Interannual-to-decadal variability of North Atlantic air-sea CO2 fluxes

[摘要] The magnitude of the interannual variability of North Atlantic air-seaCO₂ fluxes remains uncertain. Interannual extremes simulated byatmospheric inverse approaches are typically about ±0.3 Pg C yr⁻¹,whereas those from ocean models are less than ±0.1 Pg C yr⁻¹.Thusvariability in the North Atlantic is either about 60% or less than20% of the global variability of about ±0.5 Pg C yr⁻¹ (as estimatedby both approaches). Here we explore spatiotemporal variability withinthe North Atlantic basin of one ocean model in order to more fullydescribe potential counteracting trends in different regions that mayexplain why basin-wide variability is small relative to global-scalevariability. Typical atmospheric inverse approaches separate the NorthAtlantic into at most a few regions and thus cannot properly simulatesuch counteracting effects.For this study, two simulations were madewith a biogeochemical model coupled to a global ocean generalcirculation model (OGCM), which itself was forced by 55-year NCEPreanalysis fields. In the first simulation, atmospheric CO₂ wasmaintained at the preindustrial level (278 ppmv); in the secondsimulation, atmospheric CO₂ followed the observed increase.Simulated air-sea CO₂ fluxes and associated variables were thenanalysed with a statistical tool known as multichannel singularspectrum analysis (MSSA).We found that the subtropical gyre is notthe largest contributor to the overall, basin-wide variability, incontrast to previous suggestions.The subpolar gyre and theinter-gyre region (the transition area between subpolar andsubtropical gyres) also contribute with multipolar anomalies atmultiple frequencies: these tend to cancel one another in terms of thebasin-wide air-sea CO₂ flux.We found a strong correlation betweenthe air-sea CO₂ fluxes and the North Atlantic Oscillation (NAO), butonly if one takes into account time lags as does MSSA (maximumr=0.64 for lags between 1 and 3 years).The effect of increasingatmospheric CO₂ (the anthropogenic perturbation) on total variabilitywas negligible at interannual time scales, whereas at the decadal(13-year) time scale, it increased variability by 30%.

[学科分类] 海洋学与技术