Decadal Variability of the Atlantic Meridional Overturning Circulation and its Impact on the Climate


OCCI Funded Project: 2007


The Atlantic meridional overturning circulation (MOC) carries warm upper waters into northern latitudes and returns cold deep waters southward across the Equator. Its large heat transport has a substantial influence on the global as well as regional climate. Most simulations with state-of-the-art climate models suggest that the global warming will result in a slowdown of the Atlantic MOC. Some observational evidence suggests a recent slowing of the Atlantic MOC, but other observations do not.

WHOI’s Line W repeated sections suggest that the variability of the deep western boundary current is too large to detect such a downward trend from the available observations. Most climate models indeed exhibits a pronounced decadal to centennial MOC variability in the equilibrium state of the present-day control experiments as well as in global warming conditions. Long time series derived from the observations such as the Atlantic Multidecadal Oscillation (AMO) index, which is suspected to be associated with the MOC variability, and proxy data also show large low-frequency variations, but they are much too sparse to provide a complete picture of the MOC changes, whether natural or forced, and to document their impact on the global climate.As they are becoming increasingly realistic, global climate models thus provide the best available tool to investigate the processes that control the MOC variability and document its role in climate changes. Climate model still have large biases, however, so that the understanding that one derives from them needs to be put into perspective by an assessment of their degree of realism.

We propose to investigate the mechanisms of the decadal to centennial variability of the MOC in the Atlantic Ocean and its impact on the global climate using the Community Climate System Model version 3 (CCSM3) integrations in present day conditions. This should contribute to our understanding of the ocean influence in present as well as global warming conditions. CCSM3 is a state-of-the-art global climate model mainly built to produce integrations for the recently released Intergovernmental Panel on Climate Change (IPCC) 4th Assessment Report. We will emphasize the contributions from the individual components of the Atlantic MOC, such as the Gulf Stream, deep western boundary current, and high-latitude convection. We will also carefully examine the ocean-atmosphere interaction associated with the MOC variability, in terms of not only how the atmosphere drives the MOC but also how and where the MOC variability forces the atmosphere, which will be a key element in understanding the impact on the global climate. When possible, the findings will be compared with statistical signatures derived from the observations and the reanalysis, so that the reliability of the model simulation can be assessed.