Detecting Energy-Balance Modifications at the Onset of Spring

The nature of energy-flux transitions during the onset of midlatitude spring has not been widely examined, despite their critical implications for global-scale biospheric measures and climate change. Further conventional assessment of these phenomena across multiple locations is restricted by satellite-derived data limitations, and the paucity of surface measurement sites. In this paper, we explore a new phenology-based strategy for evaluating the spring surface energy-balance shift that can address these concerns. Our technique can reflect native-species responses and atmospheric surface layer change at a variety of sites, using synoptic-scale first leaf and first bloom phenology models. We test the approach at three locations with diverse climates, and within a group of stations in the state of Oklahoma, which has moderate climate variations across an east-to-west gradient. The results show that the onset of spring in midlatitudes is a modally abrupt (rather than gradual) seasonal transition in terms of energy balance (sensible and latent heat levels) and carbon-flux change that can be linked directly to vegetation phenology. The consistent temporal pattern and magnitude of flux variations across diverse sites suggest that this technique has potential as a proxy for spring energy-balance change at many locations. [Key words: phenology, spring, energy balance, North America.]