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°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...

Flight paths of seabirds soaring over the ocean surface enable measurement of fine-scale wind speed and direction

“…The extensive movement range and locomotion ability of marine mammals and seabirds enable observations to be obtained in places and scales unresolved by conventional observations. For example, instrumented seals have been providing temperature and salinity profiles in the Antarctic Ocean for more than 10 y, especially under sea ice coverage that was difficult to measure by conventional methods (10,11). Adding these data to ocean circulation models improved the accuracy of estimated mixed layer properties (12).…”

Ocean Observations Using Tagged Animals

“…The CTDsatellite relay data logger (CTD-SRDL; Figure 1b; Boehme et al, 2009; see also Box 1 in Treasure et al, 2017, in this issue), developed and built at the Sea Mammal Research Unit, University of St Andrews, UK, is to date the only existing tag that incorporates a miniaturized CTD unit, with typical accuracies of ±0.02°C for temperature and ±0.05 or better for salinity. The more than 1,000 CTD-SRDL tags deployed to date (Charrassin et al, 2008;Fedak, 2013;Roquet et al, 2014), have yielded several hundred thousand vertical profiles of temperature and salinity in a wide variety of locations.…”

Integrating the Ocean Observing System: Mobile Platforms

“…Recently, animal-borne instruments have been designed and deployed to provide in situ hydrographic data (Boehlert et al, 2001;Boehme et al, 2009). Animal-borne oceanographic instruments have enabled collection of in situ datasets with high spatial and temporal resolution even in regions that are seasonally ice-covered (Nicholls et al, 2008, Charrassin et al 2008Costa et al, 2008;Meredith et al, 2009). Some species are wideranging, while others generate quasi-Eulerian series (Boehme et al, 2008a;Meredith et al, 2009).…”

“…Marine mammals are typically not hampered by ice, as are Argo floats and gliders; rather they actively find breathing holes, where data are then transmitted. Timeseries and sections along pack ice fronts and ice shelves, and even data from within the pack ice, far from the open ocean, provide new insights into ocean dynamics (Charrassin et al, 2008;Nicholls et al, 2008).…”