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FP5

PACLIVA Résumé de rapport

Project ID: EVK2-CT-2002-00143
Financé au titre de: FP5-EESD
Pays: Switzerland

Winter precipitation over Greenland

Re-analysis data covering the period 1958 2001 were used to determine the relationship between regional, inter-annual snow accumulation variability over the Greenland Ice Sheet (GrIS) and large scale circulation patterns, cyclone frequency, and strength. Four regions of the GrIS have been identified that are highly independent with respect to accumulation variability. This result allows precipitation to be reconstructed over a longer period.

Accumulation indices of three of these regions are associated with distinct large-scale circulation patterns: Central-western GrIS reveals an inverse relationship with a NAO-like pattern, the south-west a positive correlation with a high pressure bridge from central North Atlantic to Scandinavia, and the south-eastern GrIS a positive correlation with a high-pressure anomaly over the Greenland Sea. These large scale patterns also impact European climate in different ways. Accumulation variability in north-eastern GrIS, however, is dominated by cyclones originating from the Greenland Sea. Thus, Greenland ice core accumulation records offer the potential to reconstruct various large-scale circulation patterns and regional storm activity.

The annually resolved ice core records from different regions over the GrIS were used to investigate spatial and temporal variability of calcium (Ca++, mainly from aeolian dust) and sodium (Na+, mainly from sea salt) deposition. Cores of high common inter-annual variability are grouped with an EOF analysis, resulting in regionally representative Ca++ and Na+ records for northeastern and central Greenland. Utilizing a regression and validation method including the ERA40 reanalysis data set, these common records are associated with distinct regional atmospheric circulation patterns over the North American Arctic, Greenland and Central to Northern Europe.

In the north-eastern part of the GrIS a large fraction of the Ca++ variability is connected to a circulation pattern suggesting transport from the west and probably dry deposition. This pattern is consistent with the current understanding of a predominantly Asian source of the dust deposited over the GrIS. However, our results also indicate that a significant and dominant fraction of the inter-annual dust variability in NE and Central Greenland, respectively, are determined by the complex effect of frequency and intensity of wet deposition (including a potentially dominating effect of fog) during the spring season with high atmospheric dust loading, rather than representing the variability of the Asian dust source and/or long-range transport to Greenland. The variances in the regional proxy records explained by the streamfunction patterns are high enough to attempt reconstructions of the corresponding regional deposition regimes and the associated circulation patterns.

Further, UNIBE collaborated with US colleagues on a study about the modes of temporal variability recorded in nitrate and accumulation records from six Greenland ice cores (Burkhart et al., revised manuscript submitted). Nitrate records from six Greenland ice cores covering the period 1794 to 1995 show a significant correlation in concentration for averaging periods greater than 10 years, as well as an approximately 60 percent increase in average concentration during the last 75 years.

Annual nitrate fluxes contain low-frequency trends driven primarily by changes in concentration, while higher-frequency variability is driven by changes in snow accumulation. During the period of anthropogenic influence, nitrate is positively correlated with the North Atlantic Oscillation, while prior to that the correlation is negative, but less significant.

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Thomas STOCKER, (Professor)
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