CO2 Seasonal Cycle Research Articles

Airborne in-situ measurements of vertical, seasonal and latitudinal distributions of carbon dioxide over Europe

Abstract. Airborne in-situ observations of carbon dioxide (CO2) were made during 7 intensive measurement campaigns between November 2001 and April 2003 as part of the SPURT project. Vertical profiles and latitudinal gradients in the upper troposphere/lowermost stratosphere were measured along the western shore of Europe from the subtropics to high northern latitudes during different seasons. In the boundary layer, CO2 exhibits a strong seasonal cycle with the maximum mixing ratios in winter and minimum values in summer, reflecting the strength of CO2 exchange with vegetation. Seasonal variations are strongest in high latitudes and propagate to the free troposphere and lowermost stratosphere, although with reduced amplitude. In the lowermost stratosphere, the CO2 seasonal cycle is phase-shifted relative to the free troposphere by approximately 3 months, with highest mixing ratios during the summer. Modelling studies support the interpretation that altitude gradients of CO2 are likely due to stratosphere-troposphere-transport.

A 4‐year zonal climatology of lower tropospheric CO2 derived from ocean‐only Atmospheric Infrared Sounder observations

A 4‐year zonally averaged climatology of atmospheric CO2, ocean only, between ±60° latitude has been derived from the Atmospheric Infrared Sounder (AIRS) radiances. Using only very clear fields of view, the CO2 profile in the computed radiances is scaled until agreement is found with observations. ECMWF forecast and analysis fields are used for the temperature profile in the computed radiances. The AIRS channels used to derive CO2 amounts are nominally sensitive to CO2 variability in the ∼300–800 mbar region (2–9 km), significantly lower in the atmosphere than that in previous studies using AIRS. Validation using aircraft measurements of CO2 at 650 mbar indicates that the AIRS CO2 results presented here are accurate to the 0.5–1.0 ppm level. The AIRS‐derived climatology clearly exhibits the CO2 rectifier effect, with mean CO2 values several parts per million lower than in those in the boundary layer. The AIRS CO2 seasonal cycle has a relatively constant amplitude of ∼3 ppm from +10° to +60° latitude, which matches the boundary layer seasonal cycle amplitude near +10° latitude but is about three times smaller than that in the boundary layer amplitude at +60° latitude. Phase comparisons between the AIRS and boundary layer CO2 seasonal cycles show the boundary layer phase leading AIRS in the Northern Hemisphere until ∼+10° latitude, where the phases cross and the AIRS higher‐altitude CO2 begins to lead the boundary layer phase down to ∼−10° latitude. These results may offer new insight into CO2 interhemispherical transport. Growth rates derived from the AIRS CO2 climatology are 2.21 ± 0.24 ppm/year, in good agreement with in situ measurements.

Three years of greenhouse gas column-averaged dry air mole fractions retrieved from satellite – Part 1: Carbon dioxide

Abstract. Carbon dioxide (CO2) and methane (CH4) are the two most important anthropogenic greenhouse gases. SCIAMACHY on ENVISAT is the first satellite instrument whose measurements are sensitive to concentration changes of the two gases at all altitude levels down to the Earth's surface where the source/sink signals are largest. We have processed three years (2003–2005) of SCIAMACHY near-infrared nadir measurements to simultaneously retrieve vertical columns of CO2 (from the 1.58 μm absorption band), CH4 (1.66 μm) and oxygen (O2 A-band at 0.76 μm) using the scientific retrieval algorithm WFM-DOAS. We show that the latest version of WFM-DOAS, version 1.0, which is used for this study, has been significantly improved with respect to its accuracy compared to the previous versions while essentially maintaining its high processing speed (~1 min per orbit, corresponding to ~6000 single measurements, and per gas on a standard PC). The greenhouse gas columns are converted to dry air column-averaged mole fractions, denoted XCO2 (in ppm) and XCH4 (in ppb), by dividing the greenhouse gas columns by simultaneously retrieved dry air columns. For XCO2 dry air columns are obtained from the retrieved O2 columns. For XCH4 dry air columns are obtained from the retrieved CO2 columns because of better cancellation of light path related errors compared to using O2 columns retrieved from the spectrally distant O2 A-band. Here we focus on a discussion of the XCO2 data set. The XCH4 data set is discussed in a separate paper (Part 2). In order to assess the quality of the retrieved XCO2 we present comparisons with Fourier Transform Spectroscopy (FTS) XCO2 measurements at two northern hemispheric mid-latitude ground stations. To assess the quality globally, we present detailed comparisons with global XCO2 fields obtained from NOAA's CO2 assimilation system CarbonTracker. For the Northern Hemisphere we find good agreement with the reference data for the CO2 seasonal cycle and the CO2 annual increase. For the Southern Hemisphere, where significantly less data are available for averaging compared to the Northern Hemisphere, the CO2 annual increase is also in good agreement with CarbonTracker but the amplitude and phase of the seasonal cycle show systematic differences (up to several ppm) arising partially from the O2 normalization most likely caused by unconsidered scattering effects due to subvisual cirrus clouds. The retrieved XCO2 regional pattern at monthly resolution over various regions show clear correlations with CarbonTracker but also significant differences. Typically the retrieved variability is about 4 ppm (1% of 380 ppm) higher but depending on time and location differences can reach or even exceed 8 ppm. Based on the error analysis and on the comparison with the reference data we conclude that the XCO2 data set can be characterized by a single measurement retrieval precision (random error) of 1–2%, a systematic low bias of about 1.5%, and by a relative accuracy of about 1–2% for monthly averages at a spatial resolution of about 7°×7°. When averaging the SCIAMACHY XCO2 over all three years we find elevated CO2 over the highly populated region of western central Germany and parts of the Netherlands ("Rhine-Main area") reasonably well correlated with EDGAR anthropogenic CO2 emissions. On average the regional enhancement is 2.7 ppm including an estimated contribution of 1–1.5 ppm due to aerosol related errors and sampling.

Seasonal cycle of C16O16O, C16O17O, and C16O18O in the middle atmosphere: Implications for mesospheric dynamics and biogeochemical sources and sinks of CO2

The isotopic anomaly of oxygen in atmospheric CO2 is caused by exchange reactions with isotopically anomalous O(1D) in the middle atmosphere. In the stratosphere, the major source of O(1D) is O3 photolysis; O3 is known to possess mass‐independent isotopic composition, with δ49O3 ≈ δ50O3 ≈ 100‰ relative to atmospheric O2. Higher in the mesosphere, Lyman α‐driven photodissociation of O2 provides a more important source of heavy O(1D) than O3 photolysis. Here we present a two‐dimensional simulation of the isotopic composition of CO2 from the surface to an altitude of ∼130 km that adequately reproduce the observed seasonal cycle of CO2 in the upper troposphere and the age of air in the stratosphere. Our model results suggest that stratospheric‐tropospheric exchange not only modifies the level of heavy CO2 in the troposphere, but also influences its seasonal cycle. Thus the isotopic composition of CO2 in the troposphere/biosphere could be affected by the downwelling air from the stratosphere. The predicted size of the effect is detectable by current instrumentation. Implications for the use of the isotopic composition of CO2 to constrain the gross carbon flux between the atmosphere and terrestrial biosphere and the dynamics in the remote mesosphere are discussed.

Model evaluation of CO2 and SF6 in the extratropical UT/LS region

We evaluate the transport of three‐dimensional chemical transport models in the upper troposphere and lower stratosphere applying observed distributions of CO2 and SF6. The data consist of high‐resolution in situ observations, obtained during all seasons at subtropical, middle and high latitudes over Western Europe within the SPURT (Spurenstofftransport in der Tropopausenregion) project (2001–2003). We show that the combination of the two passive tracers SF6 and CO2 with their different tropospheric characteristics and the propagation of the temporal trends of these two gases into the lower stratosphere is a powerful diagnostic for evaluation of model transport. The model evaluation shows that all models are able to capture the general features in the tracer distributions including the vertical and horizontal propagation of the CO2 seasonal cycle. However, the modeled CO2 cycles are a few months out of phase in the lowermost stratosphere due to tropospheric mixing. Two models show a too strong Brewer‐Dobson circulation causing an overestimation of the tracers in the lowermost stratosphere during winter and spring. One model displays a too strong tropical isolation leading to an underestimation of the tracers in the lowermost stratosphere during winter. All models suffer to some extend from diffusion and/or too strong mixing across the tropopause. In addition, the models show too weak vertical upward transport into the upper troposphere during the boreal summer. Sensitivity studies show that our initial conditions and boundary constraints are realistic and that a horizontal resolution higher than 2 degrees and an increase of the meteorology update frequency (from 6 to 3‐hourly) have negligible impact on the modeled CO2 and SF6 distributions.

An estimate of monthly global emissions of anthropogenic CO2: Impact on the seasonal cycle of atmospheric CO2

Monthly estimates of the global emissions of anthropogenic CO2 are presented. Approximating the seasonal CO2 emission cycle using a 2‐harmonic Fourier series with coefficients as a function of latitude, the annual fluxes are decomposed into monthly flux estimates based on data for the United States and applied globally. These monthly anthropogenic CO2 flux estimates are then used to model atmospheric CO2 concentrations using meteorological fields from the NASA GEOS‐4 data assimilation system. We find that the use of monthly resolved fluxes makes a significant difference in the seasonal cycle of atmospheric CO2 in and near those regions where anthropogenic CO2 is released to the atmosphere. Local variations of 2–6 ppmv CO2 in the seasonal cycle amplitude are simulated; larger variations would be expected if smaller source‐receptor distances could be more precisely specified using a more refined spatial resolution. We also find that in the midlatitudes near the sources, synoptic scale atmospheric circulations are important in the winter and that boundary layer venting and diurnal rectifier effects are more important in the summer. These findings have implications for inverse‐modeling efforts that attempt to estimate surface source/sink regions especially when the surface sinks are colocated with regions of strong anthropogenic CO2 emissions.

First direct observation of the atmospheric CO<sub>2</sub> year-to-year increase from space

Abstract. The reliable prediction of future atmospheric CO2 concentrations and associated global climate change requires an adequate understanding of the CO2 sources and sinks. The sparseness of the existing surface measurement network limits current knowledge about the global distribution of CO2 surface fluxes. The retrieval of CO2 total vertical columns from satellite observations is predicted to improve this situation. Such an application however requires very high accuracy and precision. We report on retrievals of the column-averaged CO2 dry air mole fraction, denoted XCO2, from the near-infrared nadir spectral radiance and solar irradiance measurements of the SCIAMACHY satellite instrument between 2003 and 2005. We focus on northern hemispheric large scale CO2 features such as the CO2 seasonal cycle and show - for the first time - that the atmospheric annual increase of CO2 can be directly observed using satellite measurements of the CO2 total column. The satellite retrievals are compared with global XCO2 obtained from NOAA's CO2 assimilation system CarbonTracker taking into account the spatio-temporal sampling and altitude sensitivity of the satellite data. We show that the measured CO2 year-to-year increase agrees within about 1 ppm/year with CarbonTracker. We also show that the latitude dependent amplitude of the northern hemispheric CO2 seasonal cycle agrees with CarbonTracker within about 2 ppm with the retrieved amplitude being systematically larger. The analysis demonstrates that it is possible using satellite measurements of the CO2 total column to retrieve information on the atmospheric CO2 on the level of a few parts per million.

The CO<sub>2</sub> tracer clock for the Tropical Tropopause Layer

Abstract. Observations of CO2 were made in the upper troposphere and lower stratosphere in the deep tropics in order to determine the patterns of large-scale vertical transport and age of air in the Tropical Tropopause Layer (TTL). Flights aboard the NASA WB-57F aircraft over Central America and adjacent ocean areas took place in January and February, 2004 (Pre-AURA Validation Experiment, Pre-AVE) and 2006 (Costa Rice AVE, CR-AVE), and for the same flight dates of 2006, aboard the Proteus aircraft from the surface to 15 km over Darwin, Australia (Tropical Warm Pool International Cloud Experiment, TWP-ICE). The data demonstrate that the TTL is composed of two layers with distinctive features: (1) the lower TTL, 350–360 K (potential temperature(θ); approximately 12–14 km), is subject to inputs of convective outflows, as indicated by layers of variable CO2 concentrations, with air parcels of zero age distributed throughout the layer; (2) the upper TTL, from θ=~360 K to ~390 K (14–18 km), ascends slowly and ages uniformly, as shown by a linear decline in CO2 mixing ratio tightly correlated with altitude, associated with increasing age. This division is confirmed by ensemble trajectory analysis. The CO2 concentration at the level of 360 K was 380.0(±0.2) ppmv, indistinguishable from surface site values in the Intertropical Convergence Zone (ITCZ) for the flight dates. Values declined with altitude to 379.2(±0.2) ppmv at 390 K, implying that air in the upper TTL monotonically ages while ascending. In combination with the winter slope of the CO2 seasonal cycle (+10.8±0.4 ppmv/yr), the vertical gradient of –0.78 (±0.09) ppmv gives a mean age of 26(±3) days for the air at 390 K and a mean ascent rate of 1.5(±0.3) mm s−1. The TTL near 360 K in the Southern Hemisphere over Australia is very close in CO2 composition to the TTL in the Northern Hemisphere over Costa Rica, with strong contrasts emerging at lower altitudes (<360 K). Both Pre-AVE and CR-AVE CO2 observed unexpected input from deep convection over Amazônia deep into the TTL. The CO2 data confirm the operation of a highly accurate tracer clock in the TTL that provides a direct measure of the ascent rate of the TTL and of the age of air entering the stratosphere.

The changing carbon cycle at Mauna Loa Observatory

The amplitude of the CO(2) seasonal cycle at the Mauna Loa Observatory (MLO) increased from the early 1970s to the early 1990s but decreased thereafter despite continued warming over northern continents. Because of its location relative to the large-scale atmospheric circulation, the MLO receives mainly Eurasian air masses in the northern hemisphere (NH) winter but relatively more North American air masses in NH summer. Consistent with this seasonal footprint, our findings indicate that the MLO amplitude registers North American net carbon uptake during the warm season and Eurasian net carbon release as well as anomalies in atmospheric circulation during the cold season. From the early 1970s to the early 1990s, our analysis was consistent with that of Keeling et al. [Keeling CD, Chin JFS, Whorf TP (1996) Nature 382:146-149], suggesting that the increase in the MLO CO(2) amplitude is dominated by enhanced photosynthetic drawdown in North America and enhanced respiration in Eurasia. In contrast, the recent decline in the CO(2) amplitude is attributed to reductions in carbon sequestration over North America associated with severe droughts from 1998 to 2003 and changes in atmospheric circulation leading to decreased influence of Eurasian air masses. With the return of rains to the U.S. in 2004, both the normalized difference vegetation index and the MLO amplitude sharply increased, suggesting a return of the North American carbon sink to more normal levels. These findings indicate that atmospheric CO(2) measurements at remote sites can continue to play an important role in documenting changes in land carbon flux, including those related to widespread drought, which may continue to worsen as a result of global warming.

Influence of atmospheric transport on the inter‐annual variation of the CO2 seasonal cycle downward zero‐crossing

Using a 3‐dimensional atmospheric transport model driven by the ECMWF winds and the non‐interannually varying seasonal biospheric flux from the CASA global ecosystem model, we show that a significant change in the downward‐zero crossing day (DZCD) of the atmospheric CO2 seasonal cycle in the Northern Hemisphere above 30°N latitude can be obtained by year‐to‐year changes in the atmospheric transport alone. The transport model is able to reproduce both the trend and standard deviation of DZCD observed at Pt. Barrow. With only atmospheric transport changing from year to year, eastern North Atlantic, Europe, southern Eurasia, southeastern United States and western North America show DZCD starting earlier by 5–10 days after 21 years of model integration. If DZCD obtained from CO2 monitoring stations is going to be used as a proxy for determining the growing season length, then it is important to take into account of that portion caused by the atmospheric transport.

Temporal variations of atmospheric carbon dioxide in the southernmost part of Japan

We present analysis of the temporal variation of atmospheric CO2 in the subtropical region of East Asia, obtained aboard a ferry between Ishigaki Island and Hateruma Island, Japan for the period June 1993–April 2005. The annual mean CO2 concentration increases from 360.1 ppmv in 1994 to 378.4 ppmv in 2004, showing an average growth rate of 1.8 ppmv yr-1. The growth rate shows interannual variations with high values duringENSOevents. The average seasonal CO2 cycle reaches the maximum in early April and the minimum in mid-September, with a peak-to-peak amplitude of 8.5 ppmv. Numerical simulations using a three-dimensional atmospheric transport model show interannual variations of the CO2 growth rate similar to the observation, but the amplitude of the seasonal cycle is larger, with maximum concentration appearing earlier than the observation by 1 month. Low CO2 values observed during the spring of 1998 are likely associated with the 1997/1998 ENSO event. A backward trajectory analysis suggests that they were due to changes in atmospheric transport whereby maritime air masses from the Pacific Ocean dominated over polluted air masses from the Asian Continent. Extreme values (either high or low) of CO2 are also occasionally observed. A comparison of backward trajectories of air parcels with CO2 concentration fields calculated using the atmospheric transport model shows that these unusual CO2 concentrations result from the transport of air affected not only by anthropogenic CO2 emissions but also by terrestrial biospheric activities mainly in China.

Seasonal exchange of CO2 and δ18O‐CO2 varies with postfire succession in boreal forest ecosystems

Seasonal cycles of atmospheric CO2 and δ18O‐CO2 at high northern latitudes have the potential to serve as indicators of ecological change in response to climate changes. Effective interpretation of these observations requires an understanding of how different species and ecosystems contribute to biosphere‐atmosphere exchange. Here we examined the effect of postfire stand age in boreal forest ecosystems on the seasonal distribution of CO2 and δ18O‐CO2 fluxes. We measured net CO2 fluxes in a 3‐year burn scar, a 15‐year trembling aspen stand, and an 80‐year black spruce stand in interior Alaska using eddy covariance. By combining measurements of the oxygen isotopic composition of ecosystem water pools at each stand with measured CO2 fluxes, we predicted half‐hourly δ18O‐CO2 fluxes and used a one‐box atmosphere model to make relative comparisons of the effect of stand age on the shape and amplitude of the seasonal cycle of CO2 and δ18O‐CO2. A shorter growing season and higher rates of net ecosystem uptake during midsummer at the 15‐year stand resulted in a larger seasonal CO2 amplitude and a delay in the drawdown of atmospheric CO2 as compared with the 80‐year stand. Reduced levels of gross primary production isoforcing from the 15‐year stand during spring and early summer caused atmospheric δ18O‐CO2 to increase more gradually between April and June as compared with fluxes from the 80‐year stand. Our analysis suggests that increased boreal forest disturbance would delay the phase of CO2 drawdown at high northern latitudes, but would advance the phase of δ18O‐CO2 drawdown.

Seasonal circulation and Mauna Loa CO2 variability

The multidecadal observational CO2 record from the Mauna Loa Observatory (MLO) in Hawaii exhibits variations at subannual frequencies associated with variations in both carbon sources and sinks and atmospheric transport. However, the atmospheric transport influence on MLO's CO2 variability in general and its seasonal characteristics in particular have received relatively little attention. In this study, we explore the impact of seasonal atmospheric circulation on MLO CO2 variability. Because of MLO's central tropical North Pacific location between zones of intense midlatitude westerly flow to the north and tropical easterly flow to the south, the air mass transport to the site broadly consists of either long‐range transport, typically originating from the west over or near the Eurasian continent, and short‐range transport, typically originating from the east over the Pacific Ocean. Climatologically, the source regions of air masses arriving at MLO reflect seasonal modulation; most notably, while the period October‐June is characterized by both the short‐ and long‐range transport pathways, with the latter more prominent, especially during Northern Hemisphere (NH) spring, the period July‐September is dominated by the short‐range pathway with little long‐range transport. At interannual timescales, the relationship between atmospheric transport and MLO CO2 also displays distinct seasonality: The correlation of observed MLO CO2 variability with simulated variability associated with cyclostationary sources interacting with an interannually varying reanalysis circulation field, i.e., an “atmospheric circulation only” estimate of MLO CO2 variability, is relatively strong during the NH cold season (November‐May) but is weak during the warm season (June‐October). Use of both Eulerian‐ and Lagrangian‐based techniques, including composite analysis and clustering of Lagrangian back trajectories, highlights distinct atmospheric circulation flow patterns associated with anomalous positive and negative excursions of MLO CO2 concentrations, with the El Niño/Southern Oscillation (ENSO) playing a significant, though seasonally variable, role. Consistent with simple correlation analysis, the strongest concurrence between circulation fields composited with respect to observed and simulated MLO CO2 is evident during the NH autumn, winter, and spring seasons. Back trajectory cluster analysis supports the physicality of the composite analysis in terms of the origins of air masses arriving at MLO. The seasonal role of atmospheric transport variability may have important implications for the interpretation of the recent downward trends in the observed CO2 seasonal cycle amplitude and maxima, features that also appear in the simulated CO2 record. Indeed, both the circulation analyses as well as available radon‐222 data for MLO point toward a decadal shift in the origin of air masses arriving at MLO during the April‐June period that favors lower CO2 concentrations.

Interactive chemistry in the Laboratoire de Météorologie Dynamique general circulation model: model description and impact analysis of biogenic hydrocarbons on tropospheric chemistry

Abstract. We present a description and evaluation of LMDz-INCA, a global three-dimensional chemistry-climate model, pertaining to its recently developed NMHC version. In this substantially extended version of the model a comprehensive representation of the photochemistry of non-methane hydrocarbons (NMHC) and volatile organic compounds (VOC) from biogenic, anthropogenic, and biomass-burning sources has been included. The tropospheric annual mean methane (9.2 years) and methylchloroform (5.5 years) chemical lifetimes are well within the range of previous modelling studies and are in excellent agreement with estimates established by means of global observations. The model provides a reasonable simulation of the horizontal and vertical distribution and seasonal cycle of CO and key non-methane VOC, such as acetone, methanol, and formaldehyde as compared to observational data from several ground stations and aircraft campaigns. LMDz-INCA in the NMHC version reproduces tropospheric ozone concentrations fairly well throughout most of the troposphere. The model is applied in several sensitivity studies of the biosphere-atmosphere photochemical feedback. The impact of surface emissions of isoprene, acetone, and methanol is studied. These experiments show a substantial impact of isoprene on tropospheric ozone and carbon monoxide concentrations revealing an increase in surface O3 and CO levels of up to 30 ppbv and 60 ppbv, respectively. Isoprene also appears to significantly impact the global OH distribution resulting in a decrease of the global mean tropospheric OH concentration by approximately 0.7×105 molecules cm-3 or roughly 8% and an increase in the global mean tropospheric methane lifetime by approximately seven months. A global mean ozone net radiative forcing due to the isoprene induced increase in the tropospheric ozone burden of 0.09 W m-2 is found. The key role of isoprene photooxidation in the global tropospheric redistribution of NOx is demonstrated. LMDz-INCA calculates an increase of PAN surface mixing ratios ranging from 75 to 750 pptv and 10 to 250 pptv during northern hemispheric summer and winter, respectively. Acetone and methanol are found to play a significant role in the upper troposphere/lower stratosphere (UT/LS) budget of peroxy radicals. Calculations with LMDz-INCA show an increase in HOx concentrations region of 8 to 15% and 10 to 15% due to methanol and acetone biogenic surface emissions, respectively. The model has been used to estimate the global tropospheric CO budget. A global CO source of 3019 Tg CO yr-1 is estimated. This source divides into a primary source of 1533 Tg CO yr-1 and secondary source of 1489 Tg CO yr-1 deriving from VOC photooxidation. Global VOC-to-CO conversion efficiencies of 90% for methane and between 20 and 45% for individual VOC are calculated by LMDz-INCA.

Long‐term record of atmospheric CO2 and stable isotopic ratios at Waliguan Observatory: Seasonally averaged 1991–2002 source/sink signals, and a comparison of 1998–2002 record to the 11 selected sites in the Northern Hemisphere

This paper investigates seasonally averaged atmospheric CO2 source or sink signals observed at Waliguan Baseline Observatory (WLG, 36°17′N, 100°54′E, 3816 m asl) in western China from 1991 to 2002. Both linear and geometric mean regressions as well as statistical significance test were performed between detrended CO2 and stable isotope monthly data by means of Keeling Model and Miller‐Tans Model. The estimated slope Δδ13C/ΔCO2 due to the seasonality in CO2 and δ13C is (−0.050 ± 0.004)‰ ppm−1, the mean source/sink isotopic signatures δs (i.e., δ13Cs and δ18Os) are (−26.159 ± 1.924)‰ and (−7.662 ± 2.113)‰, respectively, and the mean atmospheric δ13C discrimination δdis (i.e., δs minus δbg) where the δbg is the isotopic value of the background atmosphere was (−18.174 ± 1.959)‰ by the Keeling Model, in agreement with results from other continental background sites in the Northern Hemisphere. We suggest that exchange with terrestrial biosphere dominates the observed CO2, δ13C and δ18O seasonal cycles at WLG. In addition, atmospheric CO2 and δ13C data from 11 selected NH sites in the NOAA ESRL air sampling network from 1998 to 2002 were analyzed and compared to the WLG data for the same period to better address common and specific features observed in this region. The annual cycle amplitude differences, secular and seasonal Δδ13C/ΔCO2 discrepancies among sites will be useful to better understand carbon uptake and release especially on the Eurasian continent. The estimated δs during certain times at each specific site could possibly provide useful information on CO2 fluxes.

Seasonal sea-surface carbon dioxide in the Azores area

The seasonal evolution of total inorganic carbon and CO 2 air–sea fluxes in the Eastern North Atlantic Subtropical Gyre (Azores area) was investigated by means of studying a data set from 10 cruises covering a seasonal cycle. Monthly CO 2 fugacity was modelled as a function of surface temperature and month for 1998. So, the seasonal cycle of CO 2 and its air–sea fluxes were obtained using monthly average surface data in the area. Over the year, the Azores area (2.25·10 12 m 2) acts as a weak net sink of CO 2 (0.38 mmol m −2 day −1). From December to May, the zone is a rather strong sink for CO 2 (10.3 mmol m −2 day −1), while between June and November, it behaves as a CO 2 source (9.9 mmol m −2 day −1), August presents the highest outgassing (3.88 mmol m −2 day −1). Moreover, a box budget was established to evaluate the relative contribution of the physical and biological processes affecting the seasonal CO 2 variability in the mixed layer of the Azores area. The most important contributor to the average mass balance of CO 2 was the mixing with the lower layer (7.8 mmol m −2 day −1) and biological activity (−8.9 mmol m −2 day −1). Conversely, air–sea exchange (0.17 mmol m −2 day −1) and advection (1.7 mmol m −2 day −1) contribute with a very small input. There is a strong coupling between biological activity, advection, and mixing in the mixed layer depth. The biological activity is supported by mixing and advection that provide CO 2 and nutrients to the mixed layer, so we combine the three processes in only one term (ΔC AMB) that represents the net biology production in the water column, and re-evaluated the CO 2 mass balance to discriminate the importance of the physical and biological contributions. The effect of temperature, wind, and net biological process contribute in 42%, 12%, and 46%, respectively, to the explained variance of total CO 2 mass balance in the upper layer.

Global CO2 transport simulations using meteorological data from the NASA data assimilation system

We present a first analysis of atmospheric CO2 transport using meteorological data from the NASA finite volume data assimilation system (FVDAS). The analyzed meteorological fields are used along with climatological surface sources and sinks in an off‐line, forward transport simulation for 1998–2000. Analysis of model diagnostics and comparisons to previous results indicates that the model performance is consistent with that of most previous global transport models. The model interhemispheric gradients along with the timing and magnitude of the CO2 seasonal cycle are discussed, providing inferences regarding the northern biosphere, tropical land, and southern ocean fluxes. Global distributions of column‐integrated CO2 are presented to provide a basis for measurement requirements for the design of satellite‐based instruments for atmospheric CO2 column. On the synoptic scale we find a significant benefit in using the FVDAS analyzed winds for comparisons to data. At near‐equatorial observation sites, the model correctly simulates the observed atmospheric composition transition associated with the latitudinal movement of the ITCZ. Comparison to daily data from continuous analyzer sites shows the model captures a substantial amount of the observed synoptic variability due to transport changes. These results show the potential to use high temporal and spatial resolution remote sensing data to constrain CO2 surface fluxes, and they form the starting point for developing an operational CO2 assimilation system to produce high‐resolution distributions of atmospheric CO2 and quantitative estimates of the global carbon budget.

Seasonality and extent of extratropical TST derived from in-situ CO measurements during SPURT

Abstract. We present airborne in-situ trace gas measurements which were performed on eight campaigns between November 2001 and July 2003 during the SPURT-project (SPURenstofftransport in der Tropopausenregion, trace gas transport in the tropopause region). The measurements on a quasi regular basis allowed an overview of the seasonal variations of the trace gas distribution in the tropopause region over Europe from 35°-75°N to investigate the influence of transport and mixing across the extratropical tropopause on the lowermost stratosphere. From the correlation of CO and O3 irreversible mixing of tropospheric air into the lowermost stratosphere is identified. The CO distribution indicates that transport and subsequent mixing of tropospheric air across the extratropical tropopause predominantly affects a layer, which closely follows the shape of the local tropopause. In addition, the seasonal cycle of CO2 illustrates the strong coupling of that layer to the extratropical troposphere. Both, horizontal gradients of CO on isentropes as well as the CO-O3-distribution in the lowermost stratosphere reveal that the influence of quasi-horizontal transport and subsequent mixing weakens with distance from the local tropopause. The mixing layer extends to about 25 K in potential temperature above the local tropopause exhibiting only a weak seasonality. However, at large distances from the tropopause a significant influence of tropospheric air is still evident. The relation between N2O and CO2 indicates that a significant contribution of air originating from the tropical tropopause contributes to the background air in the extratropical lowermost stratosphere.

Interactive chemistry in the Laboratoire de Météorologie Dynamique general circulation model: Description and background tropospheric chemistry evaluation

We provide a description and evaluation of LMDz‐INCA, which couples the Laboratoire de Météorologie Dynamique general circulation model (LMDz) and the Interaction with Chemistry and Aerosols (INCA) model. In this first version of the model a CH4−NOx−CO−O3 chemical scheme representative of the background chemistry of the troposphere is considered. We derive rapid interhemispheric exchange times of 1.13–1.38 years and 0.70–0.82 years, based on surface and pressure‐weighted mixing ratios of inert tracers, respectively. The general patterns of the nitrogen deposition are correctly reproduced by the model. However, scavenging processes remain a major source of uncertainty in current models, with convective precipitation playing a key role in the global distribution of soluble species. The global and annual mean methane (7.9 years) and methylchloroform (4.6 years) chemical lifetimes suggest that OH is too high by about 19–25% in the model. This disagreement with previous estimates is attributed to the missing nonmethane hydrocarbons in this version of the model. The model simulates quite satisfactorily the distribution and seasonal cycle of CO at most stations. At several tropical sites and in the Northern Hemisphere during summer, the OH overestimate leads, however, to a too intense CO chemical destruction. LMDz‐INCA reproduces fairly well the distribution of ozone throughout most of the troposphere. A main disagreement appears in the Northern Hemisphere upper troposphere during summer, due to a too high tropopause in the GCM. When the GCM winds are relaxed toward assimilated meteorology, a much higher variability is obtained for ozone in the upper troposphere, reflecting more frequent stratospheric intrusions. The stratospheric influx of ozone increases from 523 Tg/yr in the base case simulation to 783 Tg/yr in the nudged version.

Interannual variation in the atmospheric CO2 growth rate: Role of atmospheric transport in the Northern Hemisphere

In this study, we examine the role of the atmospheric circulation variability in reproducing the interannual variability of the observed atmospheric CO2 growth rate in the Northern Hemisphere. Results from a series of numerical experiments performed with a three‐dimensional atmospheric transport model driven by European Centre for Medium‐Range Weather Forecasts winds from 1979 to 1999 and forced at the bottom with an annually balanced, seasonally distributed (i.e., no interannual variations) terrestrial CO2 flux calculated by the Carnegie‐Ames‐Stanford Approach (CASA) ecosystem model show changes in atmospheric CO2 growth rates that are significantly correlated with those observed at many stations located in midlatitude to high‐latitude regions in the Northern Hemisphere. Particularly, we find that the North Atlantic Oscillation (NAO) and the Pacific‐North America (PNA) indices are correlated with the observed growth rates at Alert and Point Barrow. This suggests that where there is large variability in atmospheric circulation, it needs to be taken into account when changes in observed atmospheric CO2 at various stations are to be understood in terms of changes in CO2 sources. In numerical experiments in which biospheric CO2 fluxes from the CASA model are perturbed in a known arbitrary way, we find that the Northern Hemisphere observational sites do not appear to be sensitive to changes in the CO2 seasonal flux cycle except at close proximity and thus would not be useful for inferring a Northern Hemisphere biospheric sink. Results suggest sensitivity of the locations of some monitoring stations, with respect to atmospheric circulation patterns, in detecting major changes in the biospheric CO2 flux from certain regions in the Northern Hemisphere.