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ALASKAN TEMPERATURE CHANGE OVER THE PAST MILLENNIUM: SOLAR FORCING MODULATED BY THE ARCTIC AND PACIFIC DECADAL OSCILLATIONS
WILES, GREGORY C. Department of Geology, The College of Wooster.
D'Arrigo, Rosanne D. Tree Ring Lab, Lamont-Doherty Earth Observatory.
Villalba, Ricardo Departamento de Dendrocronologia e Historia Ambiental, IANIGLA – CRICYT.
Barclay, David J. Department of Geology, State University of New York College at Cortland.
Calkin, Parker E. Institute of Arctic and Alpine Research, University of Colorado.
Well-dated histories from 130 land-terminating glaciers spanning the Little Ice Age (AD 1200-1900) are compiled from three climatically distinct regions: 1. The Arctic Brooks Range (94 glaciers), 2. The southern transitional interior to near-coastal region straddled by the Wrangell and St. Elias mountain ranges (9 glaciers), and 3. An 800-km transect along the coastal ranges of the Kenai, Chugach and St. Elias mountains (27 glaciers) bordering the Gulf of Alaska. The primary data are dendrochronologically-derived calendar dates from forests overrun by advancing ice and age estimates of moraines based on botanical methods using tree-rings and lichens.
For each of the three regions, we have generated a Glacier Expansion Index (GEI) in order to distill the glacial geologic record into time series that reflects as accurately as possible the actual intervals of glacier advance. This index is compiled from the glacial chronology and corrected for maximum (tree-ring) and minimum (moraine) age estimates of ice expansions. We have also created a composite GEI that is an average of each of the normalized GEI series from the three regions.
Correlation of GEIs with a record of solar irradiance (14C record preserved in tree rings ) suggests that multi-decadal to century-scale temperature variations in the North Pacific and Arctic sectors have been influenced by solar forcing. A Blackman-Tukey (BT) spectral analysis of the normalized composite series shows peaks at 65, 104, and 170 - 210 years. The peaks in the composite GEI occur shortly after AD 1050, and are centered on ~1250, 1450, 1650 and 1850. The 210-year peak is coherent with the deVries mode of solar variability recognized in the solar irradiance series and suggests that solar forcing was instrumental in cooling inferred from the GEI.
However, solar irradiance changes alone are not likely to be sufficient to force significant temperature change for glacier advance. We suggest that one means of enhancing the solar-induced cooling and, in part, may explain regional differences in the glacial record, is to take into account the effects of two key modes of atmosphere-ocean circulation known to impact the three regions. These are the Pacific Decadal Oscillation (PDO), in southern Alaska, and the influence of the Arctic Oscillation (AO) in the Brooks Range.
The normalized composite glacier record is compared with the solar record and a tree-ring-based reconstruction of the Pacific Decadal Oscillation (PDO) for the spring and summer months. The PDO reconstruction spans AD 1479-1977 and is based on tree-ring data from Mexico to Alaska (Cook 2003). The comparison shows that sustained intervals of low PDO indices and cold temperatures during the AD1600s and 1800s are coupled with the Maunder and Dalton solar minima and perhaps the earlier Spörer minimum as well. We postulate that a sustained negative phase of the PDO, when combined with decreased solar output, can result in the large-scale glacial advances and inferred cooling we observe in Alaska.
The decadal modes of the AO may also impact temperature, particularly at high latitudes in Alaska. In a sustained negative phase of the AO, the frigid air at high latitudes, spills southward across Alaska favoring glaciation there. Based on these relationships we suggest that during the Spörer minimum (~AD 1450) a negative phase of the AO may have had a role in the cooling to favor more intense glaciation in the Brooks Range than in the southern regions. Work underway to extend dendroclimatic reconstructions of the AO and PDO (i.e. Cook, 2002, D’Arrigo et al., 2003) will allow us to more fully compare the interaction of decadal and century-scale climate variability in the Arctic and North Pacific Sectors.
Cook, E.R., 2002, Reconstruction of Pacific Decadal Variability from long tree-ring records: Eos Trans. AGU v. 83, Spring Meet. Suppl. Abstr. GC42A-04.
D’Arrigo, R.D., Cook, E.R., Mann, M.E. and Jacoby, G.C., 2003, Tree-ring reconstructions of temperature and sea-level pressure variability associated with the warm-season Arctic Oscillation since AD 1650: Geophys. Res. Let., v. 30, p. 1549-1552.
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