耦合模式FGOALS-g2中大西洋经向翻转流对3个典型浓度路径的响应

利用2个关于大西洋经向翻转流(Atlantic Meridional Overturning Circulation,AMOC)的指数:AMOC指数(15oN~65oN、深度为500 m以下的AMOC的最大值)和AMOC扩展指数(15oN~65oN、深度为2000~2500m的AMOC的最大值),研究了耦合模式FGOALS-g2(Grid-point Version 2 of Flexible Global Ocean-AtmosphereLand System Model)中的AMOC在CMIP5(Coupled Model Intercomparison Project Phase 5)的3个典型浓度路径(Representation Concentration Pathways,RCP)(RCP2.6、RCP4.5和RCP8.5分别对应于2100年时490、650和1370 ppm的CO2浓度水平)下的响应问题,发现:在RCP2.6和RCP4.5浓度路径下,2006~2040年时间段内AMOC指数和AMOC扩展指数都呈现快速下降的趋势,2041~2100年时间段内AMOC指数逐渐恢复,AMOC扩展指数基本维持不变;在RCP8.5浓度路径下,2006~2100年时间段内AMOC指数和AMOC扩展指数都表现出快速下降的趋势。通过分析FGOALS-g2中北大西洋深水的成因发现:3个典型浓度路径下AMOC的长期变化趋势主要受到GIN(Greenland–Iceland–Norwegian)海域的深水形成率的调控,而AMOC的年代际尺度的变化则主要受到Labrador海域深水形成率的控制。同时揭示了:由于北大西洋2000 m深度附近的层结稳定性在RCP2.6和RCP4.5下(相比于1980~2005年)提高了30%~40%,使得由AMOC指数恢复产生的深水无法继续下沉,从而导致AMOC扩展指数没有出现恢复的现象。

Response of Atlantic Meridional Overturning Circulation in FGOALS-g2 Model to Three Representation Concentration Pathways

This study investigates the responses of the Atlantic Meridional Overturning Circulation (AMOC) simulated by the FGOALS-g2 (grid-point version 2 of Flexible Global Ocean-Atmosphere-Land System model) to three different RCPs (Representation Concentration Pathways) of CMIP5 (Coupled Model Intercomparison Project Phase 5). In this investigation, two indices are used: (1) The AMOC index, defines as the maximum value of AMOC between 15°N and 65°N, at depths greater than 500 m, and (2) AMOC extending index, which is the maximum value of AMOC between 15°N and 65°N, and between 2000-m and 2500-m depth. It is found that under RCP2.6 and RCP4.5, both the AMOC index and the AMOC extending index show decreasing trends from 2006 to 2040, following which the AMOC index presents a increasing trend and the AMOC extending index maintains its high value between 2041 and 2100. In contrast, under RCP8.5, rapid decreasing trends are been identified in the time series of both the AMOC index and the AMOC extending index. Through analysis of the mechanism of formation of the North Atlantic deep water, the main component of AMOC, it can be concluded that the long-term trends in AMOC are determined by deep water formation in the Greenland-Iceland-Norwegian (GIN) Seas, while the decadal variability of AMOC is modulated by deep water formation in the Labrador Sea. Meanwhile, it can be shown that due to the stratification stability at around 2000-m depth in the Northern Atlantic Ocean increasing by 30%-40% under RCP2.6 and RCP4.5, the increasing deep water, indicated by the recovering AMOC index, could not sink down any further, which is the reason for the non-recovery of the AMOC extending index.