1. Climate and Global Dynamics Division, National Center for Atmospheric Research (NCAR), Boulder, CO 80305, USA.
2. Department of Geography, University of Calgary, Calgary, Alberta, T2N 1N4, Canada.
3. Institute for the Study of Planet Earth, Department of Geosciences, and Department of Atmospheric Sciences, University of Arizona, Tucson, AZ 85721, USA.
4. Institute of Arctic and Alpine Research and Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA.