A weakened AMOC could cause southern ocean temperature and sea‐ice change on multidecadal timescales

Abstract

We present the first CMIP6-era multi-model intercomparison of the Southern Ocean (SO) temperature and sea-ice response to substantial Atlantic meridional overturning circulation (AMOC) weakening. Results are based on analysis of the North Atlantic Hosing Model Intercomparison Project, involving eight CMIP6 models under identical North Atlantic freshwater hosing. On multidecadal timescales, we find that southwards ocean heat transport into the SO increases, causing surface warming and sea-ice loss. Additionally, an atmospheric tropical-Antarctic teleconnection, identified here for the first time, causes regional temperature and sea-ice changes in the SO. Unlike previous studies, we find that the Amundsen Sea Low deepens for only some models. Overall, in the multi-model ensemble mean (multi-model range in brackets), over years 50–100 after AMOC weakening: SO surface air temperature warms by 0.3 (0.1–0.7)°C, sea level pressure (SLP) decreases by 30 (10–70) Pa, and sea-ice area decreases by 0.4 (−0.2–1.3) Mkm^(2). The teleconnection leads to regional differences between the response in the Indian sector and the Weddell Sea of 180 (80–320) Pa in SLP, 0.6 (0.5–1.4)°C in surface air temperature, and 0.1 (0.1–0.2) Mkm2 in sea-ice area. These SO heat transport, temperature, pressure, and sea-ice changes are small relative to the changes expected under future anthropogenic warming, despite the large and idealized 0.3 Sv hosing used to weaken the AMOC. Plain Language Summary. The Atlantic meridional overturning circulation (AMOC) could substantially weaken over the next century due to climate change. The Southern Ocean (SO) is a key control of global ocean circulation and climate. Here, we use the latest generation of climate models to assess the impacts of this potential AMOC weakening on the SO and Antarctic sea ice, on timescales of less than a century. Following AMOC weakening, ocean transports move heatsouthwardsinto the SO, causing SO surface warming and sea‐ice loss. We also identify a new atmospheric connection, from the tropics to Antarctica: this connection enhances warming and sea‐ice loss in one SO region, but causes cooling and sea‐ice growth in another. This shows that substantial AMOC weakening could impact the SO on multidecadal timescales. However, these SO changes resulting from AMOC collapse are much smaller than the projected direct impacts of greenhouse‐gas‐induced warming.