Research On Air-sea CO₂ Fluxes In Global And Key Oceans Using Observations And CMIP5 Models

The ocean carbon sink has a significant impact on the climate change,which can be directly reflected by the air-sea CO2 fluxes.Based on the observation-based estimates and the results from CMIP5 model runs for carbon cycle,the responses of air-sea CO2 fluxes in three critical seas – the equatorial Pacific ocean,the North Atlantic ocean and the Southern Ocean to the dominant climate modes are explored,the performance of CMIP5 models on simulating the air-sea CO2 fluxes is evaluated,and the future air-sea CO2 fluxes are projected.The main conclusions include:The Centre Pacific type of ENSO is more closely related to the air-sea CO2 fluxes in equatorial Pacific ocean.The air-sea CO2 flux anomaly in equatorial Pacific ocean is mainly in the Centre Pacific type of ENSO period,which is due to the anomaly of surface aqueous partial pressure of CO2 in the eastern equatorial Pacific ocean,the main influence factor of air-sea CO2 fluxes in equatorial Pacific ocean,is mainly occurred during the Centre Pacific type of ENSO period.None of the CMIP5 models reproduce the main effect of the Centre Pacific type of ENSO on the change of air-sea CO2 fluxes,which is due to that the models do not reproduce the Centre Pacific type of ENSO mode or the surface aqueous partial pressure of CO2 anomaly in the eastern equatorial Pacific ocean they simulated is weak during the Centre Pacific type of ENSO period.The carbon sink in the North Atlantic Ocean is positively correlated with NAO.From the view of spatial distribution,the air-sea CO2 fluxes in the Greenland Sea and the Norwegian Sea area are positively responded to the NAO index,which is mainly affected by the gas transfer rate.However,the air-sea CO2 fluxes from the Labrador Sea to the Icelandic area are negatively responded to the NAO index,which is mainly affected by the air-sea CO2 partial pressure difference dominated by concentration of dissolved inorganic carbon in the sea surface water.6 of the 13 CMIP5 models fail to reproduce the negative response of the air-sea CO2 fluxes to the NAO in the area of the Labrador Sean to Iceland,which is due to that the wind speed response is weak or the gas transfer rate dominates the air-sea CO2 fluxes.The carbon sink in the Southern Ocean is negatively correlated with SAM.The airsea CO2 fluxes in the South Pacific ocean are negatively responded to the SAM index,while the air-sea CO2 fluxes in the eastern South Pacific via the Drake Strait to the Skoche Sea are positively responded to the SAM index.They are both affected by the air-sea CO2 partial pressure difference dominated by concentration of dissolved inorganic carbon.Large difference appears in the air-sea CO2 fluxes simulated by the CMIP5 models in the Southern Ocean.The analysis of total CO2 fluxes shows that 5 models fail to correctly reproduce the mechanism of SAM influencing the air-sea CO2 fluxes in the Southern Ocean,which is due to that the air-sea CO2 partial pressure difference is positively responded to the SAM index or the influence of air-sea CO2 partial pressure difference is not dominant.Using an index of relative bias,the relative performance of CMIP5 models on the air-sea CO2 fluxes is compared.Two methods are applied to project the future air-sea CO2 fluxes and narrow the projection spread.One method involves model selection based on the ability of models to reproduce the observed air-sea CO2 fluxes from 1980 to 2005 and the other method involves constrained estimation based on the strong relationship between the historical and future air-sea CO2 fluxes.These two approaches show great agreement in the global ocean and three regional oceans of the equatorial Pacific ocean,the North Atlantic ocean and the Southern Ocean,including the average state and evolution characteristics.The multi-model ensemble mean is used to project the air-sea CO2 fluxes under various warming targets.The ocean carbon sink is projected to be 3.68-4.57 Pg C/yr at the end of 21 st century and it will stop strengthening at approximately the year of 2070,the cumulative carbon emissions of 1500 Gt C and 3.5°C warming targets.The response processes of the air-sea CO2 fluxes in critical sea to their dominant climate mode are revealed in this work.This research deepens the scientific understanding of the process of ocean carbon cycle,and provides a reference for the improvement of CMIP5 model performance on the air-sea CO2 fluxes,the formulation of the greenhouse gas emission reduction measures and the determination of the long-term warming target.