Transient versus Equilibrium Response of the Ocean’s Overturning Circulation to Warming

Much of the existing theory for the ocean’s overturning circulation considers steady-state equilibrium solutions. However, Earth’s climate is not in a steady state, and a better understanding of the ocean’s nonequilibrium response to changes in the surface climate is urgently needed. Here, the time-dependent response of the deep-ocean overturning circulation to atmospheric warming is examined using a hierarchy of idealized ocean models. The transient response to surface warming is characterized by a shoaling and weakening of the Atlantic meridional overturning circulation (AMOC)—consistent with results from coupled climate simulations. The initial shoaling and weakening of the AMOC occurs on decadal time scales and is attributed to a rapid warming of northern-sourced deep water. The equilibrium response to warming, in contrast, is associated with a deepening and strengthening of the AMOC. The eventual deepening of the AMOC is argued to be associated with abyssal density changes and driven by modified surface fluxes in the Southern Ocean, following a reduction of the Antarctic sea ice cover. Full equilibration of the AMOC requires a diffusive adjustment of the abyss and takes many millennia. The equilibration time scale is much longer than most coupled climate model simulations, highlighting the importance of considering integration time and initial conditions when interpreting the deep-ocean circulation in climate models. The results also show that past climates are unlikely to be an adequate analog for changes in the overturning circulation during the coming decades or centuries.