Abrupt climate change events during the last glacial period and the Last Interglacial resulted from changes in the Atlantic Meridional Overturning Circulation (AMOC). Over the last 50 years, the AMOC has weakened and is projected to weaken further or even collapse this century due to freshwater influx from melting glaciers driven by climate warming. Despite numerous modelling studies investigating the impacts of an AMOC shutdown, little is known about its impact on Australasian hydroclimate, particularly under a climate warmer than the pre-industrial (PI). Using the ACCESS-ESM1.5 model, we assess the processes impacting seasonal hydroclimate in the Australasian region in response to an AMOC shutdown under PI and Last Interglacial (LIG) climatic conditions. While the broad hydroclimate response to an AMOC shutdown is similar in both experiments, notable regional differences emerge, highlighting the influence of background climate states. During austral summer (DJF), the AMOC shutdown leads to drier conditions over the Maritime Continent and increased precipitation over northern Australia under both PI and LIG conditions. However, the precipitation increase over Australia is weaker under PI than LIG. During austral winter (JJA), mid to high southern latitude regions of Australia and New Zealand experience drying in response to the AMOC shutdown under PI boundary conditions, while under LIG boundary conditions, only southeastern Australia and New Zealand exhibit drier conditions, with northwestern Australia displaying wetter conditions. These results underscore the complex and region-specific responses of Australasian hydroclimate to AMOC disruptions, highlighting the importance of considering background climate states when assessing such impacts.

Antarctic ice core records suggest that atmospheric CO2 increased by 15 to 20 ppm during Heinrich stadials (HS). These periods of abrupt CO2 increase are associated with a significant weakening of the Atlantic meridional overturning circulation (AMOC), and a warming at high southern latitudes. As such, modelling studies have explored the link between changes in AMOC, high southern latitude climate and atmospheric CO2. While proxy records suggest that the aeolian iron input to the Southern Ocean decreased significantly during HS, the potential impact on CO2 of reduced iron input combined with oceanic circulation changes has not been studied in detail. Here, we quantify the respective and combined impacts of reduced iron fertilisation and AMOC weakening on CO2 by performing numerical experiments with an Earth system model under boundary conditions representing 40,000 years before present (ka). Our study indicates that reduced iron input can contribute up to 6 ppm rise in CO2 during an idealized Heinrich stadial. This is caused by a 5% reduction in nutrient utilisation in the Southern Ocean, leading to reduced export production and increased carbon outgassing from the Southern Ocean. An AMOC weakening under 40ka conditions and without changes in surface winds leads to a ~0.5 ppm CO2 increase. The combined impact of AMOC shutdown and weakened iron fertilisation is almost linear, leading to a total CO2 increase of 7 ppm. Therefore, this study highlights the need of including changes in aeolian iron input when studying the processes leading to changes in atmospheric CO2 concentration during HS.