The Atlantic Meridional Overturning Circulation (AMOC) has profound impacts on various climate phenomena. Using both observations and simulations from the Coupled Model Intercomparison Project Phase 3 and 5, here we show that most models underestimate the amplitude of low‐frequency AMOC variability. We further show that stronger low‐frequency AMOC variability leads to stronger linkages between the AMOC and key variables associated with the Atlantic multidecadal variability (AMV), and between the subpolar AMV signal and northern hemisphere surface air temperature. Low‐frequency extratropical northern hemisphere surface air temperature variability might increase with the amplitude of low‐frequency AMOC variability. Atlantic decadal predictability is much higher in models with stronger low‐frequency AMOC variability and much lower in models with weaker or without AMOC variability. Our results suggest that simulating realistic low‐frequency AMOC variability is very important, both for simulating realistic linkages between AMOC and AMV‐related variables and for achieving substantially higher Atlantic decadal predictability.

Observed Atlantic major hurricane frequency has exhibited pronounced multidecadal variability since the 1940s. However, the cause of this variability is debated. Using observations and a coupled earth system model (GFDL-ESM2G), here we show that the decline of the Atlantic major hurricane frequency during 2005–2015 is associated with a weakening of the Atlantic Meridional Overturning Circulation (AMOC) inferred from ocean observations. Directly observed North Atlantic sulfate aerosol optical depth has not increased (but shows a modest decline) over this period, suggesting the decline of the Atlantic major hurricane frequency during 2005–2015 is not likely due to recent changes in anthropogenic sulfate aerosols. Instead, we find coherent multidecadal variations involving the inferred AMOC and Atlantic major hurricane frequency, along with indices of Atlantic Multidecadal Variability and inverted vertical wind shear. Our results provide evidence for an important role of the AMOC in the recent decline of Atlantic major hurricane frequency.

Atlantic Multidecadal Variability (AMV) is a multivariate phenomenon. Here for the first time we obtain a multivariate AMV index and associated patterns using Multivariate Empirical Orthogonal Function (MEOF) analysis to explore the multivariate nature of AMV. Coherent multidecadal variability that is unique to the Atlantic is found in the observed MEOF‐extracted AMV, various AMV‐related indices, and an Atlantic Meridional Overturning Circulation fingerprint. When the signal associated with global mean sea surface temperature is removed from both observations and Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations, the residual CMIP5 forced basin‐wide sea surface temperature‐based AMV index disagrees strongly with the observed residual. Only the observed residual basin‐wide sea surface temperature‐based AMV index retains a strong AMV signal. The MEOF approach still extracts a residual CMIP5 forced AMV signal that is unique to the Atlantic, although very different from observations. Our findings suggest that the observed AMV is not dominated by external forcing.

The multidecadal variability in Mediterranean Sea surface temperature (MSMV) exerts important climate impacts on both the Mediterranean region and remote areas at hemispheric scales and has long been identified in previous studies. However, its key region and source are still unclear. For the first time, we show that the key region of the MSMV is in the eastern Mediterranean, where the MSMV can persist throughout the year. The MSMV in the central and western Mediterranean occurs mainly in summer. Comparison of the North Atlantic Oscillation (NAO) and Atlantic Multidecadal Variability (AMV) indices shows that the cumulative NAO index has a better consistency with the MSMV, suggesting that the MSMV most likely results from the cumulative effect of NAO atmospheric forcing on ocean circulation in the Mediterranean Sea. The stable lag relationship between the cumulative NAO index and the MSMV provides a natural indicator for the decadal prediction of the MSMV.

The Atlantic Niño has attracted considerable attention due to its profound climatic impacts. It has been reported that the strength of Atlantic Niño has been weakening since 2000, but it is not clear whether it would lead to a change in Atlantic Niño prediction skill. Here we find a dramatic decline in Atlantic Niño prediction skill since 2000 by evaluating the predictions of the North American Multi-Model Ensemble. The prediction skill decline is mainly associated with a climatic regime shift, which leads to a weakened El Niño-Southern Oscillation (ENSO) teleconnection to the sea surface temperature anomaly dipole mode over the South Atlantic. A systematic model deficiency may amplify the prediction skill decline. This study offers insights for understanding the Atlantic Niño predictability and for improving the simulation and prediction of Atlantic Niño events.