Transient vs. Equilibrium Response of the Deep Ocean to Atmospheric Temperature Change

Much of the existing theory of ocean circulation considers steady-state equilibrium solutions, yet Earth’s climate is not in a steady state, and hasn’t been for many of the most interesting periods in its history. A better understanding of the transient ocean response to changes in the climate on time-scales from decades to millennia is thus urgently needed.

This presentation focusses on the ocean’s response to atmospheric temperature change. Theoretical arguments and idealized numerical simulations suggest that the equilibrium response of the ocean to atmospheric warming is associated with a decreased abyssal stratification and a deepening and strengthening of the Atlantic meridional overturning circulation (AMOC) - primarily as a result of changes in the sea ice formation rate around Antarctica. The transient response to atmospheric warming instead is characterized by a shoaling and weakening of the AMOC. The distinction between the transient and equilibrium response is rarely observable in complex coupled climate models, which cannot be integrated for sufficiently long times.The mechanisms governing the patterns and time-scales of ocean adjustment in the idealized numerical simulations are discussed.