Browsing by Subject "ANTARCTICA"

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  • Tison, J. -L.; Schwegmann, S.; Dieckmann, G.; Rintala, J. -M.; Meyer, H.; Moreau, S.; Vancoppenolle, M.; Nomura, D.; Engberg, S.; Blomster, L. J.; Hendrickx, S.; Uhlig, C.; Luhtanen, A. -M.; de Jong, J.; Janssens, J.; Carnat, G.; Zhou, J.; Delille, B. (2017)
    Sea ice is a dynamic biogeochemical reactor and a double interface actively interacting with both the atmosphere and the ocean. However, proper understanding of its annual impact on exchanges, and therefore potentially on the climate, notably suffer from the paucity of autumnal and winter data sets. Here we present the results of physical and biogeochemical investigations on winter Antarctic pack ice in the Weddell Sea (R. V. Polarstern AWECS cruise, June-August 2013) which are compared with those from two similar studies conducted in the area in 1986 and 1992. The winter 2013 was characterized by a warm sea ice cover due to the combined effects of deep snow and frequent warm cyclones events penetrating southward from the open Southern Ocean. These conditions were favorable to high ice permeability and cyclic events of brine movements within the sea ice cover (brine tubes), favoring relatively high chlorophyll-a (Chl-a) concentrations. We discuss the timing of this algal activity showing that arguments can be presented in favor of continued activity during the winter due to the specific physical conditions. Large-scale sea ice model simulations also suggest a context of increasingly deep snow, warm ice, and large brine fractions across the three observational years, despite the fact that the model is forced with a snowfall climatology. This lends support to the claim that more severe Antarctic sea ice conditions, characterized by a longer ice season, thicker, and more concentrated ice are sufficient to increase the snow depth and, somehow counterintuitively, to warm the ice.
  • Biersma, E. M.; Jackson, J. A.; Hyvonen, J.; Koskinen, S.; Linse, K.; Griffiths, H.; Convey, P. (2017)
    A bipolar disjunction is an extreme, yet common, biogeographic pattern in non-vascular plants, yet its underlyingmechanisms (vicariance or long-distance dispersal), origin and timing remain poorly understood. Here, combining a large-scale population dataset and multiple dating analyses, we examine the biogeography of four bipolar Polytrichales mosses, common to the Holarctic (temperate and polar Northern Hemisphere regions) and the Antarctic region (Antarctic, sub-Antarctic, southern South America) and other Southern Hemisphere (SH) regions. Our data reveal contrasting patterns, for three species were of Holarctic origin, with subsequent dispersal to the SH, while one, currently a particularly common species in the Holarctic (Polytrichum juniperinum), diversified in the Antarctic region and from here colonized both the Holarctic and other SH regions. Our findings suggest long-distance dispersal as the driver of bipolar disjunctions. We find such inter-hemispheric dispersals are rare, occurring on multi-million-year timescales. High-altitude tropical populations did not act as trans-equatorial 'steppingstones', but rather were derived from later dispersal events. All arrivals to the Antarctic region occurred well before the Last Glacial Maximum and previous glaciations, suggesting that, despite the harsh climate during these past glacial maxima, plants have had a much longer presence in this southern region than previously thought.
  • Leppäranta, Matti; Lindgren, Elisa; Arvola, Lauri (2016)
    Thermodynamics of a seasonal supraglacial lake were examined based on field data from three summers. At maximum, the lake body consisted of an upper layer with thin ice on top, and a lower layer with slush, hard ice and sediment at the bottom. Sublimation from the upper ice surface averaged to 0.7 mm d(-1), and melting in the ice interior averaged to 9.1 mm d(-1) during summer. Albedo was on average 0.6 and light attenuation coefficient was similar to 1 m(-1). Averaged over December and January, and over 3 different years, we found that the net solar heating was 137 W m(-2), while the losses averaged to 62 W m(-2) for the longwave radiation, 16 Wm(-2) for the sensible heat flux, 24 W m(-2) for the latent heat flux and 3 W m(-2) for the bottom flux. The depth scale is determined by the light attenuation distance and thermal diffusion coefficient, and the net liquid water volume ranged from 0.5 to 1.0 m in different years. The potential winter growth is more than summer melting, and thus the lake freezes up completely in winter in the present climate.
  • Turunen, Sanni T.; Luttinen, Arto V.; Heinonen, Jussi S.; Jamal, Daúd L. (2019)
    We present geochemical and isotopic (Nd, Sr) data for a picrite lava suite from the Luenha River and adjacent areas in Mozambique. The Luenha picrites represent a previously unknown type of picrites related to the Karoo large igneous province (LIP) and are distinguished by their notably low TiO2 contents (0.3-1.0 wt%) and coupling of high Nb/Y with low Zr/Y and Sm/Yb. Relatively high CaO and low Zn/Fe point to a peridotitic mantle source. Contamination-sensitive incompatible element ratios show that one lava flow is likely to be uncontaminated by the crust and its composition suggests a mantle source with primitive mantle-like incompatible element ratios and mildly depleted isotopic ratios (initial Sr-87/Sr-86 = 0.7041 and epsilon(Nd) = +1.4 at 180 Ma). The primary melts of the Luenha picrites had MgO contents in the range of 13-21 wt%. Our preferred estimate for a primary melt composition (MgO = 18 wt%) resembles experimental melts of fertile mantle peridotite at 3-4 GPa and indicates liquidus temperature of 1445-1582 degrees C. Geochemical similarities suggest the Luenha picrites were generated from the same overall primitive mantle-like reservoir that produced the main volume of Karoo flood basalts in the Karoo, Kalahari, and Zambezi basins, whereas the previously identified enriched and depleted (upper) mantle sources of Karoo picrite suites (Mwenezi, Antarctica) were subordinate sources for flood basalts. We propose that the Luenha picrites record melting of a hot, chemically primitive mantle plume source that may have been rooted in the sub-African large low shear velocity province boundary and that such a source might have been the most significant magma source in the Karoo LIP. (C) 2019 The Author(s). Published by Elsevier B.V.
  • Leppäranta, Matti; Luttinen, Arto; Arvola, Lauri (2020)
    Shallow Antarctic surface lakes belong to the most extreme aquatic environments on the Earth. In Vestfjella, proglacial surface lakes and ponds are characterized by a 2-5 month long period with liquid water and depths <2 m. We give a detailed description of nine seasonal lakes and ponds situating at three nunataqs (Basen, Plogen and Fossilryggen) in western Dronning Maud Land. Their physical and geochemical properties are provided based on observations in four summers. Three main 'lake categories' were found: 1) supraglacial lakes, 2) epiglacial ponds and 3) nunataq ponds. Category 3 lakes can be divided into two subgroups with regards to whether the meltwater source is glacial or just seasonal snow patches. Supraglacial lakes are ultra-oligotrophic (electrical conductivity <10 mu S cm(-1), pH <7), while in epiglacial ponds the concentrations of dissolved and suspended matter and trophic status vary over a wide range (electrical conductivity 20-110 mu S cm(-1), pH 6-9). In nunataq ponds, the maxima were an electrical conductivity of 1042 mu S cm(-1)and a pH of 10.1, and water temperature may have wide diurnal and day-to-day fluctuations (maximum 9.3 degrees C) because snowfall, snow drift and sublimation influence the net solar irradiance.
  • Ruosteenoja, Kimmo; Jylhä, Kirsti; Räisänen, Jouni; Mäkelä, Antti (2018)
    The commentators regard a categorical truncation of supersaturations with respect to ice in climate model output as an inappropriate solution to the supersaturation issue. This view is supported by observational evidence from the East Antarctic Plateau. We accept this criticism to a certain degree. Even so, it is necessary to make a clear distinction between the true supersaturations measured in the atmosphere and the spurious supersaturations existing in archived model output data sets: the latter result, for example, from inconsistencies in the interpolation of temperature and specific humidity to the near-surface level. In the CMIP5 model output data sets, the largest relative humidities in near-surface air are far above those observed. Moreover, supersaturations given at the 2-m height are generally much larger than those at the lowermost tropospheric isobaric levels. Projections of relative humidity for the future may be considerably distorted if founded on unmodified output data sets.
  • Ruosteenoja, Kimmo; Jylha, Kirsti; Räisänen, Jouni; Mäkelä, Antti (2017)
    In 17 out of the 29 Phase 5 of Coupled Model Intercomparison Project (CMIP5) climate models examined in this work, near-surface air relative humidity (RH) frequently exceeded 100% with respect to ice in polar areas in winter. The degree of supersaturation varied considerably across the models, and the same evidently applies to the causes of the phenomenon. Consultations with the modeling groups revealed three categories of explanations for supersaturation occurrence: specification of RH with respect to ice rather than liquid water; inconsistencies in the determination of specific humidity and air temperature for the near-surface level; and the nonlinearity of saturated specific humidity as a function of temperature. Modeled global warming tended to reduce the artificial supersaturations, inducing a spurious negative trend in the future RH change. For example, over East Antarctica under Representative Concentration Pathway 8.5, the multimodel mean RH would decrease by about 10% by the end of the ongoing century. Truncation of overly high RHs to a maximum value of 100% cut the RH response close to zero. In Siberia and northern North America, truncation even reversed the sign of the response. The institutes responsible for the CMIP6 model experiments should be aware of the supersaturation issue, and the algorithms used to produce near-surface RH should be developed to eliminate the problem before publishing the RH output data. Plain Language Summary In the atmosphere, observed relative humidity is between 0% and 100%. However, some climate models produce spurious higher than 100% humidities. The problem only concerns polar areas in winter. As temperatures rise in the future, such model-produced excessively high relative humidities partially vanish. Unfortunately, this induces a spurious negative trend in the future humidity projections. Such a spurious component in the simulated trend complicates discerning the real physically based trend. The spurious trend could be eliminated by truncating the portion of relative humidity that exceeds 100% in the model output data. Even so, this may not be fully adequate for elaborating reliable humidity projections for polar areas. Therefore, it is highly desirable that the relative humidity calculations in the climate models would be developed so that unrealistic relative humidities would not occur in future model generations. We emphasize that this issue only concerns humidity projections and does not affect model-based predictions of temperature and precipitation change.