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Unhosed : interruptions of the Atlantic Meridional Overturning in a global coupled model , with and without freshwater forcing

It is well known that glacial periods were punctuated by abrupt climate changes, with large impacts on air temperature, precipitation, and ocean circulation across the globe. However, the longheld idea that freshwater forcing, caused by massive iceberg discharges, was the driving force behind these changes has been questioned in recent years. This throws into doubt the abundant literature on modelling abrupt climate change through ‘hosing’ experiments, whereby the Atlantic Meridional 5 Overturning Circulation (AMOC) is interrupted by an injection of freshwater to the North Atlantic: if some, or all, abrupt climate change was not driven by freshwater input, could its character have been very different than the typical hosed experiments? Here, we describe spontaneous, unhosed oscillations in AMOC strength that occur in a global coupled ocean-atmosphere model when integrated within a particular envelope of boundary conditions. We compare these unhosed oscillations 10 to hosed oscillations under a range of boundary conditions in order to examine how the global imprint of AMOC variations depends on whether or not it is the result of external freshwater input. Our comparison includes surface air temperature, precipitation, dissolved oxygen concentrations in the intermediate-depth ocean, and marine export production. The results show that the background climate state has a significant impact on the character of AMOC interruptions, including marked 15 variations in tropical precipitation and in the North Pacific circulation. Despite these differences, the first order response to AMOC interruptions is quite consistent among all simulations, implying that the ocean-sea ice-atmosphere dynamics associated with an AMOC weakening dominate the global response, regardless of whether or not freshwater input is the cause. Notable exceptions lie in the direct impact freshwater inputs can have on the strength of other polar haloclines, particularly 20 the Southern Ocean, to which freshwater can be transported relatively quickly after injection in the North Atlantic.