Predicting Dynamic Trends of the Atlantic Meridional Overturning Circulation for Transient and Stochastic Forcing Effects

Abstract

The Atlantic meridional overturning circulation (AMOC) plays a central role in regulating the climatic conditions of the North Atlantic regions. The AMOC exhibits two stable equilibrium states, one strong as in the present-day and one weak which under certain conditions ceases to exist. These states and possible transitions between them are strongly influenced by climatological changes, as past history has shown [P. U. Clark et al., Nature, 415 (2002), pp. 863--869]. Our work investigates two important factors that can alter AMOC dynamics: temperature variability and stochastic freshwater variations. These two factors in combination can lead to undesired transitions to the weak state, resulting in abrupt cooling and a reduction in precipitation in the North Atlantic and Europe [S. Rahmstorf and A. Ganopolski, Climatic Change, 43 (1999), pp. 353--367]. Our analysis is performed by implementing a two-box single-hemispheric model within which the effects of slowly varying restoring temperature conditions and stochastic freshwater forcing are analyzed numerically, as well as by use of multiple-scales approximations, given the different time scales involved. Our findings suggest that stochastic and large deterministic freshwater forcings can cause the AMOC to transition between strong and weak states under a constant temperature forcing, but large temperature gradients between boxes generally favor a strong AMOC. While there has been some concern over increased freshwater input into the Atlantic causing a weakening of the AMOC, our results suggest that if the system is forced into the weak state by large freshwater perturbations, a subsequent increase in temperature gradient can counteract this and return the system to the strong state once again.