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°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 threedimensional coupled ocean-sea-ice model. By measuring the highlatitude ocean during winter, elephant seals fill a ''blind spot'' in our sampling coverage, enabling the establishment of a truly global ocean-observing system. Antarctic Circumpolar Current ͉ instrumentation ͉ marine predators ͉ ocean observation ͉ sea-ice modeling E vidence that the polar oceans are changing is growing rapidly, particularly in the northern hemisphere, where a significant decline in sea ice (1) and changes in the freshwater budget have been observed (1, 2). In the southern hemisphere, the limited observations available suggest that the circumpolar Southern Ocean has warmed more rapidly than the global ocean average (3) and that the dense water formed near Antarctica and exported to lower latitudes has freshened in some locations (4, 5) and warmed in others (6, 7). However, studies of change in the polar oceans as well as investigations of high-latitude dynamics continue to be hampered by a paucity of observations. In particular, although satellites and profiling floats are now providing measurements of much of the global ocean (8), the ocean beneath the Antarctic sea ice remains almost entirely unobserved. At Ϸ19 million km 2 at maximum extent (9), this represents a vast area. Sea-ice cover prohibits remote sensing of the underlying ocean by satellites, prevents conventional Argo floats from surfacing to transmit data, and makes ship operations expensive, difficult, and slow. Efforts are currently underway to develop ice-capable autonomous floats (10), but existing observations are heavily biased toward summer and open water.Observations of sea ice itself are also sparse, particularly in the Antarctic. Whereas the surface characteristics of sea ice can be measured by satellite, the key climate parameters sea-ice thickness and formation rate cannot be observed by using remote sensing. The formation rate determines how much brine is released and theref...

International audienceField campaigns are instrumental in providing ground truth for understanding and modeling global ocean biogeochemical budgets. A survey however can only inspect a fraction of the global oceans, typically a region hundreds of kilometers wide for a temporal window of the order of (at most) several weeks. This spatiotemporal domain is also the one in which the mesoscale activity induces through horizontal stirring a strong variability in the biogeochemical tracers, with ephemeral, local contrasts which can easily mask the regional and seasonal gradients. Therefore, whenever local in situ measures are used to infer larger-scale budgets, one faces the challenge of identifying the mesoscale structuring effect, if not simply to filter it out. In the case of the KEOPS2 investigation of biogeochemical responses to natural iron fertilization, this problem was tackled by designing an adaptive sampling strategy based on regionally optimized multisatellite products analyzed in real time by specifically designed Lagrangian diagnostics. This strategy identified the different mesoscale and stirring structures present in the region and tracked the dynamical frontiers among them. It also enabled back trajectories for the ship-sampled stations to be estimated, providing important insights into the timing and pathways of iron supply, which were explored further using a model based on first-order iron removal. This context was essential for the interpretation of the field results. The mesoscale circulation-based strategy was also validated post-cruise by comparing the Lagrangian maps derived from satellites with the patterns of more than one hundred drifters, including some adaptively released during KEOPS2 and a subsequent research voyage. The KEOPS2 strategy was adapted to the specific biogeochemical characteristics of the region, but its principles are general and will be useful for future in situ biogeochemical surveys