Changes in North American snowpacks for 1979–2007 detected from the snow water equivalent data of SMMR and SSM/I passive microwave and related climatic factors

Abstract

[1] Changes to the North American snowpacks for 1979-2007 were detected from snow water equivalent (SWE) retrieved empirically from horizontally polarized brightness temperature (T B ) of a scanning multichannel microwave radiometer (18 and 37 GHz) and special sensor microwave imager (19 and 37 GHz) passive microwave data using the nonparametric Kendall's test. The predominant SWE trends detected agree with negative anomalies in snow cover observed in Northern Hemisphere since the 1980s, and both the SWE and snow cover results should be related to the significant increase in the surface temperature of North America (NA) observed since the 1970s. About 30% of detected decreasing trends of SWE for 1979-2007 are statistically significant, which is three times more than the significant increasing trends of SWE detected in NA. Significant decreasing SWE trends are more extensive in Canada than in the United States. The mean trend magnitudes detected for December-April are À0.4 to À0.5 mm/yr, which means an overall reduction of snow depth of about 5-8 cm in 29 years (assuming a snowpack density between 200 and 250 kg/m 3 ), which can impact regions relying on spring snowmelt for water supply. From detected increasing (decreasing) trends of gridded temperature (precipitation) based on the North American Regional Reanalysis data set and the University of Delaware data set for NA, their respective correlations with SWE data and other findings, such as global scale decline of snow cover, longer rainfall seasons, etc., it seems the extensive decreasing trends in SWE detected mainly in Canada are more caused by increasing temperatures than by decreasing precipitation. However, climate anomalies could also contribute to the detected trends, such as PC1 of NA's SWE, which is found to be correlated to the Pacific Decadal Oscillation index and marginally correlated to the Pacific North American pattern.

Temperature and Snow-Mediated Moisture Controls of Summer Photosynthetic Activity in Northern Terrestrial Ecosystems between 1982 and 2011

“…Thus, this negative response of summer photosynthetic activity to earlier spring thaw is likely a reflection of soil moisture deficits resulting from both insufficient soil moisture recharge with shallow snowpacks and increased moisture depletion with an extended evaporative season. Taken together, these emerging results suggest that recent changes in snow regimes associated with rapid climatic warming [36][37][38] are likely to have played an important, albeit unrecognized, role in the observed spatiotemporal patterns of summer photosynthetic activity across a large portion of the boreal region, particularly in North America.…”

“…A positive association has also been observed between deeper snowpack and summer NDVI in central Siberia and parts of North America [34,35]. Thus, recent declines in winter snowpack and snow cover duration documented in several regions of northern Eurasia and North America [36][37][38] are likely to have affected ecosystem function.…”

“…Ongoing trends toward thinner snowpacks and earlier snowmelt and growing seasons [9,36,38] may be further enhancing summer drying in some regions by extending the period of evapotranspiration into the spring and thus accelerating seasonal soil moisture depletion [10,35]. Indeed, pioneering modeling studies conducted in the 1980s using simple climate models suggested that, along with increasing evaporation demand, earlier snowmelt also contributed to projected summer drying at northern latitudes as a result of anthropogenic warming [75].…”

“…Recent studies based on satellite and in-situ observations have shown that rapid winter/spring warming and changes in interdecadal climate modes since around 1991 have led to a significant decline in snowpack accumulation and snow cover duration across most of Alaska and Canada despite increasing winter precipitation [36][37][38]94,95]. In some regions of the North American Cordillera, the recent loss of snowpack is unprecedented over the past millennium [96].…”

“…In some regions of the North American Cordillera, the recent loss of snowpack is unprecedented over the past millennium [96]. Estimates based on satellite retrievals for the period 1979-2007 indicate a reduction in December-April SWE of about 0.4 to 0.5 mm yr −1 over North America, which corresponds to an overall reduction of snow depth of about 5-8 cm in 29 years, depending on snow density [38]. Without increases in summer precipitation, this reduction in snowpack is sufficient to result in soil moisture deficits later in the growing season over extensive boreal regions relying on spring snowmelt for water supply (Figure 4).…”