CHANGES OF SNOW COVER, TEMPERATURE, AND RADIATIVE HEAT-BALANCE OVER THE NORTHERN-HEMISPHERE

Contemporary large-scale changes in satellite-derived snow cover were examined over the Northern Hemisphere extratropical land (NEL) areas. These areas encompass 55% of the land in the Northern Hemisphere. Snow cover (S) transient regions, the ''centers of action'' relative to in terannual variations of snow cover, were identified for the years 1972 -1992. During these years a global retreat in snow cover extent (SE) o ccurred in the second half of the hydrologic year (April-September). M ean annual SE has decreased by 10% (2.3 X 10(6) km(2)). Negative trend s account for one-third to one-half of the interannual continental var iance of SE. The historical influence of S on the planetary albedo and outgoing longwave radiation (OLR) is investigated. The mean annual re sponse of the S feedback on the radiative balance (RB) is negative and suggests a large-scale heat redistribution. During autumn and early w inter (up to January), however, the feedback of S on the planetary RB may be positive. Only by February does the cooling effect of S (due to albedo increase) dominate the planetary warming due to reduced OLR ov er the S. Despite a wintertime maximum in SE, the feedback in spring h as the greatest magnitude. The global retreat of spring SE should lead to a positive feedback on temperature. Based on observed records of S , changes in RB are calculated that parallel an observed increase of s pring temperature during the past 20 years. The results provide a part ial explanation of the significant increase in spring surface air temp erature observed over the land areas of the Northern Hemisphere during the past century. The mean SE in years with an EI Nino and La Nina we re also evaluated. El Nino events are generally accompanied by increas ed SE over the NEL during the first half of the hydrological year. In the second half of the hydrologic year (spring and summer), the EI Nin o events are accompanied by a global retreat of SE.