[1] High resolution hydrographic sections and maps of the gradient of sea surface height (SSH) reveal that the Antarctic Circumpolar Current (ACC) consists of multiple jets or frontal filaments. Here we use a 15 year time series of SSH observations to determine the circumpolar structure and distribution of the ACC fronts. The jets are consistently aligned with particular streamlines along the entire circumpolar path, confirming and extending the results of an earlier study restricted to the region south of Australia. The intensity of the fronts (as measured by the cross-front gradient of SSH) varies along the fronts and the individual branches merge and diverge, often in response to interactions with bathymetry. Maps of absolute velocity at 1000 m depth derived from Argo trajectories confirm the existence of multiple current cores throughout the Southern Ocean. High resolution hydrographic sections and profiles of temperature and salinity from Argo floats are used to show that the front locations derived from fitting SSH contours to maps of SSH gradient are consistent with locations inferred from the traditional criteria based on water mass properties, suitably modified to account for multiple frontal branches. Three regions are examined in detail: the Crozet Plateau, the Kerguelen Plateau and the Scotia Sea. These examples show how recognition of the multiple jets of the ACC can help resolve discrepancies between previous studies of ACC fronts.

We describe and model a potential re-equilibration process that can aect compositions of melt inclusions in magnesian olivine phenocrysts. This process, referred to as``Fe-loss'', can operate during natural pre-eruptive cooling of host magma and results in lower FeO t and higher MgO contents within the initially trapped volume of inclusion. The extent of Fe-loss is enhanced by large temperature intervals of magma cooling before eruption. The compositions of homogenised melt inclusions in olivine phenocrysts from several subduction-related suites demonstrate that (1) Fe-loss is a common process, (2) the maximum observed degree of re-equilibration varies between suites, and (3) within a single sample, variable degrees of re-equilibration can be recorded by melt inclusions trapped in olivine phenocrysts of identical composition. Our modelling also demonstrates that the re-equilibration process is fast going to completion, in the largest inclusions in the most magnesian phenocrysts it is completed within 2 years. The results we obtained indicate that the possibility of Fe-loss must be considered when estimating compositions of parental subduction-related magmas from naturally quenched glassy melt inclusions in magnesian olivine phenocrysts. Compositions calculated from glassy inclusions aected by Fe-loss will inherit not only erroneously low FeO t contents, but also low MgO due to the inherited higher Mg# of the residual melt in reequilibrated inclusions. We also demonstrate that due to the higher MgO contents of homogenised melt inclusions aected by Fe-loss, homogenisation temperatures achieved in heating experiments will be higher than original trapping temperatures. The extent of overheating will increase depending on the degree of re-equilibration, and can reach up to 50°C in cases where complete re-equilibration occurs over a cooling interval of 200°C.

Maps of the gradient of sea surface height (SSH) and sea surface temperature (SST) reveal that the Antarctic Circumpolar Current (ACC) consists of multiple jets or frontal filaments. The braided and patchy nature of the gradient fields seems at odds with the traditional view, derived from hydrographic sections, that the ACC is made up of three continuous circumpolar fronts. By applying a nonlinear fitting procedure to 638 weekly maps of SSH gradient (١SSH), it is shown that the distribution of maxima in ١SSH (i.e., fronts) is strongly peaked at particular values of absolute SSH (i.e., streamlines). The association between the jets and particular streamlines persists despite strong topographic and eddy-mean flow interactions, which cause the jets to merge, diverge, and fluctuate in intensity along their path. The SSH values corresponding to each frontal branch are nearly constant over the sector of the Southern Ocean between 100°E and 180°. The front positions inferred from SSH agree closely with positions inferred from hydrographic sections using traditional water mass criteria. Recognition of the multiple branches of the Southern Ocean fronts helps to reconcile differences between front locations determined by previous studies. Weekly maps of SSH are used to characterize the structure and variability of the ACC fronts and filaments. The path, width, and intensity of the frontal branches are influenced strongly by the bathymetry. The "meander envelopes" of the fronts are narrow on the northern slope of topographic ridges, where the sloping topography reinforces the ␤ effect, and broader over abyssal plains. * This paper is an addition to the collection of papers published in the February 2007 Journal of Physical Oceanography Special Issue in Honor of Carl Wunsch (see acknowledgments).

In Part 1 of this study, we showed that the Antarctic Circumpolar Current (ACC) consisted of multiple fronts, each of which was consistently associated with a particular contour of sea surface height (SSH) or approximate streamline. In Part 2 we have used maps of SSH to examine the variability of the ACC fronts between 1992 and 2007. The SSH label associated with each frontal branch is nearly constant around the circumpolar belt. The front labels are also nearly constant in time: the bands of enhanced SSH gradient (i.e., fronts) occur along the same streamlines throughout the 15 year period of observations. Both short‐ and long‐period changes of the SSH frontal labels of the ACC are small. Based on a tight relationship between dynamic height and cumulative baroclinic transport of the ACC, the baroclinic transport variability of the individual branches of the ACC is also expected to be small. The major change in the total ACC baroclinic transport occurs in the Drake Passage. The streamline associated with the northern branch of the SAF (SAF‐N) does not pass through Drake Passage and the waters carried by this branch are not observed there. Instead, the transport of the SAF‐N turns north in the Pacific to supply the export of water to the Indian Ocean north and south of Australia. Strong eddy activity in the southeast Pacific acts to dissipate the hydrographic signature of the SAF‐N there. In the Atlantic, the SAF‐N reappears as the same streamline is again associated with enhanced SSH gradients to the east of the Brazil – Malvinas confluence zone. While the large changes in SSH have occurred in the Southern Ocean between 1992 and 2007, there are strong regional differences. Because the ACC fronts are robustly associated with particular SSH contours, the changes in SSH reflect shifts in the position of the ACC fronts. In the circumpolar average, each of the ACC fronts has shifted to the south by about 60 km. The changes in SSH in the Southern Ocean are largely due to changes in ocean circulation, rather than warming and freshening by atmospheric fluxes. Much larger changes in SSH are observed in some locations of the Southern Ocean, particularly where the fronts interact with large‐scale topography. The northern branch of the PF (PF‐N) near the Kerguelen Plateau is an extreme example, where the PF‐N followed a path around the northern end of Kerguelen Plateau between 1992 and 1997, passed through the Fawn Trough after 2003, and oscillated between the two paths between 1997 and 2003.

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[1] We reexamine the relationship between circulation, bathymetry, and surface chlorophyll in the Southern Ocean, using new high-resolution maps of the frontal structure of the Antarctic Circumpolar Current (ACC) derived from satellite altimetry. The maps reveal that the ACC consists of multiple filaments or jets. By averaging surface chlorophyll measurements along streamlines, we show that the fronts define the limits of zones with similar concentrations and seasonality of surface chlorophyll. The overall pattern of surface chlorophyll is consistent with strongest upwelling of nutrient-rich deep water south of the Polar Front (PF). However, the distribution of chlorophyll in the Southern Ocean is concentrated in a number of persistent blooms, observed downstream of islands and bathymetric features. In contrast to previous studies, we find little evidence that the fronts of the ACC are associated with enhanced productivity, at least where the fronts are distant from topography. Rather, we find that most regions of elevated chlorophyll in the open Southern Ocean can be explained by upwelling of nutrients (both macronutrients and micronutrients) where the ACC interacts with topography, followed by downstream advection. The upwelling is shown to be the consequence of the bottom pressure torque established by the large-scale flow, rather than being due to small-scale instabilities of the jets. The interaction of the flow with the topography therefore establishes both the large-scale dynamical balance of the ACC and determines the productivity of the open Southern Ocean.

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...