高纬度淡水强迫增强背景下大西洋经向翻转环流的响应及其机制

Transient Response of the Atlantic Meridional Overturning Circulation to the Enhanced Freshwater Forcing and Its Mechanism

利用卑尔根海洋-大气-海冰耦合气候模式(Bergen Climate Model,简称BCM),研究在北冰洋及北欧海淡水强迫增强的背景下,大西洋经向翻转环流(Atlantic Meridional Overturning Circulation,简称AMOC)的响应及其机制,着重讨论了海表热力性质、北大西洋深层水(North Atlantic Deep Water,简称NADW)的生成率、海洋内部等密度层间的垂直混合(Diapycnal Mixing,简称DM)以及大气风场等物理过程随AMOC的响应所发生的时间演变特征。结果显示,在持续150年增强(强度为0.4 Sv)的淡水强迫下(淡水试验,FW1),AMOC的强度表现为前50年的快速减弱和在接下来100年中的逐渐恢复。同时,在淡水试验的前50年北大西洋高纬度海表盐度(Sea Surface Salinity,简称SSS)减小,海水密度降低,冬季对流混合减弱,导致NADW生成率快速减弱;在接下来的100年中,尽管增强的淡水强迫依然维持,由于海洋内部自身的调节和海气相互作用,导致了AMOC的逐渐恢复。恢复机制可以概括为:(1)随着向南的NADW的减少,大西洋中低纬度海水垂直层结逐渐减弱,DM随之逐渐增强,有利于中低纬度海盆内深层水的上升;(2)南半球西风应力增强与东风应力的减弱及北半球东风的增强使得大西洋向北的埃克曼体积通量净传输恢复;(3)大西洋向北的盐度传输逐渐恢复及次极地回旋区降水的减弱,导致SSS和NADW生成率的恢复,与之对应,AMOC逐渐恢复。研究还发现,淡水试验中,NADW的恢复主要以厄尔明格海(Irminger Sea)为主,冬季北大西洋海平面气压场(SLP)呈现类似正北大西洋涛动(NAO+)的模态,热带降水中心移到赤道以南,大西洋热带SSS增强。

The mechanism for the transient response of the Atlantic Meridional Overturning Circulation （AMOC） to the enhanced and continuous freshwater input into the Arctic Ocean and the Nordic Seas for 150 years is investigated using a fully coupled climate model （Bergen Climate Model, BCM for short）. The responses of the Sea Surface Temperature （SST）, Salinity （SSS）, Potential Density （SPD）, the North Atlantic Deep Water （NADW） formation, Diapycnal Mixing （DM） and the wind stress are analyzed. The transient response of AMOC follows a quick dropping down during the first 50 years, with a gradual recovery for the later 100 years in the freshwater perturbation experiment （FWl）. The authors find that the initial weakening of AMOC in the FW1 is mainly caused by decreasing of SSS and SPD which leads to a stable vertical stratification and then to the weakening of the NADW formation; however, AMOC recovers though the enhanced freshwater input is continuous and constant during the following 100-year integration by means of a series of feedbacks, which can be summarized as follows： 1） With the reduction in the NADW formation, the vertical density stratification in the mid-to-deep ocean at the mid-low latitudes of the North Atlantic is getting weaker, as a result, the strength of DM, parameterized by stratification-dependent method, increases and then leads to the increased upwelling; 2） the strengthened westerly and weakened easterly over the Southern Ocean and the strengthened easterly over the North Atlantic Ocean contribute together to the recovery of the northward Ekman transport; 3） the increased northward salt transport and the reduced precipitation over the North Atlantic subpolar region cause the recoveries of the SSS and SPD at northern high latitudes, and then the recovery of NADW formation, mainly in the Irminger Sea.