Atmospheric CO 2 seasonality and the air-sea flux of CO 2

Introduction Conclusions References

. Variability in the strength of these sinks is known to be large, but is presently poorly understood (e.g., Watson et al., 2009;Le Quere et al., 2007).Photosynthesis within the large, and extensively vegetated, land masses of North America and Eurasia drive a major spring-summer CO 2 uptake in the Northern Hemisphere, with leaf-fall and breakdown causing a compensating CO 2 release during autumn and winter (Machta et al., 1977).In high northern latitudes the amplitude of the CO atm 2 seasonal cycle presently exceeds 15 ppm (Peters et al., 2007), and has been increasing significantly in an apparent response to rising atmospheric CO 2 concentrations (+40 % between 1960's and 1990's) (Keeling et al., 1996).Seasonality changes are also a potential driver of glacial interglacial cyclicity (e.g., Gildor and Tziperman, 2000;Denton et al., 2005), as suggested by the close statistical coupling between variability in the earth's obliquity, and glacial terminations (Huybers and Wunsch, 2005).High obliquity means high intra-annual variability in high-latitude insolation, and appears to be a precondition for deglaciation (Loutre et al., 2004;Huybers and Wunsch, 2005;Liu et al., 2008).To understand past change, and robustly investigate potential future change, we must therefore understand interactions between seasonality and the global carbon cycle.In this paper I present two novel mechanisms linking changes in seasonality with changes in the magnitude of oceanic CO 2 sources and sinks.

Methods
Pre-industrial climate simulations (i.e.simulations without prescribed exernal forcings) were undertaken using the earth system model HadGEM2-ES, with fully interactive and coupled ocean and terrestrial biogeochemistry (Martin et al., 2011;Collins et al., 2011), and a fully coupled and well validated sea-ice component (McLaren et al., 2006).Improvements to the leaf phenology within the model now allow for good reproduction of the CO atm 2 seasonal cycle (Collins et al., 2008), and improvements to the physical and ocean-biogeochemistry model lead to good agreement between the modelled and observed spatial pattern of air-sea CO 2 flux (Fig. 1).Introduction

Conclusions References
Tables Figures

Back Close
Full Within the HadGEM2-ES model, air-sea carbon fluxes are calculated as a function of the atmospheric and ocean CO 2 concentrations, the seawater temperature and salinity (influencing the CO 2 solubility and the transfer velocity), windspeed, and sea-ice concentration, as described for a previous model version by Palmer and Totterdell 2001.To allow me to calculate the impact on air-sea fluxes of changes in the seasonal cycle of atmospheric CO 2 concentrations, without the need to spin up new model simulations to equilibrium with those seasonal cycles, I have extracted the physical and chemical fields described above calculated for the model's pre-industrial state, and undertaken air-sea CO 2 flux calculations offline.Within my offline calculations, I have varied the values relating to the atmospheric CO 2 concentration, to simulate an increased or decreased CO atm 2 seasonal cycle magnitude.The different magnitude CO atm 2 seasonal cycles were calculated by multiplying the difference between individual monthly CO atm 2 values and the annual average CO atm 2 concentrations in each latitude-longtitude box, by the specified factor (zero, one or two), and adding that to the anual mean value at that point.Within this paper, "1x seasonal cycle" therefore refers to the seasonality simulated for the preindustrial period within the model, "0x seasonal cycle" refers to a situation without any temporal variability, and "2x seasonal cycle" considers the annual variability at any each point to be twice that simulated for the preindustrial period.The calculations undertaken for this study vary from those which would be undertaken within the model only in that, annual, rather than daily mean, values for physical and chemical fields where used, and the surface wind field was calculated from wind mixing energy rather than taking values directly from the model's atmosphere.In all situations, using monthly mean values instead of continuous seasonal cycles will decrease the seasonal cycle's total magnitude, and therefore the magnitude of the results can be considered conservative.The result of these calculations will be an instantaneous value for the air-sea CO 2 flux immediately after the seasonality change, rather than an estimation of the net ocean-atmophere carbon exchange occuring in response to a change in CO atm Introduction

Conclusions References
Tables Figures

Back Close
Full

Results
Investigating the consequences of artificially doubling the amplitude of the seasonal cycle simulated within a pre-industrial earth-system-model (HadGEM2-ES (Martin et al., 2011;Jones et al., 2011;Collins et al., 2008)) climate, and calculating the instantaneous change in the air-sea flux of CO 2 , I find two contrasting impacts.The first is a relative out-gassing from the high latitude oceans, the second is a relative in-gassing from the mid-to-high-latitude oceans (Fig. 2).These two effects will initially be considered separately.
In the high latitudes, particularly of the Northern Hemisphere, the seasonal cycles of CO atm 2 concentration and sea-ice extent vary approximately in phase.The reason for the synchronous change is that they share a common driving mechanism, light.During the winter months, little light is available for either photosynthesis or heating of the surface ocean; vegetation growth therefore slows or vegetation dies back causing a net release of CO 2 , and (due to the cooling) sea-ice forms.Conversely, in the spring and summer, vegetation begins to draw-down CO 2 from the atmosphere, and, as more light reaches the ocean, sea-ice begins to melt.Making the first order assumption that sea-ice is impermeable to CO 2 (although some CO 2 flux through continuous ice has been observed (e.g., Zemmelink et al., 2006)), the average CO atm 2 concentration that the ocean sees is reduced relative to its full annual average (Fig. 3).
In the mid-to-high latitudes, equatorward of the maximum seasonal extent of sea-ice, I find that the change in air-sea CO 2 flux resulting from a change in CO atm 2 seasonality is driven by the intra-annual variability in CO 2 solubility.In common with the sea-ice mechanism, in the mid-to-high latitudes, particularly in the Northern Hemisphere, the seasonal cycle of CO 2 solubility varies approximately in phase with the seasonal cycle of CO atm 2 concentration (Fig. 4).The exchange of CO 2 between the atmosphere and the ocean occurs to bring the concentrations in each media towards equilibrium.Air-sea CO 2 concentration equilibrium occurs between the partial pressure of CO 2 in seawater multiplied by the solubility of CO 2 in that seawater, and the CO Introduction

Conclusions References
Tables Figures

Back Close
Full multiplied by the solubility of CO 2 in the seawater below.The solubility of CO 2 in seawater is a function of seawater temperature and, to a lesser degree, salinity.Again, CO 2 solubility and the CO atm 2 concentration in the high (particularly northern) latitudes share a common driver, incident light (and therefore heat).Having a high CO 2 solubility when the CO atm 2 concentration is high, and a low CO 2 solubility when the CO atm 2 concentration is low, and both CO 2 concentration and solubility always having positive values, an increase in the CO atm 2 seasonal cycle magnitude will cause an increase in the annually averaged product of the two quantities (Fig. 5).Raising the CO atm 2 concentration in potential equilibrium with seawater will cause an increased CO 2 gradient into the ocean, and promote a relative increase in the air-to-sea CO 2 flux.
Individually, neither of these two effects are likely to have a large impact on CO atm 2 concentrations.Assuming no feedbacks operate, after a change in seasonal cycle amplitude the new equilibrium CO atm 2 concentration would be unlikely to shift by more than the high latitude CO 2 seasonal cycle amplitude change.Considering the sea-ice mechanism, the dashed black line in Fig. 3 represents the average atmospheric CO 2 concentration initially in equilibrium with the underlying seawater, the red dashed line then represents the average atmospheric CO 2 concentration after a change in seasonal cycle magnitude, but prior to reaching a new air-sea equilibrium.The ocean will undergo a relative release of CO 2 to the atmosphere until the atmospheric CO 2 concentration average over the ice-free period is in equilibrium with the seawater again.
Assuming the air-sea CO 2 flux change has only a negligible effect on oceanic CO 2 concentrations, the new equilibrium would be reached when the dashed red line had raised to the concentration of the dashed black line.The problem with this reasoning is that the present high-northern-latitude ocean is not in equilibrium with the atmosphere, and is still taking up CO 2 until it sinks (Takahashi et al., 2009).The steady-state atmospheric CO 2 concentration resulting from a change in the magnitude of the CO of CO 2 resulting from a seasonal cycle shift under various conditions must therefore be quantified using coupled earth system models.A further caveat is that the seasonal melting of sea-ice can leave behind a stratified low-salinity lid which could limit the volume of water, and modify the chemistry of that water, which may come into equilibrium with the atmosphere (Cai et al., 2010).The potential supression of air-sea exchange in heavily stratified seasonally ice-covered waters may weight the net air-sea flux change towards that occuring in response to changes in the solubility mechanism.Despite, under a pre-industrial climate configuration, being unlikely to alter global CO atm 2 concentrations by more then a few ppm, the consequences of the operation of the sea-ice and solubility mechanisms under changing CO atm 2 seasonal cycle amplitude have important implications spatially and temporally.Firstly, an increase in the CO atm 2 seasonal cycle magnitude and a decrease in sea-ice extent (Intergovernmental Panel on Climate Change, 2007) over the coming decades, will cause the seasonalsolubility driven oceanic CO 2 uptake to increase and move to higher latitudes, and an increased intensity but reduced area, of seasonal-sea-ice driven relative out-gassing at the highest latitudes.These shifts will impact the location and magnitude of high latitude ocean acidification (Steinacher et al., 2009).Secondly, abrupt changes in the terrestrial biosphere, whether through natural variability (e.g.drought) or anthropogenic land-use change (e.g.deforestation), without necessarily impacting the annually averaged CO atm 2 concentration, could drive significant step changes or inter-annual variability in the high-latitude air-sea flux of CO 2 .It is possible that year-to-year changes in the CO atm 2 seasonal cycle, rather than in annually averaged CO 2 concentrations, could accounting for the observed, but largely unexplained high latitude air-sea CO 2 flux variability (e.g. Watson et al., 2009;Le Quere et al., 2007).Finally, the combination of these mechanisms have potentially interesting implications for glacial-interglacial cycling.
As previously discussed, the termination of glacial periods tends to occur at times of high orbital obliquity (Huybers and Wunsch, 2005).High obliquity causes increased high latitude insolation seasonality.A large seasonal cycle in high latitude insolation will cause high seasonal variability in sea-ice cover and high seasonal variability in Introduction

Conclusions References
Tables Figures

Back Close
Full CO 2 uptake and release by the terrestrial biosphere.The balance between the two identified mechanisms will be sensitive to the maximum annual sea-ice extent.When sea-ice is extensive, the described sea-ice mechanism dominates over the described solubility mechanism, and vice versa.During warm periods, with high seasonality, the solubility mechanism will be strong, and will pump CO 2 into the ocean, whereas during cold periods of high seasonality, the dominant mechanism will switch, and (in a relative sense) pump CO 2 into the atmosphere.The combination of these mechanisms would potentially allow the termination of a glacial to skip a number of periods of high obliquity, as is observed (Huybers and Wunsch, 2005), and only induce CO 2 release, potentially triggering warming feedbacks (e.g.Gildor and Tziperman, 2000) and deglaciation, once the glacial world has cooled enough to reach a sea-ice determined threshold.Given a slow cooling, and therefore a slow increase in maximum sea-ice extent, this mechanism also offers a possible explanation for the switch between a ∼40 kyr and ∼100 kyr glacial-interglacial periodicity at around 900 kyr before present (Raymo and Nisancioglu, 2003).To test whether the combination of these mechanisms could contribute to the timing of glacial terminations would require an appreciation of the spatial and intra-annual variability in glacial CO atm 2 concentrations.The limited temporal resolution of most palaeoclimate proxies makes this a difficult, but potentially important, challenge.Furthermore, it will be important to understand the past spatial and sub-annual variability in CO 2 solubility and sea-ice extent.Although a uniform increase in CO 2 solubility under a colder climate would make little difference to the mechanisms discussed, changes in the distribution of the reduced solubility due to reorganisation of ocean circulation may significantly amplify or reduce the climatic importance of this mechanism.Once validated for a glacial world, earth system models containing fully interactive and coupled carbon-cycle components, could be run to test whether the described mechanisms could play a role in the pacing of obliquity-driven glacial terminations.Introduction

Conclusions References
Tables Figures

Back Close
Full Presently, the seasonal cycle in earth system model CO atm 2 concentrations is often considered a way of diagnosing and benchmarking changes in model terrestrial net primary production (e.g.Cadule et al., 2010), rather than as critical prognostic variability in its own right.Given the potential sensitivity of the climate system to seasonalitydrive changes in the air-sea flux of CO 2 , it is important that the terrestrial biosphere components of earth system models consider the seasonal cycle of CO atm 2 as a critical component of the model, and develop, validate, and explore models accordingly.
Given the findings presented here, care must be taken when analysing model experiments where the CO atm 2 concentration is prescribed without either seasonal or spatial variability.Experiments of this design will contribute heavily to the conclusions reached in the IPCC's 5th climate assessment (Taylor et al., 2009).Although it will be possible to diagnose the seasonal carbon fluxes simulated within these model experiments, the individual model carbon-cycle components will not be able to feed back on each other through the mechanisms described.It is therefore imperative that fully coupled carbon cycle simulations are run and explored to quantify the feedbacks missing from the main body of simulations, and if nothing else, to show that feedbacks, such as those discussed here, are small.The feedbacks described here may help us understand the complex issue of high-latitude ocean CO 2 uptake in response to retreating sea-ice extent.

Conclusions
Despite being a prominent and dynamic feature of the carbon cycle, the climatic influence of the CO atm between the CO atm 2 and seawater CO 2 solubility seasonal cycles.The operation of the described mechanisms allows the net air-sea flux sign to switch depending on the maximum sea-ice extent, making the combination of these mechanisms of particular relevance to contemporary and glacial-interglacial climate change.One aspect of the seasonality and air-sea flux feedback mechanisms not discussed here is that of shifts in seasonal cycle phase, rather than amplitude.Various mechanisms, such as changing precipitation pattern or intensity, could shift the CO atm 2 cycle phase independently from the sea-ice and seawater CO 2 solubility cyclicity.The theory behind how changing CO 2 seasonal cycle amplitude impacts on the CO 2 air-sea flux, presented here, can equally be used to understand the response of the air-sea CO 2 flux to changing CO atm 2 seasonal cycle phase.A relative shift in the phase of the seasonal cycles of CO atm 2 concentration, and sea-ice extent or seawater CO 2 solubility may have the capacity to produce much larger changes in ocean in/out-gassing than changes in CO atm 2 concentration seasonal cycle amplitude.I have made no attempt in this study to quantify the magnitude of the highlighted carbon cycle feedbacks at steady state.To determine whether the described mechanisms could play a significant role in past or future carbon cycle change will require the spin up of (ideally) fully coupled earth system models to equilibrium with different amplitude (and potentially phase) atmospheric CO 2 seasonal cycles.If shown to be significant, the climatic impact of the seasonality-driven carbon cycle response must then be quantified.Introduction

Conclusions References
Tables Figures

Conclusions References
Tables Figures

Back Close
Full     (Peters et al., 2007), as present in http://www.esrl.noaa.gov/gmd/ccgg/carbontracker on 11/8/2010.Atmospheric CO 2 observations were detrended using a third-order polynomial, fitted (using the least squares method) to all observations at each individual site.Detrended data were averaged into a typical annual cycle, then the correlation coefficient calculated between the 12 average months of atmospheric CO 2 data and 12 months of latitudinally averaged CO 2 solubility, calculated at the latitude corresponding to the relevant atmospheric CO 2 measurement site.CO 2 solubilities in seawater were calculated from World Ocean Atlas 2009 surface temperature and salinity climatologies (Locarnini et al., 2009;Antonov et al., 2009).The solid line depicts a cubic-spline interpolated 6-point moving average through all of the data.High correlation values indicate that the annual cycles in atmospheric CO 2 concentration and CO 2 solubility in seawater vary in phase at that latitude.Introduction

Conclusions References
Tables Figures

Back Close
Full

1
Introduction Rapid cooling, high biological activity, strong winds, and the pumping of surface waters to depth, make the high latitude oceans the planet's major atmospheric CO 2 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | would therefore be a function of (to a first order) circulation, temperature change with latitude, the wind-driven rate of air-sea CO 2 exchange, time, and the spatial pattern of air-sea flux change.The net change in the air-sea flux Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

2
concentration seasonal cycle has, to the best of my knowledge, not previously been explored.I demonstrate that changes in the amplitude of the CO atm 2 seasonal cycle can impact the mid-to-high latitude air-sea flux of CO 2 .In seasonally ice-covered waters, the air-sea flux change occurs as a consequence of the synchronicity between the CO atm 2 and sea-ice seasonal cycle.Equatorward of the maximum sea-ice extent, the change in air-sea flux occurs as a result of synchronicity Introduction Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | J., Steinhoff, T., Telszewski, M., Rios, A. F., Wallace, D. W. R., and Wanninkhof, R.: Tracking the Variable North Atlantic Sink for Atmospheric CO 2 , Science to-sea CO 2 flux (Takihashi et al. 2009) model ensemble mean 1980-2000 avg.model ensemble member 1980-2000 avg.omparison of latitudinally averaged global air-sea CO 2 fluxes calculated from an n-derived climatology (Takahashi et al., 2009) (black), and from climate simulations us-dGEM2-ES model (red).Earth System model simulations were forced using greenhouse opogenic aerosol, volcanic aerosol, land-use change and solar cycle data from from the -2005 (Taylor et al., 2009; Jones et al., 2011).The observation-based climatology has lated to represent the conditions in the year 2000, but to avoid sampling internal varie model results have been presented as a mean value from the years 1980-2000.The

Fig. 1 .Fig. 2 .
Fig. 1.Comparison of latitudinally averaged global air-sea CO 2 fluxes calculated from an observation-derived climatology (Takahashi et al., 2009) (black), and from climate simulations using the HadGEM2-ES model (red).Earth System model simulations were forced using greenhouse gas, anthropogenic aerosol, volcanic aerosol, land-use change and solar cycle data from the years 1860-2005 (Taylor et al., 2009; Jones et al., 2011).The observation-based climatology has been calculated to represent the conditions in the year 2000, but to avoid sampling internal variability, the model results have been presented as a mean value from the years 1980-2000.The model's ensemble mean has been calculated as the average of three historical simulations started at 50 yr intervals from the preindustrial control simulation, and therefore considered to sample well the model's internal variability.

Fig. 3 .
Fig. 3.Explanation of how a change in the magnitude of the atmospheric CO 2 seasonal cycle can change the air-sea CO 2 flux in seasonally sea-ice covered waters.During the year, high atmospheric CO 2 concentrations occur around the time of maximum ice-cover, and are therefore prevented from exchanging freely with the ocean, whereas at times of low atmospheric CO 2 concentration, there exists no barrier to exchange.The result of this synchronicity between the seasonal cycles of atmospheric CO 2 concentration and sea-ice extent is that the annually averaged atmospheric CO 2 concentration seen by the ocean is reduced relative to that its full annual mean value, as the amplitude of the atmospheric CO 2 seasonal cycle is increased.The solid black and red curves represent the idealised annual cycles in atmospheric CO 2 concentration at one and two times the seasonal cycle amplitude respectively.The dotted black line represents the full annually averaged atmospheric CO 2 concentration.The dashed black and red lines represent the partial average of atmospheric CO 2 concentrations for the two different seasonal cycle amplitudes, over the ice-free period.

Fig. 4 .
Fig. 4.Latitudinal dependence of the phase synchronicity between the seasonal cycle of atmospheric CO 2 concentrations and the solubility of CO 2 in seawater.Correlation coefficients between observed monthly averaged atmospheric CO 2 concentration seasonal cyclicity and the calculated monthly averaged seasonal cycle of CO 2 solubility in seawater are plotted against observation latitude.Points relate to all CarbonTracker flask measurement sites containing at least five years worth of data(Peters et al., 2007), as present in http://www.esrl.noaa.gov/gmd/ccgg/carbontracker on 11/8/2010.Atmospheric CO 2 observations were detrended using a third-order polynomial, fitted (using the least squares method) to all observations at each individual site.Detrended data were averaged into a typical annual cycle, then the correlation coefficient calculated between the 12 average months of atmospheric CO 2 data and 12 months of latitudinally averaged CO 2 solubility, calculated at the latitude corresponding to the relevant atmospheric CO 2 measurement site.CO 2 solubilities in seawater were calculated from World Ocean Atlas 2009 surface temperature and salinity climatologies(Locarnini et al., 2009;Antonov et al., 2009).The solid line depicts a cubic-spline interpolated 6-point moving average through all of the data.High correlation values indicate that the annual cycles in atmospheric CO 2 concentration and CO 2 solubility in seawater vary in phase at that latitude.

Fig. 5 .
Fig. 5.Cartoon explaining how a change in the magnitude of the atmospheric CO 2 seasonal cycle can change the air-sea CO 2 flux due to coupling between the seasonal cycles of seawater CO 2 solubility and atmospheric CO 2 concentrations.Where the atmospheric CO 2 seasonal cycle and seasonal cycle of solubility are in phase, an increase in the magnitude of the atmospheric CO 2 seasonal cycle results in an annual average increase in the atmospheric CO 2 concentration in equilibrium with the underlying seawater.Dashed black and red lines represent an idealised atmospheric CO 2 cycle at 1x the normal seasonal cycle, and 2x the normal seasonal cycle respectively.The blue line represents the solubility of CO 2 in the underlying seawater throughout an idealised annual cycle.The solid black and red lines represent the atmospheric CO 2 concentration in equilibrium with seawater (the product of the atmospheric mole fraction and the solubility, assuming a total pressure of 1 atmosphere) for 1x and 2x the atmospheric CO 2 cycle amplitude respectively.The elevation of the solid red line, over the solid black line (CO 2 at 2x and 1x seasonal cycle) in the first half of the year is not cancelled by an equal decrease in the second half of the year, and consequently the annually averaged atmospheric ρCO 2 seen by the ocean is elevated by the increase in the seasonal cycle amplitude.The elevated atmospheric ρCO 2 will drive a relative flux of CO 2 into the ocean.