The imprint of surface fluxes and transport on variations in total column carbon dioxide

Abstract

Abstract. New observations of the vertically integrated CO 2 mixing ratio, CO 2 , from ground-based remote sensing show that variations in CO 2 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 CO 2 and CO 2 concentrations in the free troposphere show that large-scale spatial gradients induce synoptic-scale temporal variations in CO 2 in the Northern Hemisphere midlatitudes through horizontal advection. Rather than obscure the signature of surface fluxes on atmospheric CO 2 , these synopticscale variations provide useful information that can be used to reveal the meridional flux distribution. We estimate the meridional gradient in CO 2 from covariations in CO 2 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 CO 2 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.

Terrestrial carbon sink observed from space: variation of growth rates and seasonal cycle amplitudes in response to interannual surface temperature variability

“…The results for several latitude bands are summarised in Table 1, showing that the largest relative discrepancy is obtained for the southern temperate zone and that the best agreement is achieved for northern mid-and high latitudes where the satellite seasonal cycle is about 5 % larger compared with CarbonTracker. These seasonal cycle differences are similar to differences found between CarbonTracker and TCCON (Keppel-Aleks et al, 2012;Wunch et al, 2013).…”

Gradients of column CO<sub>2</sub> across North America from the NOAA Global Greenhouse Gas Reference Network

“…The free troposphere spatial gradient also demonstrates a wave-like pattern. A previous study on the total column CO 2 from the groundbased TCCON found strong correlation between the midlatitude column CO 2 and synoptic-scale variation of potential temperature (θ , at 700 hPa), a dynamic tracer for adiabatic air transport (Keppel-Aleks et al, 2012). Thus, they also propose that the variations in column CO 2 are mainly driven by large-scale flux and transport.…”

A method for colocating satellite <i>X</i><sub>CO<sub>2</sub></sub> data to ground-based data and its application to ACOS-GOSAT and TCCON

“…This colocation methodology is based on the one used in Inoue et al (2013), with the exception that we replace the lat-long circle with a great-distance circle to avoid warping near the poles. Wunch et al (2011b) refined the geographical method by adding midtropospheric temperature at 700 hPa as an extra threshold to take advantage of the correlation between X CO 2 and midtropospheric temperature (Keppel-Aleks et al, 2011, 2012. Their colocation methodology locates and averages all ACOS-GOSAT observations falling within ±30 • longitude, ±10 • latitude, ±5 days, and ±2 kelvin in T 700 .…”