Tuesday, January 4, 2005
Climate variability in the Northern Hemisphere over the past few hundred years: Proxy-based reconstructions of the NAM and PNA
Seasonal patterns of atmospheric variability are a strong determinant of climate of the Northern Hemisphere, especially during the winter months and on regional scales. Two of these patterns, the Northern Annular Mode (NAM) and the Pacific North American (PNA) pattern, together explain much of both the mean climate signal and variability in the Northern Hemisphere. Reconstructions of these patterns using proxy data may help us to understand climate variability before the instrumental record.
Many previous reconstructions of climate variability have combined widely disparate proxies and used complicated reconstruction algorithms. This approach tends to mask the relationship between individual proxy records and the target patterns of climate variability. We seek to address the shortcomings of such an approach by "building" a more physically-based reconstruction from the ground up using only annually-resolved records available over the past few hundred years. Before including a proxy in our reconstruction, we examine what (if any) meteorological phenomena it records and determine how its frequency response will affect its utility as a predictor.
From a comprehensive set of approaches, tree ring widths generally outperform other information available from tree cores. Using only a subset of all available tree ring records, we are able to match or exceed the skill in existing NAO reconstructions which use both tree and ice core data. Regardless of the reconstruction algorithm, more or less the same ring width sites are emphasized, implying a physical relationship between the proxy and pattern of variability. The PNA reconstruction has skill that is more sensitive to the reconstruction method, but many of the skill scores are at or near those obtained for the NAM reconstruction. As we add ice cores into the reconstructions, we anticipate an increase in the quality of our NAM and PNA reconstructions. Ultimately, we hope to provide a high-quality, physically-based reconstruction of both patterns.
Justin Wettstein is a PhD student in the University of Washington's Department of Atmospheric Sciences and the Joint Institute for the Study of the Atmosphere and Ocean.