Representing Grounding Line Dynamics in Numerical Ice Sheet Models: Recent Advances and Outlook

Recent satellite observations of the Antarctic and Greenland ice sheets show accelerated ice flow and associated ice sheet thinning along coastal outlet glaciers in contact with the ocean. Both processes are the result of grounding line retreat due to melting at the grounding line (the grounding line is the contact of the ice sheet with the ocean, where it starts to float and forms an ice shelf or ice tongue). Such rapid ice loss is not yet included in large-scale ice sheet models used for IPCC projections, as most of the complex processes are poorly understood. Here we report on the state-of-the art of grounding line migration in marine ice sheets and address different ways in which grounding line migration can be attributed and represented in ice sheet models. Using one-dimensional ice flow models of the ice sheet/ice shelf system we carried out a number of sensitivity experiments with different spatial resolutions and stress approximations. These are verified with semi-analytical steady state solutions. Results show that, in large-scale finite-difference models, grounding line migration is dependent on the numerical treatment (e.g. staggered/non-staggered grid) and the level of physics involved (e.g. shallow-ice/shallow-shelf approximation).     

Recovery of thermohaline circulation under CO2 stabilization and overshoot scenarios

In this study we examine the behavior of the thermohaline circulation, as simulated by the Community Climate System Model version 3 (CCSM3), for several centuries following CO₂ stabilization for the SRES B1 and A1B scenarios and for an “overshoot” scenario in which CO₂ levels temporarily reach the same level as in the A1B scenario before declining to an ultimate stabilization level that is identical to the B1 case. While we find no evidence for irreversible changes of the thermohaline circulation in the overshoot experiment, the interplay of the different timescales of the temperature response of the surface and interior ocean does lead to a number of differences in the long-term response of the ocean between it and the B1 stabilization scenario where the same GHG levels are approached by different paths. The stronger initial warming and its slow penetration into the deeper ocean, followed by a transient surface cooling in the overshoot scenario leads to lower static stability, deeper mixing, and a more rapid recovery of the thermohaline circulation than in the B1 stabilization scenario. While the overshoot scenario recovers surface conditions (e.g. SST, sea ice extent) very similar to the B1 scenario shortly after reaching the same GHG levels, the additional accumulation of heat in the interior ocean during the period of higher forcing causes the global mean ocean temperature and steric sea level to remain higher than in the B1 stabilization scenario for at least another several centuries.     

Isolated versus common envelope dynamos in planetary nebula progenitors

The origin, evolution and role of magnetic fields in the production and shaping of proto-planetary nebulae (PPNe) and planetary nebulae (PNe) are a subject of active research. Most PNe and PPNe are axisymmetric with many exhibiting highly collimated outflows; however, it is important to understand whether such structures can be generated by isolated stars or require the presence of a binary companion. Towards this end, we study a dynamical, large-scale α−Ω interface dynamo operating in a 3.0 M_⊙ Asymptotic Giant Branch (AGB) star in both an isolated setting and a setting in which a low-mass companion is embedded inside the envelope. The back reaction of the fields on the shear is included and differential rotation and rotation deplete via turbulent dissipation and Poynting flux. For the isolated star, the shear must be resupplied in order to sufficiently sustain the dynamo. Furthermore, we investigate the energy requirements that convection must satisfy to accomplish this by analogy to the Sun. For the common envelope case, a robust dynamo results, unbinding the envelope under a range of conditions. Two qualitatively different types of explosion may arise: (i) magnetically induced, possibly resulting in collimated bipolar outflows and (ii) thermally induced from turbulent dissipation, possibly resulting in quasi-spherical outflows. A range of models is presented for a variety of companion masses.     

Alfvén instabilities in streaming plasmas with anisotropic pressures and their relevance for the solar wind

Low frequency or Alfvén waves in streaming plasmas can become unstable when the square of the Alfvén velocity is smaller than the mean square of the bulk motion in a co-moving reference frame, (u –u )², whereu stands for the bulk velocity of each species and u is the average bulk velocity of the plasma as a whole. For these new Alfvén instabilities the streaming effects can be enhanced by a suitable pressure anisotropy. Perpendicular pressure effects are stabilizing, parallel pressure effects are destabilizing, as in the usual firehose instability. The observed velocity differences between helium and the main (hydrogen) flow in the solar wind plasma are such that the Alfvén waves are getting close to marginal instability. These new Alfvén instabilities limit the velocity differences between helium and hydrogen and thus provide a possible mechanism for accelerating the helium particles up to the order of the main flow velocity.     

Volatile halocarbons released from Arctic macroalgae

Twenty-two different species of Arctic brown, red and green macroalgae, collected in the Kongsfjord at Ny-Ålesund (Spitsbergen), were incubated under polar conditions and investigated for their release of volatile halogenated organic compounds (VHOC). Bromoform, dibromomethane, dibromochloromethane, bromodichloromethane, 1,2-dibromoethane, diiodomethane and chloroiodomethane have been identified and their net releases during incubations were determined. Generally, brown and green macroalgae showed higher VHOC release, while red macroalage had only low release. Bromoform was released in relatively large quantities from all species studied, with the highest release observed from the brown algae Dictyosyphon foeniculaceus (0.3 μg g⁻¹ wet algal weight day⁻¹) and Laminaria saccharina (0.15 μg g⁻¹ wet algal weight day⁻¹), and from the green algae Monostroma arcticum (0.3 μg g⁻¹ wet algal weight day⁻¹) and Blidingia minima (0.27 μg g⁻¹ wet algal weight day⁻¹). Dibromomethane, diiodomethane, dibromochloromethane and 1,2-dibromoethane showed lower net release during the incubations. The net release of chloroiodomethane and bromodichloromethane was very low for the most algae species investigated. Based on the distribution of these algae in the Arctic environment, Dictyosiphon foeniculaceus and Laminaria saccharina may be important sources for VHOC because of high release and high biomass. Release of VHOC could be detected from all parts of the thallus of the macroalga. This may provide some evidence for a possible role of VHOC production as a chemical protection mechanism in algae.     

An improved Ewing heat probe frame

The standard Ewing heat probe frame has been improved through the design of a new takedown model whose parts are held together by lynch pins. When a 600 lb (272 kg) unit of this type is taken apart, the heaviest single piece of the dismantled unit weighs only 75 lb (34 kg).This makes it feasible for one person to handle, transport, and assemble the new unit. A central, main support member of a heat probe using this frame can be suspended over the side of a research vessel while the heavier temperature recorder and ballast weights are attached by means of the quickaction lynch pins. This makes it possible to use this device on relatively small vessels which are not designed for coring work, having a minimum of free deck space and without launching platforms or hydraulic A-frame booms.     

History and sand budgets of the barrier island system in the Panama City,Florida, region

Shell Island and Crooked Island peninsulas comprise the Holocene barrier system in the Panama City, Florida, region. Both of these peninsulas have experienced a history of integration-fragmentation-integration superimposed on a net shoreward migration since 1779. Catastrophic events such as hurricanes are most probably responsible for the pass cutting which has fragmented these barriers. Both barrier peninsulas have very nearly segmented St. Andrew Sound into an eastern and western portion. This segmentation has been achieved through natural causes in the eastern or Crooked Island region, but an alternation in the tidal return pattern, effected by the cutting of a canal across Shell Island, is probably responsible for the ongoing segmentation of the western portion.Rates of erosion/deposition, determined by comparing bathymetries charted in 1877, 1930, and 1946, demand that these two peninsulas receive material by eastward transport. Furthermore, the long shore drift of both peninsulas is bidirectional, northwesterly and southeasterly. These two drift components are very nearly equal in magnitude immediately west of Shell Island; net easterly drift occurs on Shell Island peninsula. Crooked Island experiences net westerly and easterly drift; the easterly drift terminates in the Bell Shoal area and does not reach the adjacent St. Joe Spit.