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\title{Complex basal conditions and their influence on ice flow at the onset of
the Northeast Greenland Ice Stream}



\author[1,1,1]{Steven Franke}%
\author[2,2,2]{Daniela Jansen}%
\author[3,3,3]{Sebastian Beyer}%
\author[2,2,2]{Tobias Binder}%
\author[4,4,4]{John Paden}%
\author[2,2,2]{Olaf Eisen}%
\author[5,5]{Niklas Neckel}%
\affil[1]{Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany}%
\affil[2]{Alfred Wegener Institute}%
\affil[3]{MARUM - Center for Marine Environmental Sciences}%
\affil[4]{Center for Remote Sensing of Ice Sheets (CReSIS)}%
\affil[5]{Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany}%


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  \date{May 29, 2024}


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\begin{abstract}
The ice stream geometry and large ice surface velocities at the onset
region of the Northeast Greenland Ice Stream (NEGIS) are not yet well
reproduced by ice sheet models. The quantification of basal sliding and
a parametrisation of basal conditions remains a major gap. In this
study, we assess the basal conditions of the onset region of the NEGIS
in a systematic analysis of airborne ultra-wideband radar data. We
evaluate basal roughness and basal return echoes in the context of the
current ice stream geometry and ice surface velocity. We observe a
change from a smooth to a rougher bed where the ice stream widens, and a
distinct roughness anisotropy, indicating a preferred orientation of
subglacial structures. In the upstream region, the excess ice mass flux
through the shear margins is evacuated by ice flow acceleration and
along-flow stretching of the ice. At the downstream part, the generally
rougher bed topography correlates with a decrease in flow acceleration
and lateral variations in ice surface velocity. Together with basal
water routing pathways, this hints to two different zones in this part
of the NEGIS: the upstream region collecting water, with a reduced basal
traction and downstream, where the ice stream is slowing down and is
widening on a rougher bed, with a distribution of basal water towards
the shear margins. Our findings support the hypothesis that the NEGIS is
strongly interconnected to the subglacial water system in its onset
region, but also to the subglacial substrate and morphology.%
\end{abstract}%



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