ROCKY MOUNTAIN HYDROCLIMATIC VARIABILITY AT MULTI-DECADAL TO MILLENNIAL TIME SCALES

Over the period of instrumental observations it has become evident that western North American climatology is dominated by the interactions between the El Niño Southern Oscillation (ENSO), which has a typical period of 12 to 18 months, and the Pacific Decadal Oscillation (PDO), which has a period of 20 to 30 years. Although multi-decadal climate variations are incompletely understood, there is agreement that PDO frequency and spatial structure are related to ocean-atmosphere interactions. Both the ENSO and PDO phenomena result in similar spatial climate patterns. The few existing long-term Holocene proxy records from the North American Rocky Mountains have documented broad, multi-millennial-scale climate states, but until recently both hydroclimatic variability at multi-decadal frequencies and changes in the seasonality of precipitation were largely unknown. Model-based projections have proposed future changes in the seasonal precipitation balance. Holocene records provide a means for testing this idea over a period when changes in global energy balance due to solar insolation are known to have occurred.

This talk will review recently obtained sediment-carbonate oxygen isotope records with decade-to-century resolution from lakes within the North American cordillera. Individually, the records reflect some degree of variance related to their respective regional hydroclimate, whereas their comparison elucidates similarities that are best explained by regional hydroclimatic change. This overview of the records will summarize new insights into broader North Pacific atmospheric circulation patterns and shifts at multi-decadal to millennial time scales that are shown to coincide with changes in solar insolation, ENSO/PDO and the North American Monsoon. Additional comparisons are made with marine proxy data along the northeast Pacific margin and tree-ring networks. Recognizing and understanding the natural behavior of Pacific atmosphere-ocean dynamics is of particular importance for climate model verification and evaluating forest ecosystems and fire. Advances in our understanding of these relationships will require a long-term perspective that Holocene proxy records of hydroclimate can provide.