Marlene Schramm, Friedrich-Alexander-Universität Erlangen-Nürnberg, Department of Physics, Erlangen, Germany; Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany and Alexander Haumann, Alfred Wegener Institute Helmholtz-Center for Polar and Marine Research Bremerhaven, Bremerhaven, Germany; Alfred Wegener Institute Helmholtz-Center for Polar and Marine Research, Bremerhaven, Germany