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San Rafael Glacier and Northern Patagonia Icefield surface mass balance estimation from different approaches

Abstract : The Northern Patagonia Icefield (NPI) have been losing mass at an accelerated rate during the last decades compared to the mean losses recorded since the Little Ice Age. Here we look for accurate estimates of the mean climate variables, surface mass balance (SMB) and ice dynamics of NPI. Due to the lack of available data in the area, the approach is based on physical models for both the atmosphere and the ice dynamics, in order to properly attribute the components of the glacial mass budget (mass balance, surface mass balance and ice discharge).First, the San Rafael Glacier (SRG) ice dynamics are modelled using the full-Stokes model Elmer/Ice. The flow model is initialised and constrained using the most up-to-date observations of surface velocities and bedrock elevation. The model is forced with several parametrisations of thealtitudinal SMB distribution to obtain consistency between the changes in SMB and ice dynamics. We show that previous studies have proposed excessive accumulation values on the icefield plateau, and that SRG imbalance is largely controlled by a large ice discharge (-0.83 +- 0.08 Gt/a compared to a slightly positive glacier-wide SMB (0.08 +- 0.06 Gt/a). This allows for an estimation of a committed mass loss of 0.34 +- 0.03 Gt/a for the next century. This value likely constitutes a minimum wastage in the future global climate change situation.Second, we model the SMB of the SRG and the NPI using the regional circulation model MAR (Modèle Atmosphérique Regional). This model is forced by the ERA-Interim reanalysis outputs and adapted to accurately reproduce accumulation on the icefield. In addition to accumulation, attention is paid to estimating accurate ablation and albedo values on the plateau. The modelled temperature and precipitation are also validated using data from weather stations in the valleys around the icefield. For the period 1980-2014, the modelled mean SMB of the SRG and the NPI are 0.86 Gt/a and -1.84 Gt/a, respectively, with a large inter-annual variability of 1.4 Gt/a and 6.1 Gt/a, which is induced by temperature and snowfall variability. Due to the hypsometry of the SRG, small changes in the punctual SMB around the ELA have impacts over large areas and have significant consequences on the final glacier-wide SMB. However, in-situ data above the ELA are lacking, leading to important uncertainty in accumulation. Nevertheless, our SMB estimates for the 3 largest non-calving glaciers of the icefield are similar to mass balance values given by geodetic techniques. This validation shows that our estimate of SMB, which is the first negative proposed at the scale of NPI, is accurate. Combined with the ice discharge proposed in the first section of this study, this SMB could have contributed to the observed mass loss from the 1980s.Finally, the study of the surface energy balance given by MAR reveals the key role of the albedo and the shortwave radiation budget in the variations of the SMB. Albedo variations indirectly justify the existing correlations between temperature and melting as air temperature controls the phase of precipitation on the plateau. Thus, changes in solid precipitation explain most of the SMB variations. Consequently, past conditions with higher solid precipitation may have explained larger glaciers in Patagonia. The current instability is possibly a result from the location of glacier fronts which are well below the necessary altitude for equilibrium in the current icefield topography.This study allows us to conclude that well constrained atmospheric and ice dynamic modelling leads to SMB values which are in better agreement with the mass balance of the icefield. Nevertheless, new field data is necessary to better constrain SMB estimates of the SRG and the NPI in order to improve our understanding of past and future climate change impacts on these glaciers.
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Gabriela Collao Barrios. San Rafael Glacier and Northern Patagonia Icefield surface mass balance estimation from different approaches. Glaciology. Université Grenoble Alpes, 2018. English. ⟨NNT : 2018GREAU044⟩. ⟨tel-02050432⟩

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