectories, and float trajectories simulated in numerical models (section below) to select float deployment locations likely to produce at least one annual cycle before entering multiyear ice.…”

“…During PS89 float planning, we examined historical Argo floats from south of 50°S that included 27 floats from the University of Washington in the Weddell gyre (Chamberlain et al, ), hence under sea ice part of the year, as well as all historical tracks within the ACC (supporting information Figure S1a). (For current SOCCOM planning, we use all available Argo trajectories.)…”

Profiling Floats in SOCCOM: Technical Capabilities for Studying the Southern Ocean

“…This procedure makes the inherent assumption that the profiles collected by the floats while under the ice represent the properties of the water column at horizontal scales larger than the distance that the float traverses over the winter. In tests using measured summertime positions and particle simulation experiments in models similar to Wang et al (), Chamberlain et al () found that such interpolation might yield uncertainties of up to 100 km in the estimated location of floats by the end of winter, with the error being cumulative from the period of ice formation to ice melting; some improvement in the linearly‐interpolated position estimates can possibly be found using a Kalman filter. Occasionally, it is possible to attempt to actually track the floats under the ice using acoustic methods; beginning in 2007, a suite of 17 UW Argo floats equipped with RAFOS receivers tested this concept by recording acoustic travel times in the Weddell Sea transmitted by an array of 8 moored acoustic sources maintained by colleagues at the Alfred Wegener Institute for Marine and Polar Research in Germany.…”

“…These difficulties in the ice regime are due to the general lack of a sound channel in the water column at high latitudes as well as the absorption and scattering of the acoustic signals by the ice itself. Chamberlain et al () has examined these issues in considerable detail and found that for addressing scientific questions related to the large‐scale circulation, a precise knowledge of the under‐ice positions may not be a strong requirement; for making objective maps of more local under‐ice properties or estimating heat and freshwater fluxes, the problem is likely more severe. It is clear that making substantial improvements over linear interpolation under the ice at a reasonable cost will be a difficult undertaking, one that deserves a great deal of thought and experimentation in the future.…”