LONG-TERM VARIABILITY IN TROPICAL- AND HIGH-LATITUDE CIRCULATION MODES OF CLIMATE IN THE AMERICAS INFERRED FROM TREE-RINGS

Instrumental records show that the climate system is characterized by low- and high-latitude patterns of variability such as the equatorial Pacific and the polar annular modes. These modes of climate variability fluctuate at many different temporal scales. ENSO and PDO are respectively linked to the interannual- and decadal-scale climatic variability in the tropical Pacific. The Arctic (AO) and Antarctic (AAO) Oscillations, which are the dominant modes of variability at higher latitudes, also exhibit short- and long-term modes of variability.

Tree-ring chronologies from a treeline transect paralleling the western coast of the Americas have been used to track past climatic variations in the tropical Pacific that have impacted the extratropical regions of North and South America during the past four centuries. Significant correlated records from the coast of Alaska and northern Patagonia show the existence of common oscillatory modes for temperature variations at 9, 13, and 50 years. Tree-ring chronologies from precipitation-sensitive regions also reveal decadal-scale oscillations, centered at 8 and 10-18 years, which have simultaneously influenced climatic conditions in the Midwest-Southern United States and Central Chile. Spatial correlation patterns between tree-ring records and SST show that variations in climate-sensitive records are strongly connected with SST anomalies in the equatorial Pacific and off the western coast of subtropical America. These correlation patterns resemble the spatial signature of the PDO over the Pacific.

Past influences of the annular modes of climate variability at high latitudes in the Americas were evaluated by the comparison of temperature reconstructions for the Arctic and sub-Antarctic regions. For the past 400 years, striking similarities in temperature fluctuations are observed in both regions. The most notable feature of temperature change revealed by most high-latitude records is the continuous transition from anomalous cold conditions in the mid-19th century to anomalous warm in the mid-20th century. In contrast, global and hemispheric mean temperatures show almost no trend between the late 1850s to the 1910s, suggesting that high latitudes in both hemispheres share common patterns of temperature changes that are not present at global scales.