Synoptic Patterns Associated with Evapotranspiration and Carbon Dioxide Flux in a Northern New England Forest

How water and carbon cycle through forest ecosystems has huge implications for the evolution of local and global climate. Synoptic scale weather patterns affect local meteorological variables, such as vapor pressure deficit (VPD), temperature, and insolation, that are known to influence evapotranspiration (ET) and net CO₂ flux (FC). However, little research exists that links synoptic scale patterns to ET and FC. In this study, we seek to understand how synoptic scale patterns influence ET, a critical component of the water cycle, and FC for the temperate mixed-hardwood forest in Hubbard Brook Experimental Forest (HBEF) in New Hampshire, U.S.A.

Water vapor, carbon dioxide fluxes, and other meteorological variables were measured above canopy at the US-HBK Ameriflux site beginning in 2016. We use self-organizing maps to identify the most common synoptic pattern types impacting the HBEF during the 2016-2021 growing seasons and determine how ET and FC vary with these synoptic pattern types.

Our analysis reveals that the highest ET and most negative FC days occur for the synoptic pattern phases starting after the departure of a low pressure system and lasting through the cresting of a high pressure system over the region. These high magnitude fluxes can persist after the ridge moves east of HBEF, but ET will decrease if the high pressure is centered to the northeast and allows an east wind to advect a maritime airmass over the region. Both ET and FC decrease in magnitude when the HBEF is located between high pressure to the east and low pressure to the west implying humid southerly flow decreases VPD and insolation. Our results suggest that future changes in the frequency of passing low pressure systems and pathways of high pressure systems could impact the fluxes of water and CO₂ from this forest.