Inter-annual to Multi-centennial Variabilities of Simulated AMOC in an Earth System Model

The stability and variability of the Atlantic Meridional Overturning Circulation (AMOC) are focus of recent observational and numerical studies. A declining trend of observed AMOC for the past decade is widely recognised. However, the significance of this trend and its attribution is still unclear. It is important to understand the natural variability of the AMOC to predict its potential anthropogenic forced change and climate impacts in the future. The natural variability of the AMOC is simulated in a multi-millennial control integration of the Norwegian Earth System Model (NorESM). The simulated AMOC yields a long-term mean of 20 Sv and decadal variability of 3 Sv. The simulated AMOC trends, in randomly 12-year time windows, reach about 0.2 Sv/year, which is comparable to the observed declining trend of 0.265 Sv/year from 2004-2015. The variability of simulated AMOC displays a rich power spectrum, showing enhanced power at different frequencies from inter-annual to multi-centennial time scales. To investigate the how these variabilities originate, the AMOC at 26.5N is decomposed into three components: the Ekman transport (Ekman), the Upper-middle ocean transport (UMO) and the Florida Current (FC). It shows that the components of Ekman, UMO and FC have enhanced power of variability at time scales less than 5 years, between 6 and 20 years, and between 60 and 200 years, and the corresponding variabilities (standard deviation) are 0.3, 0.7 and 0.4 Sv respectively. Cross-correlation analysis demonstrates that the Ekman transport is in-phase with the first principal component (PC1) of the atmospheric variability over the North Atlantic (NAO), the UMO is corrected with the PC2 of the Subpolar Gyre circulation (SPG), and the FC variability is lag-correlated with the circulation of the Southern Ocean by about 50 to 80 years. It is found that the currently observed change of AMOC falls within the range of natural variability, but future changes of sea ice covers in the Antarctic under global warming may substantially alter the longterm trend of AMOC.