Transient Global Overturning Following the Shutdown of North Atlantic Deep Water Formation

Abstract

The Atlantic Meridional Overturning Circulation (AMOC), a key component of the climate system, is projected to weaken under climate warming, raising concerns about a potential collapse and its climatic consequences. However, it has recently been suggested that the wind‐driven upwelling of the Southern Ocean may prevent the AMOC from collapsing even under climate extremes. Here we analyze a climate model under CO2 ${\text{CO}}_{2}$ quadrupling and show that North Atlantic Deep Water (NADW) formation can cease under extreme warming. Despite the shutdown of NADW formation, a cross‐hemispheric overturning circulation persists for centuries, reflecting a transient adjustment of the global ocean that arises from progressive deepening of isopycnal surfaces rather than sustained diapycnal downwelling in the interior ocean. This adjustment explains the emergence of clockwise overturning circulation in the Indo‐Pacific despite the absence of deepwater formation. These features are reproduced in an idealized 1.5‐layer reduced‐gravity model. The transient adjustment of ocean overturning has important implications for the climate system. In particular, it can effectively redistribute heat into intermediate depths, thereby mitigating surface warming, and reorganize ocean tracers, altering global patterns of nutrients and ventilation that may feed back to the climate system.