Detecting the onset of spring: a possible application of phenological models

The onset of the spring season is a phenomenon associated with systematic changes in the land surface-atmosphere interface, which will likely b e modified by climate change. A measure of this complex feature, or 'spring index', should reflect the variations in both weather events and plant physiology usually identified with mid-latitude spring. In this study, variables representing plant phenology, synoptlc weather system frequency, surface temperature-thickness relationship 'breaks' (difference between surface maximum temperature and thickness-derived layer mean temperature), and last freeze date (0C or lower) are compared and examined as components of a comprehensive measure of the onset of spnng. Two small networks of stations in the eastern and central United States are used as test data over the period 1961 to 1980. The phenology-synoptic weather component (P-SW), derived from regression-based models of first leaf emergence, has the strongest relationship l a t h other measures, and can be produced from generally available surface data. This component is proposed as a first approximation of a comprehensive spring index, suitable for analyzing the impact of past and future climate changes on the onset of spring.