The Response Of The North Atlantic Carbon Cycle To The North Atlantic Oscillation

The North Atlantic is an important sink of atmospheric CO₂,which is affected by North Atlantic Oscillation?NAO?.In order to better understand the relation between the North Atlantic air-sea carbon flux?fCO₂?and the NAO,the following studies of three aspects are conducted:?1?to compare the difference between the NAO indexes with different winter durations,?2?to explore the influences of the NAO on the North Atlantic physical fields and CO₂ flux and their corresponding mechanisms,and?3?to evaluate the simulation of NAO and North Atlantic carbon flux by CMIP5 Earth System Models?CMIP5-ESMs?.The following conclusions are drawn:?1?The different definition of winter durations can lead to different signs of the NAO indexes in some years which mainly appear to be in the period of the decadal phase transition.The NAO defined by different winter durations also can generate different descriptions of the NAO action centers,including the position and the movement,which greatly impacts on the site-based and EOF?Empirical Orthogonal Function?–analysis-based NAO indexes.The NAO indexes with different winter durations display different cycles.The longer the winter duration,the weaker the inter-annual variation.?2?According to the significant periods of the NAO indexes,based on the time scales of 2-6 years and 8 years or more to represent the inter-annual scale and decadal scale and the above time scale,the relation between the NAO indexes and North Atlantic physical variables is analyzed.The results show that the relationship between the NAO and North Atlantic meridional flow field is statistically significant on the long time scale?>8 years?,and that on this scale the relationship between the NAO and the Atlantic meridional overturning circulation?AMOC?is opposite to that between the NAO and the subpolar sea surface temperature?SST?,indicating that the change of SST probably has a driving effect on AMOC at this time scale.On the inter-annual scale?2-6 years?,SST anomalies in the subtropical and subpolar regions are positively correlated with the NAO,although the mechanisms may be different,and SST anomalies do not drive the response of circulation fields to the NAO.?3?Based on the observed data,on the inter-annual scale,the main difference for the responses of fCO₂ to the NAO between winter and summer appears to be in the subtropical area.This seasonal difference is mainly due to the different response mechanism of fCO₂ anomalies to the NAO:in winter the NAO-driven change in wind speed is an important controlling factor of the fCO₂ anomalies,while in summer,especially in the subtropical region,the NAO-driven change in sea surface temperature is an important factor of fCO₂ through the impact on surface-ocean CO₂partial pressure(pCO_2sea).?4?An evaluation of the simulation of the winter NAO by 13 CMIP5-ESMs indicates that the largest difference between the model-based and observation-based NAO index is incarnated in decadal signals.Compared with observations,the model-simulated high-pressure action center by all 13 CMIP5-ESMs is more southern,which makes that the corresponding area where the fCO₂ anomalies have a strong negative correlation with the NAO is also more southern.The strong CO₂ sinks in the North Atlantic can be simulated by most of the CMIP5-ESMs,whereas the CO₂source south of Iceland is poorly simulated.The influences of the simulated NAO on the North Atlantic CO₂ flux have some common features with the observations:the models that can simulate the carbon source are able to simulate the negative response center of fCO₂ anomalies to the NAO in the carbon source region.