Freshwater Flux Variability Lengthens the Period of the Low‐Frequency AMOC Variability

Atlantic Meridional Overturning Circulation (AMOC) exhibits interdecadal to multidecadal variability, yet the role of surface freshwater flux (FWF) variability in this AMOC variability remains unclear. This study isolates the contribution of FWF variability in modulating AMOC through a partially coupled experiment, in which the effect of the interactive FWF is disabled. It is demonstrated that the impact of the coupled FWF variability enhances the persistence of density and deep convection anomalies in the Labrador Sea (LS), thus lengthening the period of the AMOC oscillation on multidecadal timescale and suppressing its ∼30‐year periodicity. Further lead‐lag regressions illuminate that the more persistent LS density anomalies are maintained by two mechanisms: (a) The local temperature‐salinity coupling through the evaporation and (b) a downstream propagation along the East Greenland Current of the extra salinity anomaly due to the sea ice melting changes associated with an atmosphere forcing over the southern Greenland tip. Plain Language Summary The long‐term variability of Atlantic Meridional Overturning Circulation (AMOC) has a profound impact on the Earth's climate. However, the extent to which they are regulated by variability in surface freshwater fluxes (FWF) remains largely elusive. Here, we use a partial‐coupling technique to isolate the contribution of the FWF variability to the low‐frequency AMOC variability. In the absence of interactive FWF, the density anomalies important to the deep convection in the Labrador Sea (LS) are dominated by the temperature anomalies being advected around the subpolar gyre, with the salinity anomalies working to partially compensate the temperature effect. With interactive FWF, however, convection anomalies in the LS initialized by temperature changes are extended and reinforced by salinity changes and persist over a much longer time, thus extending the periodicity of the AMOC variability on multidecadal timescale. The more persistent deep convection in the LS is found to be achieved through a local and a remote mechanism. First, the local density anomalies initialized by temperature changes are extended and reinforced by evaporation‐induced salinity changes. In addition, salinity anomalies in the upstream Irminger Sea due to sea ice melting anomalies can propagate into LS to affect LS density anomalies. Key Points The surface freshwater flux (FWF) variability lengthens the period of the Atlantic Meridional Overtu