Simulation Differences And Cause Analysis Of Multi-decadal Fluctuations In The North Atlantic

Based on the multi-model comparative analysis of the phase 3 to 6 of coupled model intercomparison plan(CMIP3-6),there are a large difference in the simulation period of the Atlantic Multidecadal Oscillation(AMO)in each stage of the model,which have not been significantly improved from CMIP3 to CMIP6.This paper analyzes the periodic simulation relationship between AMO and Atlantic Meridional Overturning Circulation(AMOC)interdecadal fluctuation.Then,the theoretical Rossby wave trans-basin time scale calculated from the ocean dynamic environment simulated by each model is compared and analyzed with the AMOC period.Based on the above analysis,the interpretive framework of ocean dynamic environment,ocean Rossby wave,AMOC interdecadal fluctuation and AMO is constructed.In addition,through the results of pi Control experiments in the CCSM4,the paper analyzing simulating oceanic Rossby waves.As well as,experiments are conducted in the MITGCM model to analyze the role of interdecadal variability of atmospheric forcing fields(heat flux,freshwater flux,and wind stress)on interdecadal fluctuations of AMOC.Thereby,the role of ocean dynamical processes and atmospheric forcing played in the possible determination mechanism of AMO is discussed.The results show the following:(1)This large inter-model discrepancy and lack of improvement is also present in the simulation of the AMOC interdecadal fluctuation,and there is a certain correlation between the model biases of the AMO and AMOC interdecadal fluctuation,especially in the CMIP6.The paper proposed that the model biases of upper ocean temperature and salinity in the North Atlantic may cause large differences of the AMO(AMOC interdecadal variability)period simulated by different models,through distorting the oceanic Rossby waves.(2)The temperature is higher,the density is more sensitive to temperature,and the temperature is lower,salinity plays more important role in denisty.Certain differences in the temperature and salt climate values simulated by each model lead to density differences in response to the same temperature and salinity variations.Density response caused by temperature and salinity variation may take part in modulating the AMO period mechanism.Therefore,the model simulating temperature and salinity have different sensitivities to density,and to a certain extent the model bias of temperature and salinity climatic state values can affect the simulating period and amplitude of AMO.(3)During the westward propagation of simulated oceanic Rossby wave,the simulated oceanic Rossby wave modulates the AMOC interdecadal fluctuation,which conclusions are consistent with the theoretical Rossby waves calculated by the ocean dynamical environment.(4)The interdecadal variability of the atmospheric forcing field originates from ocean dynamics processes,but the feedback of the atmospheric forcing field to the ocean has influence on stabilizing the interdecadal variability of the ocean.When the interdecadal variability is removed from the atmospheric forcing field in the sensitivity experiment,the simulated variability of the interdecadal fluctuation of AMOC cannot be maintained stably.This paper systematically studyes the causes of the bias in the AMO(AMOC)interdecadal fluctuation model simulations from the perspectives of ocean dynamics processes and atmospheric forcing field,which deepens our understanding of the AMO dynamics mechanism and provides direction for the improvement of AMO simulation capability.