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  • 1
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    Light Induced Physical-biogeochemical Implications in the Arctic Ocean: a Coupled Sea-ice – Ocean – Ecological Modeling Study (2020)
    AGU
    In:  EPIC3Ocean Sciences Meeting 2020, San Diego, CA, USA, 2020-02-16-2020-02-21San Diego, CA, USA, AGU
    Details
    Publication Date: 2020-05-14
    Description: Currently, the most rapid increase in near-surface air temperature takes place in the Arctic, accompanied by reduced sea ice concentration. Under the resulting retreat of sea ice, the underwater shortwave radiation and, thus, the amount and types of phytoplankton may change. In this study, we use a coupled sea-ice – ocean – ecological model (Darwin-MITgcm) to simulate the variability of the ocean’s major optically active constituents, comprising six phytoplankton functional types (PFTs) and colored dissolved organic matter (CDOM), in response to Arctic amplification. We further set up the general circulation model to account for the biogeochemical processes, in terms of light attenuation, so that their feedback on Arctic Ocean’s physical and biogeochemical properties can be assessed. Here, for the first time, CDOM is included in the underwater light attenuation scheme as a prognostic model variable that interacts with the changes induced by its presence. The coupled model simulation, allowing to consider explicitly the optical constituents, is compared with a constant attenuation depth formulation corresponding to Jerlov water type I, which is the case in most ocean models. Our findings suggest that the presence of CDOM and phytoplankton, by modulating the vertical distribution of the incoming light, affects significantly the upper ocean thermal structure. The promotion of heat-trapping near the surface results in summertime warming, locally even in more than 1°C, and to sea ice reduction. These changes have implications to upper ocean stratification and are accompanied by changes in nutrients supply, as well as in total but also partial PFTs chlorophyll-a.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 2
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    Observed interannual changes beneath Filchner-Ronne Ice Shelf linked to large-scale atmospheric circulation (2021)
    Springer Nature
    In:  EPIC3Nature Communications, Springer Nature, 12(1), ISSN: 2041-1723
    Details
    Publication Date: 2021-06-16
    Description: Floating ice shelves are the Achilles’ heel of the Antarctic Ice Sheet. They limit Antarctica’s contribution to global sea level rise, yet they can be rapidly melted from beneath by a warming ocean. At Filchner-Ronne Ice Shelf, a decline in sea ice formation may increase basal melt rates and accelerate marine ice sheet mass loss within this century. However, the understanding of this tipping-point behavior largely relies on numerical models. Our new multi-annual observations from five hot-water drilled boreholes through Filchner-Ronne Ice Shelf show that since 2015 there has been an intensification of the density-driven ice shelf cavity-wide circulation in response to reinforced wind-driven sea ice formation in the Ronne polynya. Enhanced southerly winds over Ronne Ice Shelf coincide with westward displacements of the Amundsen Sea Low position, connecting the cavity circulation with changes in large-scale atmospheric circulation patterns as a new aspect of the atmosphere-ocean-ice shelf system.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev , info:eu-repo/semantics/article
    Format: application/pdf
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  • 3
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    FRIS Revisited in 2018: On the Circulation and Water Masses at the Filchner and Ronne Ice Shelves in the Southern Weddell Sea (2021)
    In:  EPIC3Journal of Geophysical Research: Oceans, 126(6), pp. e2021JC017269
    Details
    Publication Date: 2021-06-16
    Description: The Filchner-Ronne Ice Shelf (FRIS) is characterized by moderate basal melt rates due to the near-freezing waters that dominate the wide southern Weddell Sea continental shelf. We revisited the region in austral summer 2018 with detailed hydrographic and noble gas surveys along FRIS. The FRIS front was characterized by High Salinity Shelf Water (HSSW) in Ronne Depression, Ice Shelf Water (ISW) on its eastern flank, and an inflow of modified Warm Deep Water (mWDW) entering through Central Trough. Filchner Trough was dominated by Ronne HSSW-sourced ISW, likely forced by a recently intensified circulation beneath FRIS due to enhanced sea ice production in the Ronne polynya since 2015. Glacial meltwater fractions and tracer-based water mass dating indicate two separate ISW outflow cores, one hugging the Berkner slope after a two-year travel time, and the other located in the central Filchner Trough following a ∼six year-long transit through the FRIS cavity. Historical measurements indicate the presence of two distinct modes, in which water masses in Filchner Trough were dominated by either Ronne HSSW-derived ISW (Ronne-mode) or more locally derived Berkner-HSSW (Berkner-mode). While the dominance of these modes has alternated on interannual time scales, ocean densities in Filchner Trough have remained remarkably stable since the first surveys in 1980. Indeed, geostrophic velocities indicated outflowing ISW-cores along the trough's western flank and onto Berkner Bank, which suggests that Ronne-ISW preconditions Berkner-HSSW production. The negligible density difference between Berkner- and Ronne-mode waters indicates that each contributes cold dense shelf waters to protect FRIS against inflowing mWDW.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev , info:eu-repo/semantics/article
    Format: application/pdf
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  • 4
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    Amplified Arctic Surface Warming and Sea Ice Loss Due to Phytoplankton and Colored Dissolved Material (2021)
    Details
    Publication Date: 2021-07-05
    Description: Optically active water constituents attenuate solar radiation and hence affect the vertical distribution of energy in the upper ocean. To understand their implications, we operate an ocean biogeochemical model coupled to a general circulation model with sea ice. Incorporating the effect of phytoplankton and colored dissolved organic matter (CDOM) on light attenuation in the model increases the sea surface temperature in summer and decreases sea ice concentration in the Arctic Ocean. Locally, the sea ice season is reduced by up to one month. CDOM drives a significant part of these changes, suggesting that an increase of this material will amplify the observed Arctic surface warming through its direct thermal effect. Indirectly, changing advective processes in the Nordic Seas may further intensify this effect. Our results emphasize the phytoplankton and CDOM feedbacks on the Arctic ocean and sea ice system and underline the need to consider these effects in future modeling studies to enhance their plausibility.
    Description: Plain Language Summary: The amount of microalgae and colored dissolved organic material in the ocean determines how much light is absorbed in the surface waters and how much can reach greater depths. The vertical distribution of energy affects the upper ocean temperature and general circulation. Here, we use a numerical ocean model with biogeochemistry and sea ice, in which the individual effects of microalgae and colored dissolved organic matter can be turned on and off separately. When both effects are turned on, the summertime surface temperatures in the Arctic are larger and consequently more sea ice melts, so that the sea ice season is shorter by up to one month. We find that, to a large extent, the colored dissolved material is responsible for these changes. An increase of this material due to climate change will amplify the observed Arctic surface warming. For better projections of climate change, new models should account for the effect of these light‐absorbing water constituents.
    Description: Key Points: Colored dissolved material is responsible for a significant part of the induced surface warming and sea ice loss in the Arctic Ocean. The combined effect of optical constituents reduces the sea ice season by up to one month. Considering the properties of optical constituents and their variability will enhance the plausibility of future modeling studies.
    Description: Federal Agency for Scientific Organizations (FASO) Russia http://dx.doi.org/10.13039/501100013176
    Description: German Research Foundation (DFG) http://dx.doi.org/10.13039/501100001659
    Description: Helmholtz Climate Initiative (REKLIM)
    Keywords: 551,9 ; phytoplankton ; CDOM ; Arctic Ocean ; colored dissolved organic matter ; radiative effect ; light attenuation
    Type: article
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  • 5
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    FRIS Revisited in 2018: On the Circulation and Water Masses at the Filchner and Ronne Ice Shelves in the Southern Weddell Sea (2021)
    Details
    Publication Date: 2021-09-29
    Description: The Filchner‐Ronne Ice Shelf (FRIS) is characterized by moderate basal melt rates due to the near‐freezing waters that dominate the wide southern Weddell Sea continental shelf. We revisited the region in austral summer 2018 with detailed hydrographic and noble gas surveys along FRIS. The FRIS front was characterized by High Salinity Shelf Water (HSSW) in Ronne Depression, Ice Shelf Water (ISW) on its eastern flank, and an inflow of modified Warm Deep Water (mWDW) entering through Central Trough. Filchner Trough was dominated by Ronne HSSW‐sourced ISW, likely forced by a recently intensified circulation beneath FRIS due to enhanced sea ice production in the Ronne polynya since 2015. Glacial meltwater fractions and tracer‐based water mass dating indicate two separate ISW outflow cores, one hugging the Berkner slope after a two‐year travel time, and the other located in the central Filchner Trough following a ∼six year‐long transit through the FRIS cavity. Historical measurements indicate the presence of two distinct modes, in which water masses in Filchner Trough were dominated by either Ronne HSSW‐derived ISW (Ronne‐mode) or more locally derived Berkner‐HSSW (Berkner‐mode). While the dominance of these modes has alternated on interannual time scales, ocean densities in Filchner Trough have remained remarkably stable since the first surveys in 1980. Indeed, geostrophic velocities indicated outflowing ISW‐cores along the trough's western flank and onto Berkner Bank, which suggests that Ronne‐ISW preconditions Berkner‐HSSW production. The negligible density difference between Berkner‐ and Ronne‐mode waters indicates that each contributes cold dense shelf waters to protect FRIS against inflowing mWDW.
    Description: Plain Language Summary: We visited the largest floating Antarctic ice shelf in the southern Weddell Sea in 2018 with an icebreaker expedition, and measured ocean temperature, salinity, meltwater content, and other parameters in front of the FRIS. We found that the ocean conditions were still dominated by the very cold and dense waters needed to protect the ice shelf from inflowing warm waters from the deep ocean. We compared the 2018 conditions with earlier surveys since the 1980s and concluded that, in spite of climate change and in contrast to other Antarctic regions, the water masses on the southern Weddell Sea shelf remained relatively stable overall. We found that most of the stations we visited near the Filchner Ice Shelf edge were dominated by cold ISW, which forms when water masses interact with the underside of the shelf ice. Our measurements helped improve our understanding regarding the currents and water masses on the southern Weddell Sea continental shelf.
    Description: Key Points: Hydrographic status update with the first comprehensive CTD survey along the entire FRIS front since 1995. Strong and stable presence of High Salinity Shelf Water in Ronne Depression over decades. Dominance of Ronne‐sourced Ice Shelf Water in Filchner Trough in 2018 points to intensified sub‐FRIS circulation.
    Description: Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) http://dx.doi.org/10.13039/501100003207
    Keywords: 551.46 ; Ocean circulation ; ocean‐ice shelf interaction ; water masses ; Weddell Sea ; Filcher and Ronne shelves
    Type: map
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    CITATION GEO-LEO
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