ALBERT

Description: The reflection of a train of two-dimensional finite-amplitude internal waves propagating at an angle β to the horizontal in an inviscid fluid of constant buoyancy frequency and incident on a uniform slope of inclination α is examined, specifically when β 〉 α. Expressions for the stream function and density perturbation are derived to third order by a standard iterative process. Exact solutions of the equations of motion are chosen for the incident and reflected first-order waves. Whilst these individually generate no harmonics, their interaction does force additional components. In addition to the singularity at α = β when the reflected wave propagates in a direction parallel to the slope, singularities occur for values of a and β at which the incident-wave and reflected-wave components are in resonance; strong nonlinearity is found at adjacent values of a and β. When the waves are travelling in a vertical plane normal to the slope, resonance is possible at second order only for α 〈 8.4° and β 〈 30°. At third order the incident wave is itself modified by interaction with reflected components. Third-order resonances are only possible for α 〈 11.8° and, at a given a, the width of the β -domain in which nonlinearities connected to these resonances is important is much less than at second order. The effect of nonlinearity is to reduce the steepness of the incident wave at which the vertical density gradient in the wave field first becomes zero, and to promote local regions of low static stability remote from the slope. The importance of nonlinearity in the boundary reflection of oceanic internal waves is discussed. In an Appendix some results of an experimental study of internal waves are described. The boundary layer on the slope is found to have a three-dimensional structure. © 1987, Cambridge University Press. All rights reserved.

Description: In January and February 2000, four ships conducted an extensive hydrographic survey of the Scotia Sea as part of the CCAMLR 2000 Survey. There were 169 CTD stations to at least 1000 m depth, making this the largest synoptic dataset since 1981. A hydrographic section at Drake Passage was used to define water masses and ocean fronts. In 2000, the Subantarctic Front and the Polar Front were unusually close, and the entire survey occurred to the south of the Polar Front. The survey area was bisected by the Subantarctic Circumpolar Current Front and the Southern Boundary of the Antarctic Circumpolar Current. In Drake Passage, these fronts were widely spaced. A further two hydrographic sections to the east of Drake Passage show that the relative location of these fronts changes east of Drake Passage. Horizontal maps across the survey area show that close to Drake Passage, properties are aligned in a southwest to northeast direction. At approximately 35°W, properties become orientated in a north–south direction. A map of geopotential anomaly shows the flow field across the survey area and allows identification of oceanic fronts. In months previous to the survey, the giant icebergs A22B and B10A crossed the Scotia Sea and closely followed the geopotential field from the CCAMLR 2000 dataset. The SACCF is not the only important front for transporting biological matter from the Antarctic Peninsula to South Georgia; an interaction between the SBACC and the SACCF is also likely to be important.

Description: Published

Description: In January and February 2000, four ships conducted an extensive hydrographic survey of the Scotia Sea as part of the CCAMLR 2000 Survey. There were 169 CTD stations to at least 1000 m depth, making this the largest synoptic dataset since 1981. A hydrographic section at Drake Passage was used to define water masses and ocean fronts. In 2000, the Subantarctic Front and the Polar Front were unusually close, and the entire survey occurred to the south of the Polar Front. The survey area was bisected by the Subantarctic Circumpolar Current Front and the Southern Boundary of the Antarctic Circumpolar Current. In Drake Passage, these fronts were widely spaced. A further two hydrographic sections to the east of Drake Passage show that the relative location of these fronts changes east of Drake Passage. Horizontal maps across the survey area show that close to Drake Passage, properties are aligned in a southwest to northeast direction. At approximately 35°W, properties become orientated in a north–south direction. A map of geopotential anomaly shows the flow field across the survey area and allows identification of oceanic fronts. In months previous to the survey, the giant icebergs A22B and B10A crossed the Scotia Sea and closely followed the geopotential field from the CCAMLR 2000 dataset. The SACCF is not the only important front for transporting biological matter from the Antarctic Peninsula to South Georgia; an interaction between the SBACC and the SACCF is also likely to be important.

Description: Published

Description: A comparison between hydrographic observations and output from two realistically forced z-level global ocean circulation models (OCCAM and POCM_4C) in the Scotia Sea, South Atlantic, is described. The study region includes the southern part of the Antarctic Circumpolar Current (ACC) and the northern Weddell Gyre. Despite similar formulations, the models have different strengths and weaknesses. OCCAM simulates well the horizontal circulation around South Georgia but loss of Antarctic Bottom Water distorts the mean circulation in the central Scotia Sea. A poorer bathymetric dataset in POCM_4C means that the circulation is not adequately topographically steered leading to greater zonal flow and a southward shift of the fronts of the southern ACC. In a comparison with sea surface height variability data, OCCAM overestimates and POCM_4C underestimates the maximum values. Both models have higher background variability than the satellite data. Mean monthly model output is compared with a meridional hydrographic section from the study region. The regional water masses at the time of the hydrographic section (April 1995) are recognisably reproduced in both models despite some discrepancies. The surface waters are too saline in OCCAM (by 0.12-0.40) and too warm in POCM_4C (by 〉2 °C) suggesting problems with the air-sea surface heat and freshwater fluxes used to force both models and the models' vertical mixing parameterisations. Anomalous mixed layer properties in winter lead to inaccurate Winter Water characteristics in both models. Slumping of Circumpolar Deep Water occurs in OCCAM, associated with the loss of the bottom water. Subsurface restoration to climatology at buffer zones prevents this slumping in POCM_4C although the densest waters are not reproduced. The models overestimate the baroclinic transport of the section by up to a factor of two and simulate a significant barotropic component of transport. Overall, both models can be used in this region in ways that utilise their strengths. Further improvements are likely to come from better bathymetric representations, surface fluxes, and bottom water formation processes, elimination of spurious diapycnal mixing, improvement of vertical mixing parameterisations, and higher resolution

Description: Published

Description: Conductivity-temperature-pressure satellite relay data loggers (CTD-SRDLs) were attached to southern elephant seals (Mirounga leonina) on the island of South Georgia. During the animals' migration the CTD-SRDLs recorded and transmitted hydrographic profiles at a rate of approximately two profiles per day to an average depth of about 547 m, representing transect-type sections with a spatial resolution of 16-47 km along the migratory routes of the seals. Two sections are used to clearly identify the locations of the Antarctic Circumpolar Current fronts across Drake Passage, providing in situ data complementary to satellite and other techniques. An empirical relationship between upper ocean temperature and baroclinic mass transport is used to determine the transport through Drake Passage at the times of the sections, and these transports are compared with estimates derived by other techniques. An absolute geostrophic velocity section across Drake Passage is calculated using CTD-SRDL data and data of absolute geostrophic surface velocities from altimetry. The mean total baroclinic transports in June 2004 and April 2005 are estimated to be 124 X 10(6) +/- 14 X 10(6) m(3) s(-1) and 112 X 10(6) +/- 14 X 10(6) m(3) s(-1) respectively.

Description: Published

Description: Antarctic krill (Euphausia superba) and Ice krill (Euphausia crystallorophias) are key species within Southern Ocean marine ecosystems. Given their importance in regional food webs, coupled with the uncertain impacts of climate change, the on-going recovery of krill-eating marine mammals, and the expanding commercial fishery for Antarctic krill, there is an increasing need to improve current estimates of their circumpolar habitat distribution. Here, we provide an estimate of the austral summer circumpolar habitat distribution of both species using an ensemble of habitat models and updated environmental covariates. Our models were able to resolve the segregated habitats of both species. We find that extensive potential habitat for Antarctic krill is mainly situated in the open ocean and concentrated in the Atlantic sector of the Southern Ocean, while Ice krill habitat was concentrated more evenly around the continent, largely over the continental shelf. Ice krill habitat was mainly predicted by surface oxygen concentration and water column temperature, while Antarctic krill was additionally characterized by mixed layer depth, distance to the continental shelf edge, and surface salinity. Our results further improve understanding about these key species, helping inform sustainable circumpolar management practices.