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Science Rendezvous > Posters
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Synoptic-Scale Atmospheric Forcing of Frozen Ground in the Eurasian High Latitudes
Oliver W. Frauenfeld, Tingjun Zhang
CIRES/NSIDC
Seasonal freezing and thawing processes of the ground thermal regime in cold regions play an important role in ecosystem diversity, productivity, and the Arctic hydrologic system. Long-term changes in seasonal freeze and thaw depths are useful indicators of climate change. In this analysis we improve on a previous 1956-1990 assessment of seasonal freeze depths based on only 242 sites by employing a greatly expanded station database of soil temperatures for 423 sites with updated observations for 1930-2000. The addition of 181 sites throughout the Russian Arctic combined with 37 more years of observations allows for a significantly more comprehensive evaluation. The addition of 1991-2000 in particular allows us to quantify changes in frozen ground during a decade when accelerated climate warming has occurred.
We find a statistically significant overall change in seasonal freeze depth of -29 cm for 1930-2000, averaged across the Eurasian high latitudes. This time series also indicates some patterns of interdecadal variability: increasing freeze depths until ~1970, followed by a sharp decrease until ~1995. From ~1995 on, freeze depths may actually be increasing again. Exploring potential climatic drivers of these changes, we find that seasonal freeze depth correlates most strongly with the annual freezing index, and also with air temperature. However, when all potential drivers are working together to jointly influence ground freezing, snow depth becomes the most important driver, more so than temperature.
The most prominent feature in this time series is the strong 1970-1995 decrease, which coincides with a strong positive phase in the North Atlantic Oscillation (NAO). We find that when the NAO is in a strong positive phase, as it was from the late 1960s to the mid 1990s, it accounts for 80% of the variability in frozen ground conditions. It could thus be argued that the prominent decrease in freeze depths from ~1970-1995 is due to internal variability in the atmosphere, as represented by the NAO.
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