Abstract

Abstract. New observations of the vertically integrated CO2 mixing ratio, ⟨CO2⟩, from ground-based remote sensing show that variations in CO2⟩ are primarily determined by large-scale flux patterns. They therefore provide fundamentally different information than observations made within the boundary layer, which reflect the combined influence of large-scale and local fluxes. Observations of both ⟨CO2⟩ and CO2 concentrations in the free troposphere show that large-scale spatial gradients induce synoptic-scale temporal variations in ⟨CO2⟩ in the Northern Hemisphere midlatitudes through horizontal advection. Rather than obscure the signature of surface fluxes on atmospheric CO2, these synoptic-scale variations provide useful information that can be used to reveal the meridional flux distribution. We estimate the meridional gradient in ⟨CO2⟩ from covariations in ⟨CO2⟩ and potential temperature, θ, a dynamical tracer, on synoptic timescales to evaluate surface flux estimates commonly used in carbon cycle models. We find that simulations using Carnegie Ames Stanford Approach (CASA) biospheric fluxes underestimate both the ⟨CO2⟩ seasonal cycle amplitude throughout the Northern Hemisphere midlatitudes and the meridional gradient during the growing season. Simulations using CASA net ecosystem exchange (NEE) with increased and phase-shifted boreal fluxes better fit the observations. Our simulations suggest that climatological mean CASA fluxes underestimate boreal growing season NEE (between 45–65° N) by ~40%. We describe the implications for this large seasonal exchange on inference of the net Northern Hemisphere terrestrial carbon sink.

Highlights

  • Accurate determination of surface fluxes of carbon dioxide is important both to predict future climate and increasingly to support climate and energy policy

  • We focus on sensitivity simulations with perturbed boreal fluxes because the large seasonality of boreal net ecosystem exchange (NEE) has a large imprint on the seasonal cycle in CO2 (KWS11, Randerson et al, 1997); the KWS simulations show that changing NEE in the subtropics or midlatitudes has a small effect on the seasonal cycle amplitude (SCA) of CO2

  • NEE inferred from column drawdown (Eq 5) shows little correlation with NEE inferred from eddy covariance on daily timescales (Fig. 5a)

Read more

Summary

Introduction

Accurate determination of surface fluxes of carbon dioxide is important both to predict future climate and increasingly to support climate and energy policy. Terrestrial biospheric carbon fluxes can be measured quite accurately via inventory (e.g., changes in carbon stocks, Saatchi et al, 2007) and by atmospheric observations (e.g., eddy-covariance measurements, Baldocchi, 2008) Such measurements are, representative only of small spatial scales (

Ground based total column data
Net ecosystem exchange estimated from column drawdown
Eddy covariance flux data
Aircraft data
Simulations with a transport model
Diurnal variations
Horizontal gradients as sources of synoptic variations in CO2
Free tropospheric data from aircraft
Comparison with models
Conclusions
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call