Frost Rings, Volcanism and Santorini Revisited – New Data and Exploration of Mechanistic Linkages INTRODUCTION In 1984 LaMarche and Hirschboeck first postulated a linkage between frost damage recorded in the rings of subalpine bristlecone pines in western United States and anomalous cooling attributed to major climatically effective explosive volcanic eruptions. It was shown statistically that the observed joint occurrence of major eruptions and the subsequent formation of notable (widespread and/or severe) frost rings was unlikely to have occurred by chance alone. Our study of frost ring / eruption linkages based on a new compilation of frost-ring dates aims to address these issues more thoroughly. 6 th International Conference on Dendrochronology, Quebec City, Quebec, Canada, August, 2002 A tree-ring response to an eruption is filtered through complex radiative and circulation processes in the atmosphere and can only provide evidence of anomalous local or regional climatic behavior, not direct evidence of the eruption that may be forcing the unusual climatic response. Katherine K. Hirschboeck & Christine L. Hallman Laboratory of Tree-Ring Research The University of Arizona Notable Frost Event No Notable Frost Event A BCD FROST RING SEVERITY SCALE ACKNOWLEDGEMENTS The new frost-ring chronology was developed by co-author Christine Hallman for her Masters thesis: Spatial Relationships in Frost-Damaged High Elevation Pines and Links to Major Volcanic Eruptions (2001) University of Arizona, Department of Geosciences. SANTORINI REVISITED LaMarche & Hirschboeck (1984) proposed an alternative date ( BC) for the cataclysmic eruption of Santorini, based on the evidence of a very severe frost event in 1627 BC (-1626 tree-ring date) at Campito Mt. in the White Mountains of California. This date was younger than the conventionally accepted date at that time ( BC) and contributed to a growing controversy over the exact timing of the Santorini eruption. METHODOLOGY Approximately 900 tree-ring samples bristlecone (Pinus aristata, Pinus longaeva & Pinus balfournia) from 16 sites were examined or re- analyzed by Christine Hallman to compile a list of 503 individual frost- ring years, four light-ring years, and 12 frost-damaged years (compared to 116 total frost-ring years reported in La Marche and Hirschboeck, 1984). The ages of the samples ranged from 1692 BC to AD A new feature of the updated compilation was a categorization of three different degrees of within-ring frost damage to scale the severity of frost injury at a given site or sites FR GB SN AZ "Notable Frost Events" (NFE) were so designated when both of the following criteria were met in a given year: (1)frost rings were found in greater than or equal to 25% of the trees at a given site (2)frost rings occurred in multiple sites. These criteria were more spatially stringent than those of La Marche and Hirschboeck (1984) who designated an event as "notable" when frost rings occurred at two or more localities, or in 50% or more of sampled trees in any one locality. NFE years were compared with the dates of climatically effective volcanic eruptions and the probability of joint occurrence of the two phenomena was evaluated using a chi-square test. Chi-square tests based on the new compilation of frost ring dates indicated that the joint occurrence of a volcanic eruption and subsequent notable frost event is statistically significant during some time periods, but not others. MECHANISTIC LINKAGES The frost ring / eruption linkage was examined from a mechanistic basis so as to provide a more robust foundation for the reasoning underlying the proposed linkage. The spatial distribution of sites with frost rings during each notable frost event year was matched with the synoptic atmospheric circulation pattern likely to have been associated with the delivery of unseasonably cold temperatures during the growing season at that combination of sites. From these patterns, a "signature" circulation pattern for western United States frost-ring occurrence was defined. Frost-ring evidence recorded in the cells of a tree during its growing season is unique in that it represents a very short-lived and anomalously cold weather event of 1 to 3 days that (in the observed record) has a distinct, "signature" synoptic-scale circulation pattern and may also be associated with a larger-scale, seasonal circulation anomaly. NOTABLE FROST EVENTS based on the new chronology RESULTS Based on the new criteria and expanded database, 17 NFE years were identified, 9 of which included frost-ring evidence at sites spanning much of western United States from the Sierra Nevada / Great Basin to the Front Range of the Rockies. The new compilation supports the existence of a frost ring / eruption connection as first suggested in La Marche and Hirschboeck (1984). Of the 17 NFE frost-ring years, 13 corresponded with major volcanic eruptions that occurred up to 3 years prior to the frost event. Even with this expanded frost ring dataset, the sample depth prior to the 1st century BC was low, hence detailed analysis of the Santorini eruption period was limited. Nevertheless, the updated compilation did yield a (1627 BC) Category 3 frost ring at a new locality (Sheep Mt.). Adding this event to the Campito Mt. frost-ring event (from LaMarche and Hirschboeck, 1984), allows us to consider 1627 BC as a notable frost event year as well, under the new compilation's more spatially stringent criteria of frost-ring occurrence at multiple sites. Although no widespread frost events that fit the spatially stringent criteria were identified after the 1982 eruption of El Chichón or the 1991 eruption of Pinatubo, frost ring evidence indicates that a more mild or less widespread frost event did occur after these recent events at three sites in the Great Basin region. DISCUSSION The greatest challenge in linking the climatic forcing of a major eruption with subsequent tree-ring evidence is the lack of a direct physical connection between an eruption and a tree's response. The meteorology, climatology, atmospheric circulation, and energy balance dynamics through which such a link might operate must be better understood. Our systematic examination of the spatial patterns of frost-ring occurrence during notable frost-ring years provides a basis for describing the unique long-wave circulation pattern of these events and suggests that a more meridional summer and fall circulation pattern with unseasonable incursions of cold air steered by deep upper-level troughs provides a mechanistic explanation for the processes involved in a frost ring / eruption linkage. SELECTED NOTABLE FROST EVENTS The spatial distribution of frost-ring occurrence varies, indicating different synoptic circulation patterns for the freeze events. Weighted DVI & Notable Frost Events (NFE) only Weighted DVI values based on Mann et. al (1998) available at ftp://eclogite.geo.umass.edu/pub/mann/WEBB/volcanic.dat Weighted DVI & Frost Ring Occurrence (all years with Category 3 frost rings) Weighted DVI values based on Mann et. al (1998) available at ftp://eclogite.geo.umass.edu/pub/mann/WEBB/volcanic.dat Comparison of these two plots indicates that the eruption / frost-ring linkage is only evident when filtering the frost- ring data using the “Notable Frost Ring” criteria. The detailed spatial network of the new frost ring chronology can be used to investigate the patterns and driving mechanisms for anomalous freezing episodes at a variety of spatial scales.