The Tree-Ring Record of False Spring in the Southcentral USA

Frost injuries are common in the annual rings of deciduous oaks of the southcentral United States, and can be identified microscopically by unique anatomical criteria. A chronology of 70 frost ring years between 1650 and 1980 has been developed from 42 collection sites in the Southern Plains. False spring conditions cause frost rings in oaks, and include both an abnormally warm winter and the subsequent severe freeze in spring (temperatures must fall to ≤ 23 °F or -5 °C). Major circulation changes over North America often occur from the warm to cold phase of false spring. An upper level trough over Southern California and surface high over the Southeast favor warm air advection into the Southern Plains during the warm phase. This pattern is usually replaced by a deep upper level trough over the central USA and a strong surface ridge often extending from Canada to Mexico during the cold phase. The resulting cold air advection often causes heavy damage to crops and native vegetation which are prematurely advanced by the preceding mild weather. These false spring episodes include both climatological and meteorological signals, and the consistent registration of specific weather conditions by frost rings establishes the feasibility of “dendrometeorology”. Frost rings in oaks often form during La Nina events, and may reflect a tropical influence on both above and below average winter temperatures in the Southern Plains and Canada, respectively. Warm winters in the Southern Plains favor premature growth, and cold Canadian winters may help explain the severity of the late cold wave which terminates false spring. False spring occurrence has been nonrandom over the past 331 years, and the many one- and three-year intervals between events may partially reflect La Nina forcing. In contrast, an El Nino influence on the formation of bristlecone pine frost rings and light rings in Canadian black spruce has been detected in previously published chronologies. The co-occurrence of oak frost in spring followed by light rings in summer often reflects an enhanced La Nina-El Nino cycle. An amplified El Nino-Southern Oscillation (ENSO) may be suggested by the four such co-occurrences from 1814 to 1819, which could help explain many ambiguities in the worldwide temperature response to the cataclysmic eruption of Tambora in 1815.