Explore the vast range of titles available.

Geopolitics and climate change now have immediate consequences for national and international security interests across the Arctic and Antarctic. The world’s polar regions are contested and strategically central to geopolitical rivalry. At the same time, rapid political, social, and environmental change presents unprecedented challenges for governance, environmental protection, and maritime operations in the regions. With chapters that raise awareness, address challenges, and inform policy options, Polar Cousins reviews the state of strategic thinking and options on Antarctica and the Southern Oceans in light of experience in the circumpolar North. Prioritizing strategic issues, it provides an essential discussion of geostrategic thinking, strategic policy, and strategy development. Featuring contributions from international defence experts, scientists, academics, policymakers, and decisionmakers, Polar Cousins offers key insights into the challenges unique to the polar regions.

Although the Drake Passage has been considered a critical component of ocean circulation and climate, its initial opening age remains controversial due to the weak constraints on the paleoposition of the Antarctic Peninsula. Here, new zircon U‐Pb geochronological studies are conducted on the Barchans Islands, providing a critical age constraint on the paleopole (Latitude = 76.9°S, Longitude = 332.1°E, A95 = 5.9°) of the peninsula at ca. 59 Ma. Geochronological and paleomagnetic studies on Anagram Island and King George Island provide a new paleopole (Latitude = 77.4°S, Longitude = 23.5°E, A95 = 4.4°) of the Antarctic Peninsula at ca. 55 Ma. When combined with existing evidence, the initial separation between the Antarctic Peninsula and the Patagonian Andes is restricted to be between ca. 62 and 59 Ma, caused by the clockwise rotation of the Antarctic Peninsula. The separation induced the initial opening of the Drake Passage and the formation of the shallow proto‐Antarctic circumpolar current, as well as Paleocene global cooling.

An area of ocean centered on 179°E, 46°S has warmed to full depth since 2006, with surface warming around 5 times the global rate. This Subtropical Front area is associated with a confluence of warm, salty, subtropical water from the north carried in a western boundary current and cold, fresh, subantarctic water from the south carried in the northernmost branch of the Antarctic Circumpolar Current. Temperature and salinity changes observed from Argo floats indicate that the Subtropical Frontal Zone has moved west ∼120 km, creating this area of strong warming analogous to changes in extension regions of other western boundary currents. The warming is a result of changes in the local flows of subantarctic water, evident in satellite altimeter data and 1,000 m Argo trajectories, which in turn likely result from changes in meridional ocean heat content and winds. The warming has placed this biologically‐significant region in almost perpetual marine heatwave conditions. Plain Language Summary An area of ocean east of New Zealand has warmed strongly since 2006 through the full ocean depth. The warming has been driven by a change in Southern Ocean currents, which, in turn appear to result from changes in the ocean heat content gradient between mid and high latitudes and changes in wind. The change is occurring in a biologically highly‐productive area of importance to Orange Roughy and Hoki fisheries. Key Points There has been strong, full‐depth ocean warming since 2006 in a region south of Chatham Islands, New Zealand South of Chatham Islands, the Subtropical Frontal Zone has moved 120 km west The warming is a result of diminished Subantarctic Water flows along northern Campbell Plateau and around Bounty Trough

Aim: We assess biogeographical patterns, population structure and the range of species in the pulmonate genus Siphonaria across the sub-Antarctic. We hypothesized that locally endemic cryptic species will be found across the distribution of these direct-developing limpets in the sub-Antarctic. Location: The sub-Antarctic coasts of the Southern Ocean including South America, the Falkland/Malvinas, South Georgia, Kerguelen and Macquarie Islands. Methods: Multi-locus phylogenetic reconstructions, mtDNA time-calibrated divergence time estimations and population-based analyses of Siphonaria populations were used at the scale of the Southern Ocean. Results: We resolve two widely distributed lineages of Siphonaria (S. lateralis and S. fuegiensis) across the sub-Antarctic. MtDNA divergence time estimates suggest that they were separated around 4.0 Ma (3.0 to 8.0 Ma). Subsequently both species followed different evolutionary pathways across their distributions. Low levels of genetic diversity characterize the populations of both species, reflecting the role of Quaternary glacial cycles during their respective demographic histories, suggesting high levels of dispersal among geographically distant localities. Main conclusions: Siphonaria lateralis and S. fuegiensis constitute sister and broadly co-distributed species across the sub-Antarctic. Unexpected transoceanic similarities and low levels of genetic diversity in both these direct-developing species imply recurrent recolonization processes through long-distance dispersal to isolated sub-Antarctic islands. For such groups of Southern Ocean invertebrates, rafting may be more effective for long-distance dispersal than a free-living planktotrophic larval stage. This biogeographical model may explain why many marine species lacking a dispersal phase exhibit broad distributions, low genetic diversity and low population structure over thousands of kilometres.

Transit time distributions (TTDs) for the Antarctic Intermediate Water (AAIW) in the South Atlantic Ocean are estimated from an eddying ocean model with a twofold scope: validation of the TTD method and identifying pathways of the AAIW. The TTDs are inferred both from Lagrangian particle backtracking and the modeled CFC-11 concentrations, under the assumption that the TTDs can be described with an inverse Gaussian function. A bimodal distribution is obtained for the Lagrangian TTDs with four major subduction regions identified: near the Agulhas retroflection, south of New Zealand, west of the Drake Passage (smallest mean age Γ = 13 years), and in the Argentine basin (largest mean age Γ = 25 years). With the Southern Ocean as source region, the inverse Gaussian is a reasonable representation for the TTDs in the eastern Atlantic basin (40°–35°S, 0°–20°E), whereas the fit for region west (40°–35°S, 60°–40°W) of the mid-Atlantic ridge is not as good and overestimates the TTDs for transit times < 15 years. Mean ages from the modeled CFC-11 are mostly larger (up to 12 years) in the eastern Atlantic basin, and they are mostly smaller than the Lagrangian mean ages in the west. Both methods yield mean ages smaller in the western than in the eastern Atlantic basin and an aging of AAIW from the 1990s to the 2000s that is consistent with reduced flow velocities. The Antarctic Circumpolar Current appears to be the prime determinant of the transit times. The results suggest that the inverse Gaussian, despite assuming 1D advection–diffusion with constant mean flow and diffusivity, is a surprisingly good fit.

Aim We use an integrative biogeographical approach to further understand the evolution of an important Southern Ocean marine benthic element, the limpet genus Nacella (Mollusca: Patellogastropoda). Location Southern Ocean. Methods We used multi-locus time-calibrated phylogeny of Nacella at the scale of the whole Southern Ocean to elucidate the underlying processes involved in the origin and diversification of the genus. Results Divergence-time estimates suggest that soon after its origin during the mid-Miocene (c. 12.5 Ma), Nacella separated into two main lineages currently distributed in (1) South America and (2) Antarctica and the sub-Antarctic islands. We identified two pulses of diversification, during the late Miocene (8 to 5.5 Ma) and the Pleistocene (< 1 Ma). Main conclusions Major periods of climatic and oceanographical change strongly affected the biogeography of Nacella and demonstrate both the long- and short-term influence of the Antarctic Circumpolar Current across the Southern Ocean. Our analyses support the validity of all currently recognized Nacella species and reveal a new South-American lineage. This work constitutes the most detailed molecular-based study of an ecologically important, nearshore invertebrate Southern Ocean group and in so doing contributes to the improved understanding of the underlying patterns and processes in the origin and diversification of marine benthic fauna across this globally important region.

The M 2 internal tide in the Tasman Sea is investigated using sea surface height measurements made by multiple altimeter missions from 1992 to 2012. Internal tidal waves are extracted by two-dimensional plane wave fits in 180 km by 180 km windows. The results show that the Macquarie Ridge radiates three internal tidal beams into the Tasman Sea. The northern and southern beams propagate respectively into the East Australian Current and the Antarctic Circumpolar Current and become undetectable to satellite altimetry. The central beam propagates across the Tasman Sea, impinges on the Tasmanian continental slope, and partially reflects. The observed propagation speeds agree well with theoretical values determined from climatological ocean stratification. Both the northern and central beams refract about 15° toward the equator because of the beta effect. Following a concave submarine ridge in the source region, the central beam first converges around 45.5°S, 155.5°E and then diverges beyond the focal region. The satellite results reveal two reflected internal tidal beams off the Tasmanian slope, consistent with previous numerical simulations and glider measurements. The total energy flux from the Macquarie Ridge into the Tasman Sea is about 2.2 GW, of which about half is contributed by the central beam. The central beam loses little energy in its first 1000-km propagation, for which the likely reasons include flat bottom topography and weak mesoscale eddies.

Satellite data for the past three decades reveal a record‐high sea surface temperature (SST) anomaly within a large mid‐latitude region of the south‐central Pacific (SCP) during the mature phase of the 2009–10 El Niño, with a peak magnitude that is 5 times the standard deviation of local SST anomaly and is warmer than the concurrent tropical‐Pacific SST anomaly. The SCP oceanic warming was confined to the upper 50 meters and is associated with an extreme and persistent anticyclone. Wind changes associated with the anticyclone caused the oceanic warming with surface heat flux and ocean processes playing equally important roles. The anticyclone diverted circumpolar westerlies and warm air towards Antarctica. Austral‐summer SST in the Bellingshausen Sea also reached a three‐decade high. The extreme atmospheric and oceanic anomalies in the South Pacific may have been fueled by the 2009–10 El Niño because of its record‐high SST anomaly in the central‐equatorial Pacific.

Potential sources of decadal climate variability over southern Africa are examined by conducting in-depth analysis of available datasets and coupled general circulation model (CGCM) experiments. The observational data in recent decades show a bidecadal variability noticeable in the southern African rainfall with its positive phase of peak during 1999/2000. It is found that the rainfall variability is related to anomalous moisture advection from the southwestern Indian Ocean, where the anomalous sea level pressure (SLP) develops. The SLP anomaly is accompanied by anomalous sea surface temperature (SST). Both SLP and SST anomalies slowly propagate eastward from the South Atlantic to the southwestern Indian Ocean. The analysis of mixed layer temperature tendency reveals that the SST anomaly in the southwestern Indian Ocean is mainly due to eastward advection of the SST anomaly by the Antarctic Circumpolar Current. The eastward propagation of SLP and SST anomalies are also confirmed in the 270-yr outputs of the CGCM control experiment. However, in a sensitivity experiment where the SST anomalies in the South Atlantic are suppressed by the model climatology, the eastward propagation of the SLP anomaly from the South Atlantic disappears. These results suggest that the local air–sea coupling in the South Atlantic may be important for the eastward propagation of the SLP anomaly from the South Atlantic to the southwestern Indian Ocean. Although remote influences from the tropical Pacific and Antarctica were widely discussed, this study provides new evidence for the potential role of local air–sea coupling in the South Atlantic for the decadal climate variability over southern Africa.

The distribution of the Southern Ocean (SO) biota is the result of major geological, oceanographic, and climate changes during the last 50 million years (Ma). Several groups of marine benthic organisms exhibit marked taxonomic similarities between the Antarctic Peninsula and southern South America, where families, genera, and even species are currently co-distributed in these continents. Several species of macroalgae including Gigartina skottsbergii, Plocamium cartilagineum, and Iridaea cordata are currently found on both sides of the Drake Passage. Advances in molecular techniques have allowed estimating phylogenetic relationships, levels of differentiation and divergence time estimates between populations from these continents in order to determine whether they constitute separate evolutionary units. In this study, we determine whether Iridaea cordata represents the same evolutionary unit in southern South America and the Antarctic Peninsula or if populations on the two sides of the Drake Passage represent different genetic lineages. According to our results, I. cordata populations from the Antarctic Peninsula and South America are clearly distinguishable evolutionary units with 8.31% and 3.17% mtDNA and cpDNA molecular divergence, respectively. The separation between Antarctic and South American populations of I. cordata occurred at the end of the Miocene, between 5 Ma (rbcL) and 9 Ma (COI-5P). These results are similar to those reported in G. skottsbergii on both sides of the Drake Passage. Thus, I. cordata populations on the two sides of the Drake Passage should be considered two sister species. Cryptic speciation plays an important role in the evolution of the Southern Ocean; thus, the systematics, biogeography, and biodiversity of the region require major revisions.