Long-term measurements of turbulent surface fluxes and carbon dioxide in Canadian Arctic at Eureka climate observatory

The Arctic region is experiencing unprecedented changes associated with increasing average temperatures (faster than the pace of the globally-averaged increase) and significant decreases in both the areal extent and thickness of the Arctic pack ice. These changes are early warning signs of shifts in the global climate system that justifies increased scientific focus on this region. The increase in atmospheric carbon dioxide has raised concerns worldwide about future climate change. Recent studies suggest that huge stores of carbon dioxide (and other climate relevant compounds) locked up in Arctic soils could be unexpectedly released due to global warming. Observational evidence suggests that atmospheric energy fluxes are a major contributor to the decrease of the Arctic pack ice, seasonal land snow cover and the warming of the surrounding land areas and permafrost layers. To better understand the atmosphere-surface exchange mechanisms, improve models, and to diagnose climate variability in the Arctic, accurate measurements are required of all components of the net surface energy budget and the carbon dioxide cycle over representative areas and over multiple years. In this study we analyze variability of turbulent fluxes including water vapor and carbon dioxide transfer based on long-term measurements made at Eureka observatory (80.0 N, 85.9 W) located near the coast of the Arctic Ocean (Canadian territory of Nunavut). Turbulent fluxes and mean meteorological data are continuously measured and reported hourly at various levels on a 10-m flux tower. Sonic anemometers are located at 3 and 8 m heights while high-speed Licor-7500 infrared gas analyzer (water moisture and carbon dioxide measurements) at 7.5 m height. Tower-based eddy covariance measurements provided a long-term near continuous temporal record of hourly average mass and energy fluxes. The data show that sensible heat flux, water vapor and carbon dioxide fluxes were small and mostly irregular in the cold season while the ground remained completely covered with snow. However the turbulent fluxes increase rapidly upon air temperatures rise above 0 deg C during spring melt and eventually reach a summer maximum. According to our data, strong upward sensible and latent (water vapor) heat fluxes observed during summer months. This indicates unstable (convective) conditions on average. This study shows that the sensible heat flux, water vapor, and carbon dioxide fluxes exhibited clear diurnal cycles in Arctic summer. This behavior of the sensible heat flux is similar to the diurnal variations in mid-latitudes in summer. However, according to our study the turbulent flux of carbon dioxide was mostly negative (uptake by the surface) in summer, i.e. the Eureka site was a net sink for atmospheric CO2 during the growing seasons. It is also found that in a summer period the carbon dioxide and water vapor turbulent fluxes have different directions (CO2 -downward and H2O - upward). During late summer and early autumn all turbulent fluxes rapidly decreases in magnitude when the air temperature decreases and falls below freezing.