Stability of the AMOC depends on the background climate

English Abstract:

Atlantic Meridional Overturning Circulation (AMOC) is an important global scale oceanic circulation which contributes significantly to the global heat balance. Previous theoretical and modeling studies suggest that the stability of this circulation can affect the regional and global climate and may be the primary cause of the abrupt climate change event in the ice core records. Here we use two version of the Community Climate System Model (CCSM3 and CCSM4) to study the hysteresis of the AMOC under present day and mid-glacial conditions with two ensembles for each background condition. This is the first time that we clearly demonstrate the AMOC’s hysteresis not only depending on the background climate but also depending on the status of the Bering Strait. With an open Bering Strait, the AMOC strength is only determined by the strength of the external haline or thermal forcing, not the background climate, and does not have hysteresis behavior. However, with a closed Bering Strait, the AMOC could collapse/restart suddenly and induces abrupt climate change events regardless the background climates. Further, the width of the AMOC hysteresis loop is affected by the background climates with a much narrower width under glacial conditions, suggesting that with a closed Bering Strait, the AMOC is much more sensitive to the freshwater perturbation in the subpolar North Atlantic than under present day condition. For the future warmer climate with an open Bering Strait, the AMOC could collapse due to strong greenhouse gas effect, but would restart as soon as the greenhouse gas forcing weakens, indicating the absence of the AMOC hysteresis. We conclude that the AMOC is possibly mono-stable under present day condition with an open Bering Strait, and multiple stable under glacial condition when Bering Strait is closed.