Chaotic and regular trajectories in the Antarctic Circumpolar Current

Methods from chaos theory are applied to the analysis of the circulation in the Southern Ocean, using velocity fields produced by a realistic global ocean model. We plot the intersections of individual trajectories encircling Antarctica with a vertical plane in the Drake passage. This so-called Poincaré section shows a drastic difference between regular trajectories in a core region of the Antarctic Circumpolar Current (ACC), and chaotic, mixing trajectories in the surrounding region. It also shows that there is a region with overturning circulation of approximately 3.5 Sv in the ACC, with downwelling on the northern side and upwelling on the southern side, which may be related to the Deacon cell.     

Long-Term Linear Meridional Shift of the Jet Structure of the Antarctic Circumpolar Current South of Africa

Oceanology - The meridional shift of the absolute dynamic topography (ADT) gradient field structure in the zone of the Antarctic Circumpolar Current (ACC) south of Africa (10° E–25°...     

Interaction between a synoptic eddy and a Rossby wave

The behaviour of an isolated synoptic eddy on the -plane in the field of a plane Rossby wave is reported. As is shown, the eddy exchanges its energy intensively with Rossby waves having zero frequency in the frame of axes moving with the eddy. If the Rossby wave has an amplitude exceeding some critical value, a transition from average translational eddy motion along the meridian to oscillations in the vicinity of the equilibrium latitude is possible.Translated by Mikhail M. Trufanov. UDK 551.465.535:551.466.     

The Southern Antarctic Circumpolar Current Front: physical and biological coupling at South Georgia

The coupling of physics and biology was examined along a 160 km long transect running out from the north coast of South Georgia Island and crossing the Southern Antarctic Circumpolar Current Front (SACCF) during late December 2000. Surface and near surface potential TS properties indicated the presence of three water types: a near-shore group of stations characterised by water which became progressively warmer and fresher closer to South Georgia, an offshore grouping in which sea surface temperatures and those at the winter water level were relatively warm (1.8°C and 0.5°C, respectively), and a third in which surface and winter water temperatures were cooler and reflected the presence of the SACCF. The transect bisected the SACCF twice, revealing that it was flowing in opposite directions, north-westward closest to South Georgia and south-eastwards at its furthest point from the island. The innermost limb was a narrow intense feature located just off the shelf break in 2000–3500 m of water and in which rapid surface baroclinic velocities (up to 35 cm s⁻¹) were encountered. Offshore in the outermost limb, shown subsequently to be a mesoscale eddy that had meandered south from the retroflected limb of the SACCF, flow was broader and slower with peak velocities around 20 cm s⁻¹. Chlorophyll a biomass was generally low (<1 mg m⁻³) over much of the transect but increased dramatically in the region of the innermost limb of the SACCF, where a deepening of the surface mixed layer was coincident with a subsurface chlorophyll maximum (7.4 mg m⁻³) and elevated concentrations down to 100 m. The bloom was coincident with depleted nutrient concentrations, particularly silicate, nitrate and phosphate, and although ammonium concentrations were locally depleted the bloom lay within an elevated band (up to 1.5 mmol m⁻³) associated with the frontal jet. Increased zooplankton abundance, higher copepod body carbon mass and egg production rates all showed a strong spatial integrity with the front. The population structure of the copepods Calanoides acutus and Rhincalanus gigas at stations within the front suggested that rather than simply resulting from entrainment and concentration within the jet, increased copepod abundance was the result of development in situ. Estimates of bloom duration, based on silicate and carbon budget calculations, set the likely duration between 82 and 122 d, a figure supported by the development schedule of the two copepod species. Given this timescale, model outputs from FRAM and OCCAM indicated that particles that occurred on the north side of South Georgia in December would have been in the central-southern Scotia Sea 2–3 months earlier, probably in sea ice affected regions.     

Energy Exchange between Jets of the Antarctic Circumpolar Current and Synoptic Eddies in the Drake Passage and Scotia Sea

We study the energy exchange between jets of the Antarctic Circumpolar Current (ACC) and synoptic eddies generated by them in the surface layer of the ocean in the Drake Passage and Scotia Sea based on 22-year-long satellite altimetry time series from the French CLS Agency (DT Global–MADT–Upd product, http://www.aviso.altimetry.fr) under the assumption, based on observations, that each of the jets is confined between two fixed contour lines of the absolute dynamic topography of the ocean. We calculate and analyze the 22-year evolution of the kinetic energy of each ACC jet and cyclonic and anticyclonic eddies generated by it. We demonstrate the fundamental dependence of fluctuations in jet energy on the phase of their meander and eddy formation, as well as their back absorption by jets. We calculate the mean and extreme energetic characteristics of jets and eddies and compare the jets in terms of the intensity of the generated eddies.     

The Scotia Sea and the Drake Passage as an orographic barrier for the Antarctic Circumpolar Current

It is shown on the basis of the data of the Russian Academy of Sciences expeditions in 2003–2010, the historical CTD database, the WOCE climatology, and the satellite altimetry that the area of the Scotia Sea and the Drake Passage is even a greater significant orographic barrier for the eastward Antarctic Circumpolar Current (ACC) than was previously thought. It is the current concept that this barrier is the most important for the ACC; it consists of three obstacles: the Hero Ridge with the Phoenix Rift, the Shackleton Ridge, and the North Scotia Ridge with the relatively shallow eastern part of the Scotia Sea. Despite the fact that all three obstacles are permeable for the layer of the Circumpolar Bottom Water (CBW; 28.16 < γ ⁿ < 28.26) being considered the lower part of the circumpolar water, the circulation in this layer throughout the Scotia Sea and the Drake Passage quite substantially differs from the transfer by the surface-intensified ACC jets. Herewith, the upper CBW boundary is the lower limit of the circumpolar coverage of the ACC jets. This result is confirmed by the near zero estimate of the total CBW transport according to the three series of the LADCP measurements on the sections across the Drake Passage. It is shown that the transformation (cooling and freshening) of the CBW layer, which occurs owing to the flow of the ACC over the Shackleton Ridge, is associated with the shape and location of the ridge in the Drake Passage. The high southern part of this ridge is a partially permeable screen for the eastward CBW transport behind which the colder and fresher waters of the Weddell Sea and the Bransfield Strait of the same density range as the CBW penetrate into the ACC zone. The partial permeability of the Shackleton Ridge for the CBW layer leads to the salinization of this layer on the eastern side of the ridge and to the CBW’s freshening on the western side of this ridge, which is observed across the entire Drake Passage.     

CO2 in the Weddell Gyre and Antarctic Circumpolar Current: austral autumn and early winter

Quasi-continuous fugacity of CO₂ (fCO₂) data were collected in the eastern Weddell Gyre and southern Antarctic Circumpolar Current (ACC) of the Southern Ocean during austral autumn 1996. Full depth Total CO₂ (TCO₂) sections are presented for austral autumn and winter (1992) cruises. Pronounced fCO₂ gradients were observed at the Southern Ocean fronts. In the Weddell Gyre, fCO₂ regimes appeared to coincide with surface and subsurface hydrographic regimes. The southern ACC was supersaturated with respect to CO₂, as was part of the northern Weddell Gyre. The southern Weddell Gyre was markedly undersaturated. The great potential of autumn cooling for generating undersaturation and CO₂ uptake from the atmosphere was demonstrated. In the northeastern Weddell Gyre, upwelling of CO₂- and salt-rich deep water was shown to play a role as the horizontal fCO₂ distribution closely resembled that of the surface salinity. The total uptake of atmospheric CO₂ by the Weddell Gyre in autumn (45 days) was calculated to be 7·10¹² g C. The deep TCO₂ distribution noticeably reflected the different water masses in the region. A new deep TCO₂ maximum was detected in the ACC, which apparently characterizes the boundary between the equatorward flowing Antarctic Bottom Water (AABW) and the Circumpolar Deep Water (CDW). East of the Weddell Gyre, the AABW stratum is much thicker (>2000 m) than more to the west, on the prime meridian (<300 m).     

Climatic variability of transport in the upper layer of the Antarctic Circumpolar Current from hydrological and satellite data

Based on the satellite altimetry dataset of sea level anomalies, the climatic hydrological database World Ocean Atlas-2009, ocean reanalysis ECMWF ORA-S3, and wind velocity components from NCEP/NCAR reanalysis, the interannual variability of Antarctic Circumpolar Current (ACC) transport in the ocean upper layer is investigated for the period 1959–2008, and estimations of correlative connections between ACC transport and wind velocity components are performed. It has been revealed that the maximum (by absolute value) linear trends of ACC transport over the last 50 years are observed in the date-line region, in the Western and Eastern Atlantic and the western part of the Indian Ocean. The greatest increase in wind velocity for this period for the zonal component is observed in Drake Passage, at Greenwich meridian, in the Indian Ocean near 90° E, and in the date-line region; for the meridional component, it is in the Western and Eastern Pacific, in Drake Passage, and to the south of Africa. It has been shown that the basic energy-carrying frequencies of interannual variability of ACC transport and wind velocity components, as well as their correlative connections, correspond to the periods of basic large-scale modes of atmospheric circulation: multidecadal and interdecadal oscillations, Antarctic Circumpolar Wave, Southern Annual Mode, and Southern Oscillation. A significant influence of the wind field on the interannual variability of ACC transport is observed in the Western Pacific (140° E–160° W) and Eastern Pacific; Drake Passage and Western Atlantic (90°–30° W); in the Eastern Atlantic and Western Indian Ocean (10°–70° E). It has been shown in the Pacific Ocean that the ACC transport responds to changes of the meridional wind more promptly than to changes of the zonal wind.     

Fronts, baroclinic transport, and mesoscale variability of the Antarctic Circumpolar Current in the southeast Indian Ocean

Fronts,baroclinic transport,and mesoscale variability of the Antarctic Circumpolar Current(ACC) along 115°E are examined on the basis of CTD data from two hydrographic cruises occupied in 1995 as a part of the World Ocean Circulation Experiment(WOCE cruise I9S) and in 2004 as a part of CLIVAR/CO2 repeat hydrography program.The integrated baroclinic transport across I9S section is(97.2×106±2.2×106) m3/s relative to the deepest common level(DCL).The net transport at the north end of I9S,determined by the south Australian circulation system,is about 16.5×106m3/s westward.Relying on a consistent set of water mass criteria and transport maxima,the ACC baroclinic transport,(117×106 ±6.7×10 6)m3/s to the east,is carried along three fronts:the Subantarctic Front(SAF) at a mean latitude of 44°-49°S carries(50.6×10 6 ±13.4×106)m3/s;the Polar Front(PF),with the northern branch(PF-N) at 50.5°S and the southern branch(PFS) at 58°S,carries(51.3×106 ±8.7×106)m3/s;finally,the southern ACC front(SACCF) and the southern boundary of the ACC(SB) consist of three cores between 59°S and 65°S that combined carry(15.2×106 ±1.8×106)m3/s.Mesoscale eddy features are identifiable in the CTD sections and tracked in concurrent maps of altimetric sea level anomalies(SLA) between 44°-48°S and 53°-57°S.Because of the remarkable mesoscale eddy features within the SAF observed in both the tracks of the cruises,the eastward transport of the SAF occurs at two latitude bands separating by 1°.Both the CTD and the altimetric data suggest that the mesoscale variability is concentrated around the Antarctic Polar Frontal Zone(APFZ) and causes the ACC fronts to merge,diverge,and to fluctuate in intensity and position along their paths.     

Energy of the Jets of the Antarctic Circumpolar Current and of Their Eddies in the Surface Layer of the Southern Ocean

Oceanology - The kinetic energy of six jets of the Antarctic Circumpolar Current (ACC) and of the cyclonic and anticyclonic mesoscale eddies generated by these jets is studied in application to the...     

Circumpolar connections between Antarctic krill (Euphausia superba Dana) populations: Investigating the roles of ocean and sea ice transport

Antarctic krill, Euphausia superba Dana, has a heterogeneous circumpolar distribution in the Southern Ocean. Krill have a close association with sea ice which provides access to a critical food source and shelter, particularly in the early life stages. Advective modelling of transport pathways of krill have until now been on regional scales and have not taken explicit account of sea ice. Here we present Lagrangian modelling studies at the circumpolar scale that include interaction with sea ice. The advection scheme uses ocean velocity output from the Ocean Circulation and Climate Advanced Modelling (OCCAM) project model together with satellite-derived sea ice motion vectors to examine the potential roles of the ocean and sea ice in maintaining the observed circumpolar krill distribution. We show that the Antarctic Coastal Current is likely to be important in generating the large-scale distribution and that sea ice motion can substantially modify the ocean transport pathways, enhancing retention or dispersal depending upon location. Within the major krill region of the Scotia Sea, the effect of temporal variability in both the ocean and sea ice velocity fields is examined. Variability in sea ice motion increases variability of influx to South Georgia, at times concentrating the influx into pulses of arrival. This variability has implications for the ecosystem around the island. The inclusion of sea ice motion leads to the identification of source regions for the South Georgia krill populations additional to those identified when only ocean motion is considered. This study indicates that the circumpolar oceanic circulation and interaction with sea ice is important in determining the large-scale distribution of krill and its associated variability.     

Drifting and Meandering of Olive Ridley Sea Turtles in the Bay of Bengal: Role of Oceanic Rossby Waves

Olive Ridley turtles in the Bay of Bengal are previously thought to migrate southward from their nesting ground, along the east coast of India (Orissa coast), towards Sri Lanka. Surprisingly, three of the four Platform Transmitter Terminal (PTT) attached turtles in April-June 2001 meandered off the east coast of India for about two months. It is found that these turtles meandered at the peripherals of cold core cyclonic eddy surrounded by warm core eddies on either side. Concentrations of prey for the turtles in those frontal regions are known to be abundant. Only one of the four PTT attached turtles migrated to the south along the frontal regions in the direction of geostrophic currents. It is found that the locations of these thermal fronts in the Bay of Bengal are primarily determined by the Oceanic Rossby waves and local Ekman pumping.     

The East Caledonian Current: A Case Example for the Intercomparison between AltiKa and In Situ Measurements in a Boundary Current

This paper presents an assessment of SARAL/AltiKa satellite altimeter for the monitoring of a tropical western boundary current in the south-western Pacific Ocean: the East Caledonian Current. We compare surface geostrophic current estimates obtained from two versions of AltiKa along-track sea level height (AVISO 1 Hz and PEACHI 40 Hz) with two kinds of dedicated in situ datasets harvested along the satellite ground tracks: one deep-ocean current-meter mooring deployed in the core of the boundary current and five glider transects. It is concluded that the AltiKa-derived current successfully captures the velocity of the boundary current, with a standard error of 11 cm/s with respect to the in situ data. It also appears important to reference AltiKa sea level anomaly to the latest mean dynamic topography available in our area. Doing so, Ka-band altimetry provides a satisfactory representation of the western boundary current. Thereby, it usefully contributes to observing its variability in such a remote and under-observed ocean region. However, the rather long repeat period of SARAL (35 days) in comparison to the high frequency variability seen in the flow velocity of the boundary current calls for a combined use of SARAL with the other satellite altimetry missions.     

Potential Vorticity in the Ocean: Ertel and Rossby Approaches with Estimates for the Lofoten Vortex

Izvestiya, Atmospheric and Oceanic Physics - The potential vorticity (PV) in the ocean is considered, including the history of the term. Various aspects and basic formulas used to calculate the PV...     

A proof and applications of the expansion theorem for the Rossby normal modes in a closed rectangular basin

An expansion theorem is derived for Rossby normal modes in a closed rectangular basin and the set of Rossby normal modes is proved to be complete. This theorem provides a general linear solution to the initial value problem as well as to the response problem. In particular, the Green's function is obtained for the instantaneous localized torque anywhere in the basin. Weakly nonlinear versions are solved also by the combination of the general linear solution with the asymptotic expansion in terms of small amplitude. Further, an application is suggested to the spectral method of numerical simulation based on Rossby normal modes relevant to the more nonlinear evolution equation on a-plane, instead ofsin functions or Chebyshev polynomials, which have been employed conventionally for this purpose.     

ENSO to multi-decadal time scale changes in East Australian Current transports and Fort Denison sea level: Oceanic Rossby waves as the connecting mechanism

The connection between East Australian Current (EAC) transport variability and Australia’s east coast sea level has received little treatment in the literature. This is due in part to the complex interacting physical processes operating in the coastal zone combined with the sparsity of observations available to improve our understanding of these possible connections. This study demonstrates a statistically significant (at the >90% level) relationship between interannual to decadal time scale variations in observed estimates of the EAC transport changes and east coast sea level measured at the high-quality, long record Fort Denison tide-gauge in Sydney Harbour, Australia (33°51′18″S, 151°13′32″E). We further demonstrate, using a linear reduced-gravity ocean model, that ENSO to decadal time-scale variations and the ocean-adjusted multi-decadal trend (approx. 1 cm/decade) in observed sea level at Fort Denison are strongly connected to modulations of EAC transports by incoming westward propagating oceanic Rossby waves. We show that EAC transport and Fort Denison sea level vary in a manner expected from both Tasman Sea generated Rossby waves, which account for the interannual and multi-annual variability, and remotely forced (from east of New Zealand) Rossby wave connections through the mid-latitudes, accounting for the ocean-adjusted multi-decadal trend observed at the New South Wales coast - with the regional-Tasman Sea forcing explaining the greatest overall proportion of EAC transport and sea-level variances.     

Nutrient distributions and new production in polar regions: parallels and contrasts between the Arctic and Antarctic

Polar regions are poorly understood components of global biogeochemical cycles. This paper discusses the influences of nitrate and ammonium concentrations on nitrate uptake (and hence new production), particularly with regard to data collected within marginal ice zones in the Arctic and Antarctic. Subsurface ammonium maxima in waters over 150 m are frequently encountered in the Arctic and occasionally in the Antarctic. Such maxima result from the heterotrophic remineralization of organic matter, and because stratified environments occur more frequently in the Arctic, significant concentrations of ammonium accumulate as a result of lower diffusive losses. Causal agents (bacteria vs. Zooplankton) may also be different in the Arctic from those in the Antarctic. Elevated ammonium concentrations significantly reduce nitrate uptake, and it is suggested that this nutrient interaction may play a significant role in controlling new production, particularly in open water regions. The new production of the Southern Ocean is estimated, and it is suggested that the production and flux from the surface layer is significant relative to the world's oceans.     

Interaction between the surface and internal waves: The modulation and maser mechanisms

Two feedback mechanisms are considered which emerge from the interaction between the surface and internal waves. The energy exchange between the wave systems is coupled with the periodic variations of momentum and the losses of the wind wave momentum during their interaction with the internal wave (IW) current field. These mechanisms result in the IW attenuation which is the strongest when IW and wind propagate in the same direction. When attenuating, the IWs give rise to the inertial currents comparable to the Ekman background current.Translated by Mikhail M. Trufanov. UDK 551.466.3: 551.466.8.     

Exploitation of Antarctic marine living resources: a brief history and a possible approach to managing the krill fishery

Four major phases in the exploitation of Antarctic marine living resources are identified and briefly described, viz. exploitation of seals, whales, finfish and krill Euphausia superba Dana. The pattern of exploitation has shifted from one group of resources to another. The recent efforts of the Commission for the Conservation of Antarctic Marine Living Resources are considered in relation to management of finfish exploitation and to the development of a scientifically based management plan for krill. Special emphasis is given to the data required for the effective management of the krill fishery, per se, given the current limited knowledge of the potential yield of the resource and in the face of uncertainty concerning key aspects of the species' biology/ecology. A recent South African initiative aimed at developing a practical management strategy for the Antarctic krill resource is described.