大西洋热盐环流年代际变化机制研究Ⅱ.热盐环流年际和年代际变化机制研究

摘要

A climate model (ECHAM5/MPI-OM) newly developed for the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC) at Max-Planck Institute for Meteorology is used to study the variations of the Atlantic Thermohaline Circulation (THC). Especially, the mechanism of the inter-decadal variability of the THC is discussed in details. The result of this study shows that: (1) the dominant period of interannual variability of THC is 4 years, while the dominant period of interdecadal variability of THC is 24 years, which appears as the strongest signal and the first principal component. (2) The interdecadal variability of the THC works as follows: when the THC has the weakest strength, negative temperature anomalies are induced in the up upper ocean north of the Gulf Stream region due to the reduced northward ocean heat transport. About 5 years later, the negative temperature anomalies reach its maximum. Associated with the build up of negative upper ocean temperature anomalies positive anomalies in the upper ocean density and an acceleration of the subpolar gyre and North Atlantic Current (NAC) take place. After another 3 years the subpolar gyre and NAC receive their strongest values. Stronger subpolar gyre and NAC mean increased transport of salinity into the Greenland—Iceland—Norway (GIN) seas and will lead to a maximum in the upper ocean density in GIN. After 1 further year enhanced convection is triggered, leading to an increase in the rate of deep water formation and acceleration of the THC. The THC reaches its maximum approximately 4 years after the maximum of subpolar gyre and NAC strength. The total time for the phase reversal is 12 years, consistent with a period of about 24 years.%基于德国Max-Planck气象研究所的最新大气海洋环流模式(ECHAM5/MPI-OM),对控制试验(control run)下热盐环流(THC)年际及年代际变化进行了分析,揭示了年代际变率的产生机制.研究表明:(1)THC年际振荡的主导周期是4 a,年代际振荡的主导周期是24 a,THC的年代际振荡信号最强,是第一主成分.(2)THC的年代际振荡机制为:首先从大西洋径向翻转环流(MOC)强度最小开始,由于MOC强度处于较弱状态,从低纬度向高纬度输送的热量偏少,副极地海区海表温度出现负异常,持续5 a之后,北大西洋副极地海区海表温度达到最大负异常.此时副极地流环中心(北大西洋)的表层海水变冷,密度增加,海表面下降,产生从副极地流环边缘指向副极地流环的中心的压强梯度力,根据地转平衡关系,北大西洋副极地海区的上层海洋会出现一个气旋式的环流异常(副极地流环得到加强),北大西洋暖流(NAC)同时得到加强.在副极地海区海表温度达到最大负异常的3 a之后,副极地流环和NAC达到最强.由此,作为NAC延伸的法鲁海峡入流水增强,更多的高盐法鲁海峡入流水进入格陵兰-冰岛-挪威海(GIN)海域,使GIN海域层结稳定性减弱.1 a后,GIN海域深层对流增强,格陵兰-苏格兰海脊溢流水增加.在GIN海域深层对流达到最强的3 a之后,MOC强度达到最大.整个状态翻转过程完成的时间大约为12 a,THC年代际振荡的整个周期大约是24 a.