Southern Ocean frontal structure and sea-ice formation rates revealed by elephant seals

Abstract

Polar regions are particularly sensitive to climate change, with the potential for significant feedbacks between ocean circulation, sea ice, and the ocean carbon cycle. However, the difficulty in obtaining in situ data means that our ability to detect and interpret change is very limited, especially in the Southern Ocean, where the ocean beneath the sea ice remains almost entirely unobserved and the rate of sea-ice formation is poorly known. Here, we show that southern elephant seals (Mirounga leonina) equipped with oceanographic sensors can measure ocean structure and water mass changes in regions and seasons rarely observed with traditional oceanographic platforms. In particular, seals provided a 30-fold increase in hydrographic profiles from the sea-ice zone, allowing the major fronts to be mapped south of 60 degrees S and sea-ice formation rates to be inferred from changes in upper ocean salinity. Sea-ice production rates peaked in early winter (April-May) during the rapid northward expansion of the pack ice and declined by a factor of 2 to 3 between May and August, in agreement with a three-dimensional coupled ocean-sea-ice model. By measuring the high-latitude ocean during winter, elephant seals fill a "blind spot" in our sampling coverage, enabling the establishment of a truly global ocean-observing system.

Fig. 1.

Circumpolar distribution of hydrographic profiles and temperature at 200 m from the Coriolis database and data collected by elephant seals in the Southern Ocean during 2004–2005. (A) Data from the Coriolis database consisting in Argo floats, XBTs, and research vessels. (B) Data collected by elephant seals equipped with CTD-SRDLs at South Georgia (SG), and South Shetland (SS), Kerguelen, (KER), and Macquarie (MAC) islands. Red points indicate profiles collected in sea ice. Color stars indicate positions of time series collected in sea ice by four different seals (see Table 1).

Fig. 2.

Temperature field at 500 m during 2004–2005 from the Coriolis database and from the merged Coriolis and elephant seal databases. Mean front positions during the same period derived from Coriolis (A) or Coriolis and seal temperature field at 500 m (B) (thick lines), and from altimetry (thin lines in A and B). Plotted fronts are Bdy, southern branch of sACCf, and central branches of PF and SAF. Note the increased level of detail in the combined plots.

Fig. 3.

Time series of hydrological properties collected by an elephant seal in sea ice over the continental shelf. (A) Positions of CTD profiles collected by a seal near 84°E in April–May 2004 (color dots), and in August 2004 (black dots). The outline of the West Ice Shelf was obtained from Moderate Resolution Imaging Spectroradiometer (MODIS) data. (B) T and S measured by this seal in April–May 2004 near 84°E (indicated by a yellow star on Fig. 1B); for T and S time series, small vertical bars indicate profiles collection; the large vertical bars on S time series delimit the period over which the sea-ice formation rate was estimated.

Fig. 4.

Sea-ice net freezing rates derived from the seal data and from a coupled sea ice–ocean model (FESOM) (24). Thin lines indicate daily net freezing rates from the model, extracted at four grid points closest to the respective seal positions (Fig. S7) and smoothed with a 31-d running mean. Thick horizontal line segments correspond to sea-ice formation rates inferred from the salinity budgets measured by seal 1, 2, 3, and 4 at 34°E, 54°E, 84°E, and 103°E, respectively (Table 1). Length of segment indicates the averaging interval.