Interdecadal Variations And Their Mechanisms Of Atlantic Meridinonal Overturning Circulations With Different Intensities

Atlantic Meridional Overturning Circulation(AMOC)plays an important role in the global climate system.Based on the simulations by CCSM3(Community Climate System Model,version 3)and CMIP5 models,this paper aims to examine the characteristics and differences of air-sea elements of the North Atlantic under different AMOC average intensities,and to put forward an interdecadal mechanism by analyzing the interdecadal characteristics of different average intensities as well as the differences between them.It also investigates the interdecadal scales'climate response to different AMOC average intensities in China.Here listed are the main results in this study:1)The CCSM3 model developed by the National Center for Atmospheric Research was used to study the characteristics of AMOC with freshwater disturbance experiment.In this trial,the features of AMOC were analyzed using different mean AMOC intensities.It showed that both the climate mean state of ocean and atmospheric elements over the North Altantic are significantly different between the simulations.Compared to the results under the high average intensity,the SST(sea surface temperature),SSS(sea surface salinity),SSD(sea surface density),and SAT(surface air temperature)over the North Atlantic are weakened under the low average intensity,with the largest negative anomaly appeared in GIN(Greenland sea-Iceland sea-Norwegian sea).The abnormal cold high pressure can be found over the North Atlantic,with elevation of SLP(sea level pressure)and cooling of SST and SAT.In addition,sea ice distribution area expands and the precipitation in the western tropical Atlantic decreases,while the ITCZ(intertropical convergence zone)moves southward.Under the different AMOC average intensities,the area between SST,SSS and SSD interannual anomalies is different.For instance,under the high average intensity the most significant area is in GIN,while under the low average intensity it is in the Labrador waters.Under the high average intensity,the dominant variability mode of SST over the North Atlantic locates in GIN,which does not exist in the low average intensity.The dominant variability mode of SLP over North Atlantic is NAO(North Atlantic Oscillation)-like mode,which can be more significant under high average intensity.2)The simulation with CCSM3 model showed that the interdecadal fluctuation of AMOC decreases significantly in accordance with the decrease of average intensity of AMOC.Under the high average intensity,the interdecadal fluctuations of 20-30 a is significant,instead of 12-20 a under the low average intensity.The simulation by CMIP5 climate models further suggested that the impact of AMOC changes on more than 100 years scale is significanton its interdecadal fluctuations.Under high average intensity,AMOC interdecadal fluctuation is longer and more significant.3)The responses of SST and SLP to different AMOC intensities in North Atlantic are obviously distinct.Under the high average intensity,the dominant correlation mode between North Atlantic SST and AMOC index exhibits a meridional tripole pattern,while it shows a NAO-like pattern between North Atlantic SLP and AMOC index.However,the two modes do not co-exist under the low average intensity.Under high average intensity,the correlation between AMOC and the various elements is more significant.4)The mechanism of interdecadal AMOC variability is different between varied mean AMOC intensities for the reason of the differences of ocean fluctuations and air-sea interactions in the North Atlantic Ocean.Relative to the high average intensity,the stratification in the North Atlantic ocean is strengthened under the low average intensity,with accelerated first baroclinic Rossby wave speed and shortened time for Rossby wave crossing the North Atlantic,which consequently lead to a shorter interdecadal cyle of the AMOC.It is founded that the propagation process and path of the signal of AMOC's interdecadal oscillation are different from the relationship between the air-sea interactions in North Atlantic Ocean under different intensities.The mechanism of interdecadal AMOC variability is the whole basin scale air-sea coupling mechanism under the high average intensity,while it is the local ocean internal wave mechanism in the North Atlantic ocean under the low average intensity.5)On the interdecadal scale,the relationship between AMOC changes and changes of China's surface air temperatures and precipitation is clearly close,but theclimate responsesin China under different average intensitiesare different.Under the high average intensity,the surface temperature increases over the whole region of China,but the precipitation only rises in the north of China,which is vice versa under the low average intensity.Meanwhile,the distribution of the first eigenvector of surface air temperature and precipitation in Chinadisplays significant differencesbetween high and low average intensities.Under the high average intensity,surface air temperature shows unipole pattern in China and precipitation presents " ???" type from north to south of China.However,under the low average intensity,surface air temperature shows north-south dipole pattern and precipitation presents "—?" type from the north to the south.Furthermore,the interdecadal scale of the time coefficient of the first mode is longer under the high average intensity than that under the low average intensity.