Future changes in internal variability of the Atlantic Meridional Overturning Circulation

The response of the internal variability of the Atlantic Meridional Overturning Circulation (MOC) to enhanced atmospheric greenhouse gas concentrations has been estimated from an ensemble of climate change scenario runs. In the model, enhanced greenhouse forcing results in a weaker and shallower MOC with reduced internal variability. At the same time at 55 degrees N between 0 and 1,000 m the overturning increases as a result of a change in the area of convection. In a warmer world, new regions of deepwater formation form further north due to the poleward retreat of the sea-ice boundary. The dominant pattern of internal MOC-variability consists of a monopole centered around 35 degrees N. Due to anthropogenic warming this monopole shifts poleward. The shift is associated with a stronger relation between MOC-variations and heat flux variations over the subpolar gyre. In old convective sites (Labrador Sea) convection becomes more irregular which leads to enhanced heat flux variability. In new convective sites heat flux variations initially are related to sea-ice variations. When the sea-ice coverage further decreases they become associated with (irregular) deepwater formation. Both processes act to tighten the relation between subpolar surface heat flux variability and MOC-variability, resulting in a poleward shift of the latter.