Suchergebnisse

An overturning circulation, driven by prescribed buoyancy forcing, is used to set a zonal volume transport in a reentrant channel ocean model with three isopycnal layers. The channel is designed to represent the Southern Ocean such that the forced overturning resembles the lower limb of the meridional overturning circulation (MOC). The relative contributions of wind and buoyancy forcing to the zonal circulation are examined. It is found that the zonal volume transport is strongly dependent on the buoyancy forcing and that the eddy kinetic energy is primarily set by wind stress forcing. The zonal momentum budget integrated over each layer is considered in the buoyancy-forced, wind-forced, and combined forcing case. At equilibrium, sources and sinks of momentum are balanced, but the transient spinup reveals the source of momentum for the current. In the buoyancy-forced case, the forcing creates a baroclinic shear with westward flow in the lower layer, allowing topographic form stress and bottom friction to act as the initial sources of eastward momentum, with bottom friction acting over a longer time frame. In the wind-forced and combined forcing cases, the surface wind stress dominates the initial momentum budget, and the time to reach equilibration is shorter in the combined forcing simulation. These results imply that future changes in the rate of formation of Antarctic Bottom Water may alter the volume transport of the Antarctic Circumpolar Current. ; EH gratefully acknowledges the support of an A. L. Hales Scholarship provided by RSES. AMH was supported by an Australian Research Council Future Fellowship FT120100842. This research was undertaken on the NCI National Facility in Canberra, Australia, which is supported by the Australian Commonwealth Government.

Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) control the abyssal limb of the global overturning circulation and play a major role in oceanic heat uptake and carbon storage. However, current general circulation models are unable to resolve the observed AABW and NADW formation and transport processes. One key process, that of overflows, motivates the application of overflow parameterisations. We present a sensitivity study of both AABW and NADW properties to three current parameterisations using a z*-coordinate ocean-sea ice model within a realistic-topography sector of the Atlantic Ocean.Overflow parameterisations that affect only tracer equations are compared to a fully dynamical Lagrangian point particle method. An overflow parameterisation involving partial convective mixing of tracers is most efficient at transporting dense NADW water downslope. This parameterisation leads to a maximum mean increase in density in the north of 0.027kgm-3 and a decrease in age of 525years (53%). The relative change in density and age in the south is less than 30% of that in the north for all overflow parameterisations. The reduced response in the south may result from the differing dense water formation and overflow characteristics of AABW compared to NADW. Alternative approaches may be necessary to improve AABW representation in z*-coordinate ocean climate models. ; BMS was supported by the Australian Government Department of the Environment, and CSIRO through the Australian Climate Change Science Programme. AMH was supported by an Australian Research Council Future Fellowship FT120100842. SMH was supported by the ARC Centre of Excellence for Climate System Science (Grant CE110001028)

Antarctica has long been considered biologically isolated 1. Global warming will make parts of Antarctica more habitable for invasive taxa, yet presumed barriers to dispersal—especially the Southern Ocean's strong, circumpolar winds, ocean currents and fronts—have been thought to protect the region from non-anthropogenic colonizations from the north1,2. We combine molecular and oceanographic tools to directly test for biological dispersal across the Southern Ocean. Genomic analyses reveal that rafting keystone kelps recently travelled >20,000 km and crossed several ocean-front 'barriers' to reach Antarctica from mid-latitude source populations. High-resolution ocean circulation models, incorporating both mesoscale eddies and wave-driven Stokes drift, indicate that such Antarctic incursions are remarkably frequent and rapid. Our results demonstrate that storm-forced surface waves and ocean eddies can dramatically enhance oceanographic connectivity for drift particles in surface layers, and show that Antarctica is not biologically isolated. We infer that Antarctica's long-standing ecological differences have been the result of environmental extremes that have precluded the establishment of temperate-adapted taxa, but that such taxa nonetheless frequently disperse to the region. Global warming thus has the potential to allow the establishment of diverse new species—including keystone kelps that would drastically alter ecosystem dynamics—even without anthropogenic introductions. ; The oceanographic modelling was undertaken on the National Computational Infrastructure in Canberra, Australia, which is supported by the Australian Commonwealth Government. The research was funded by Australian Research Council grants to C.I.F. (DE140101715 and FT170100281) and A.K.M. (DE170100184), and Fondap-IDEAL grant 15150003 from CONICYT-Chile to E.C.M. and N.V.

7 pages, 2 figures ; High-resolution satellite images of ocean color and sea surface temperature reveal an abundance of ocean fronts, vortices and filaments at scales below 10 km but measurements of ocean surface dynamics at these scales are rare. There is increasing recognition of the role played by small scale ocean processes in ocean-atmosphere coupling, upper-ocean mixing and ocean vertical transports, with advanced numerical models and in situ observations highlighting fundamental changes in dynamics when scales reach 1 km. Numerous scientific publications highlight the global impact of small oceanic scales on marine ecosystems, operational forecasts and long-term climate projections through strong ageostrophic circulations, large vertical ocean velocities and mixed layer re-stratification. Small-scale processes particularly dominate in coastal, shelf and polar seas where they mediate important exchanges between land, ocean, atmosphere and the cryosphere, e.g., freshwater, pollutants. As numerical models continue to evolve toward finer spatial resolution and increasingly complex coupled atmosphere-wave-ice-ocean systems, modern observing capability lags behind, unable to deliver the high-resolution synoptic measurements of total currents, wind vectors and waves needed to advance understanding, develop better parameterizations and improve model validations, forecasts and projections. SEASTAR is a satellite mission concept that proposes to directly address this critical observational gap with synoptic two-dimensional imaging of total ocean surface current vectors and wind vectors at 1 km resolution and coincident directional wave spectra. Based on major recent advances in squinted along-track Synthetic Aperture Radar interferometry, SEASTAR is an innovative, mature concept with unique demonstrated capabilities, seeking to proceed toward spaceborne implementation within Europe and beyond ; CG and AM received funding from the United Kingdom Centre for Earth Observation Instrumentation SEASTAR+ project (Contract No. RP10G0435A02). PVL was supported by the European Research Council (ERC) CUNDA project 694509 under the European Union Horizon 2020 Research and Innovation Program ; Peer Reviewed