Response of northern North Atlantic and Atlantic meridional overturning circulation to reduced and enhanced wind stress forcing

Surface wind stress strongly influences Atlantic meridional overturning circulation (AMOC) variability on interannual time scales. On longer time scales, however, its role in AMOC variations is less clear. Here, we show a nonlinear AMOC response to globally reduced and enhanced wind stress forcing, based on sensitivity experiments with MPI-ESM1.2. Under reduced wind stress forcing, the AMOC strength strongly decreases. In contrast, under enhanced wind stress forcing the AMOC strength increases only in the first decades and then decreases, stabilizing at a value similar to the reference simulation. To reveal possible mechanisms underlying this response, we assess the response of the northern North Atlantic circulation and climate to the changed wind stress forcing. Initially, the response is linear: reduced wind stress forcing weakens the gyre circulation and the associated heat and salt transport, leading to larger winter sea ice extent and a shutdown of subpolar deep convection. In the Nordic Seas, the fresher and lighter subsurface state leads to a decrease in the baroclinic pressure and the overflow strength. Under enhanced wind stress forcing, initially, the opposite is happening. However, eventually, subpolar surface density anomalies are determined by warmer temperature rather than increased salinity, leading to a decrease in surface density and a weakening of subpolar deep convection. The resulting AMOC weakening reduces the Atlantic inflow salinity, and subsequently the Nordic Seas baroclinic pressure and overflow strength. The quasi-equilibrium response of the northern North Atlantic circulation and climate under enhanced wind stress forcing differs from the reference simulation, even though the AMOC strength converges. show a nonlinear response of the AMOC to reduced and enhanced wind stress forcing, based on sensitivity experiments with the MPI-M climate model. Under reduced wind stress forcing, the AMOC strength strongly decreases. In contrast, under enhanced wind stress forcing the AMOC strength increases only initially, then decreases and stabilizes at a value similar to the AMOC strength under unchanged wind stress forcing. The initial linear AMOC response to reduced (enhanced) wind stress forcing involves a shutdown (intensification) of both subpolar deep water formation and export of deep water masses from the Nordic Seas. The afterward different response under enhanced wind stress forcing is mainly caused by the fact that eventually subpolar surface density anomalies are determined by temperature (decrease in density due to warming) rather than salinity anomalies (increase in density due to increased salinity), which leads to a decrease in surface density and to a weakening of subpolar deep water formation. Key to the response of the northern North Atlantic circulation and climate described in our study is the changed wind stress forcing over the North Atlantic. Thus our results indicate possible impacts of a intensification or weakening of the northern hemispheric jet stream and associated North Atlantic storm track on the large-scale North Atlantic circulation and climate.