Hellmer, Hartmut
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Hellmer, Hartmut
Official Name
Hellmer, Hartmut
Alternative Name
Hellmer, Hartmut H.
Hellmer, Hartmut Heinz
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GND
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Item type:Publication, Analysis and modeling of the distribution of tracers of glacial melt in the Southern Ocean with focus on the Southeast Pacific sector(2015-02-17); ; ; In the Pacific sector of the Southern Ocean, the ice shelves and glaciers of the West Antarctic Ice Sheet (WAIS) are rapidly thinning, especially in the Amundsen Sea (AS) and Bellingshausen Sea (BS). The high basal melting of these relative small ice shelves is caused by warm Circum- polar Deep Water (CDW) that, based on observations, mainly intrudes via submarine glacial troughs located at the continental shelf break. When CDW reaches the grounding line of the fringing glaciers, strong basal melting supports a buoyant melt water plume. As the glacial melt becomes part of the shelf circulation, it may cause a freshening of the shelf water locally as well as remotely in the Ross Sea (RS). In the RS, on the other hand, shelf water salinity has declined by 0.03 per decade over the past 50 years. Thus, the objective of this doctoral thesis is to simu- late the spreading of glacial melt water using the Finite-Element Sea-ice/ice-shelf/Ocean Model (FESOM) and investigate whether the ice shelf melting in the AS is one of the main reasons for the observed shelf water freshening in the RS. However, there are two main difficulties for the simulation of the spreading of glacial melt water: (1) the number of observations on the Amundsen Sea continental shelf and thus our un- derstanding of CDW intrusions, ice shelf basal melting, and spreading of glacial melt water is limited and (2) none of the high-resolution global models resolving all small ice shelves around Antarctica is able to simulate CDW intrusions realistically. Thus, in this doctoral study the hy- drographic data of the cruise ANTXXVI/3 from the eastern AS continental shelf was analyzed, focusing on intrusions of CDW, basal melting of Pine Island Ice Shelf (PIIS), and spreading of glacial melt water. The data analysis was followed by several model sensitivity experiments using FESOM. They revealed that the model configuration such as horizontal resolution and atmospheric forcing is important for the reproduction of CDW intrusions. With these modifications, FESOM reproduces the bottom temperatures in the AS and BS and basal melt rates of AS and BS ice shelves much closer to reality. Therefore, I tested whether the observed freshening of the RS is a consequence of enhanced basal melting of AS ice shelves. Using several independent virtual passive tracers to identify pathways of the glacial melt, I found that the melt water from the ice shelves in the AS mainly flows towards the Ross Ice Shelf front. After 10 years of simulation, about half of the melt water in the RS originates from the Getz Ice Shelf. Further, I investigated the sensitivity of the melt water transport into the RS associated with the strength of the basal melt water flux. When this flux is increased by 30%, the transport of glacial melt into the RS nearly doubles, supporting the idea that the basal melting of AS and BS ice shelves is one of the main reasons for the observed freshening on the RS continental shelf. This may imply that the observed freshening of the RS in the late 1970's/early 1980's, prior to the remotely sensed increase in ice shelf mass losses, might have been caused by enhanced basal melting already in the 1960' s.doctoral thesis750 188 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Über die Tiefenwasserausbreitung im Weddellmeer und in der Scotia-See: Numerische Untersuchungen der Transport- und Austauschprozesse in der Weddell-Scotia-Konfluenz-Zone(2002-05-02); ; ; ; The deep Scotia Sea is filled with ventilated Weddell Sea Deep Water (WSDW). This in turn is an essential contributor to the ventilation of the World Ocean abyss. A primitive equation, hydrostatic, ocean general circulation model (BRIOS1.1) with terrain-following coordinate is used to investigate the water mass export from the Weddell Sea. The model is circumpolar focusing on the Weddell Sea, with particularly high resolution (~20 km) in the DOVETAIL area. The northern limb of the Weddell Gyre exhibits an eastward Weddell Sea Deep Water transport across 44°W of 24 Sv. Export rates of Weddell Sea Deep Water through gaps in theSouth Scotia Ridge are estimated to be 6.4 Sv with a semi-annual cycle of ± 0.6 Sv, which can be correlated to atmospheric cyclone activity and Weddell Gyre strength. Sensitivity studies considering extreme sea ice conditions in the Weddell Sea show higher (lower) exports in years of minimum (maximum) winter sea ice extent. This can be attributed to the local change of the surface stress achieved by wind and ice. Lagrangian particle trajectories illustrate the pathways of water masses from the inner Weddell Sea into the Scotia Sea through the major gaps in the South Scotia Ridge. They support the existing flow divergence on the northwestern continental shelf with one branch entering Bransfield Strait and the other continuing eastwards subsequently filling the deep Weddell and Scotia seas. Floats highlight the interannual variability of the flow field. Water masses flowing through the major gaps originate from the southwestern and southeastern Weddell Sea continental shelves.Water masses formed east of the Weddell Sea (e.g., Prydz Bay) also seem to feed the deep Scotia Sea. The propagation of dense water masses spreading from the southern continental shelf to the South Atlantic Ocean is estimated to be about 8 years not including residence times on the continental shelf.doctoral thesis657 230