Identifying the Timing of Sudden Shifts in Meridional Overturning Circulation and Coupled Climate Model Equilibration

Abstract

Currently, there exists no Pacific Meridional Overturning Circulation (PMOC). There does however appear to be geological evidence for a PMOC in the Pliocene epoch thought to have disappeared around 2.7 Million years ago with the inception of Northern Hemisphere glaciation. As changes in thermohaline circulation have important consequences for regional and global climate, it is important to develop our understanding of how the ocean’s Meridional Overturning Circulation (MOC) responds to, and interacts with, changes in climate forcings (e.g. global warming due to increased CO2 concentrations) to better our understanding of both paleoclimate change and the ocean’s response to current human-induced global warming. In this project, we examine the temporal evolution in long, ~3000-year simulations of oceanic adjustment to abrupt changes in climate forcing, and either atmospheric CO2 concentrations or clouds properties. We focus on the relationship between MOC strength and different variables, such as temperature, salinity, and the global-mean surface heat flux (SHF) into the ocean, to determine when the simulated MOC has equilibrated and will no longer change. Our results show that as the total global-mean SHF approaches zero, the global MOC equilibrates. These findings have important implications for the climate modeling community, highlighting the need to run models until a near-zero global mean SHF is reached in order to evaluate the full consequences of an abrupt change in climate forcing on the MOC.