Integrated satellite assessment of regional trends in snowmelt and frozen ground conditions over the pan-Arctic

Abstract:

We analyzed satellite climate data records (CDRs) documenting frozen (FR) season and snow cover extent (SCE) changes from 1979 to 2011 over the pan-Arctic (≥45°N). A global Earth System Data Record of daily landscape freeze-thaw status (FT-ESDR) derived from satellite microwave remote sensing was used to define the FR season [days] from the number of classified FR or transitional (AM frozen, PM non-frozen) days. SCE duration [%] was estimated from NOAA/NCDC CDR northern hemisphere weekly SCE data records. We find that relatively strong declines in FR and SCE durations in spring and summer are partially offset by increasing trends in fall and winter. These contrasting seasonal trends result in relatively weak decreasing trends in annual FR and SCE durations. A dominant SCE retreat response to FR season decreases coincides with widespread warming (0.44°C decade-1), while the sign and strength of this relationship is spatially complex, varying by latitude, regional snow cover and climate characteristics. The spatial extent of FR conditions exceeds SCE in early spring and is smaller during snowmelt in late spring and early summer, while FR ground in the absence of snow cover is widespread in the fall. To gain new insight on the spatial and temporal characteristics of seasonal FR ground and snowmelt changes, we conducted an integrated analysis of the independent FT-ESDR and SCE data records. Snowmelt was defined for grid cells where the SCE record indicated snow cover presence, but the FT-ESDR indicated transitional FT conditions. Frozen ground cells were defined for coincident FR and snow-free SCE conditions. The integrated satellite record reveals a general increasing trend in annual snowmelt duration from 1.3 to 3.3 days decade-1 (p<0.01), occurring largely in the fall. The annual duration of frozen ground conditions in the absence of insulating snow cover is declining from 0.8 to 1.3 days decade-1. These changes imply extensive biophysical impacts to northern snow cover, soil and permafrost regimes, with potentially large climate feedbacks, while ongoing satellite microwave missions provide an effective means for regional monitoring.