VARIABILITY OF FREEZING LEVELS, MELTING SEASON INDICATORS, AND SNOW COVER FOR SELECTED HIGH-ELEVATION AND CONTINENTAL REGIONS IN THE LAST 50 YEARS HENRY F. DIAZ 1 , JON K. EISCHEID 2 , CHRIS DUNCAN 3 and RAYMOND S. BRADLEY 3 1 Climate Diagnostics Center, NOAA, 325 Broadway, Boulder, CO 80305, U.S.A. E-mail: hfd@cdc.noaa.gov 2 Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, U.S.A. 3 Department of Geosciences, University of Massachusetts, Amherst, MA 01003, U.S.A. Abstract. We have used NCEP/NCAR Reanalysis data and a Northern Hemisphere snow cover data set to analyze changes in freezing level heights and snow cover for the past three to five decades. All the major continental mountain chains exhibit upward shifts in the height of the freezing level surface. The pattern of these changes is generally consistent with changes in snow cover, both over the course of the year and spatially. We examined different free-air temperature parameters (dry bulb temperature, virtual temperature, and 700–500 hPa thickness) using the Reanalysis grid point values located over the different mountain areas as defined in this study. The different trend values were in reasonably good agreement with each other, particularly over the second half of the record. Freezing level changes in the American Cordillera are strongly modulated by the El Niño/Southern Oscillation (ENSO) phenomenon and the freezing level heights (FLH) respond to both interannual and decadal-scale change in tropical Pacific sea surface temperature (SST). The ∼0.5 ◦ C increase in SST recorded in the tropical Pacific since the 1950s accounts for approximately half of the increase in FLH in tropical and subtropical latitudes of the Cordilleran region during that same time. 1. Introduction In the last 20 years, a great deal of interest has been focused on climatic varia- tions and changes in the mountain regions of the Earth, and on the effects of such changes on water resources, economic development, and ecosystem health, to name just a few impact areas (Beniston, 1994; Beniston et al., 1997; Messerli and Ives, 1997; Mountain Agenda, 1998). The Intergovernmental Panel on Climate Change (IPCC) considered the potential impacts of climate change on mountain regions as part of its Second Assessment Report (Beniston and Fox, 1995). In the IPCC Third Assessment Report (IPCC, 2001) there are several chapters, which touch upon the effects of global climate change in mountainous regions (e.g., in Chapters 4 and 5, on hydrology and water resources, and ecosystems, respectively). Studies by Diaz and Graham (1996), Diaz and Bradley (1997), Parmesan et al. (1999), Thompson (2000), Pounds et al. (1999), and Still et al. (1999), for Climatic Change 59: 33–52, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.