Interannual variability across sites: Bridging the gap between flux

towers and flasks

GoalsObtain a mechanistic understanding of tower-scale interannual variability in NEE of CO2 across many

AmeriFlux/Fluxnet sites.

Link observations of interannual variability in tower fluxes with the global CO2 flask network.

Understand the mechanisms that govern interannual changes in the atmospheric CO2 budget.

Progress

• Seasonal anomalies in tower fluxes are coherent over large spatial scales and are linked to climate.

• Seasonal anomalies in fluxes are reflected in [CO2] data at AmeriFlux towers and at the oceanic flask sites(?).

Early leaf-out, 1998, Wisconsin

Impact on atmospheric [CO2]

Spatial coherence of seasonal flux anomalies

A similar pattern isseen at several fluxtowers in N. Americaand Europe.

Three sites have high-quality [CO2] measurements + dataat Fluxnet (NOBS,HF, WLEF).

The spring 98 warm period and a later cloudy period appear at all 3 sites.

Detection of the spring 98 anomaly via oceanic flasks?

2 Alaskan flasksites have slightlyhigher [CO2] inthe spring of 98.

Mace Head, Irelandshows a depression of [CO2] in thespring of 98.

Potential exists to link flux towers with seasonal inverse studies.

More progress

• Net radiation and precipitation become more correlated with NEE as the time scale increases. “Dryness” parameter as an indicator or large-scale interannual variability in NEE of CO2?

Respiration and temperature

Correlation betweennighttime tower flux and air temperature is very highon daily, monthly and seasonal time scales.

Correlation breaks downon interannual scales.

Data from the Walker Branch tower, Baldocchi,Wilson et al.

Precipitation andNEE of CO2

Poorly correlated on monthlyand seasonal time scales.

Correlation increases at annualtime scales.

Walker Branch example.

Net radiation andNEE of CO2

Net radiation shows correlationwith NEE of CO2 at monthly,seasonal and annual timescales.

Walker Branch example.

Dryness parameterand annual NEE of CO2

Correlation exists betweendryness ( = Rnet / precip)and annual NEE of CO2 atall sites tested (WBranch,NOBS, HF, WLEF).

Sign of the correlation changes!Mechanisms are not obvious.

Research needed

• Construct a continental [CO2] network.

• Get more multi-year flux tower data sets into the AmeriFlux/Fluxnet data base. Add [CO2].

• Continued analysis as the data base grows. • Conduct the North American Carbon Project.

Deliverables• Mechanistic understanding of the causes of fluctuations in

the global CO2 budget.