The Atlantic Meridional Overturning Circulation under Climate Forcing: An Analysis of the Community Earth System Model

The Atlantic Meridional Overturning Circulation (AMOC) is a key component in the Earth System. Given its important role in the climate system, variability in the AMOC strength is expected to have great impact on the global climate. The current observational timeseries are not long enough to make climate projections for the end of the century or even longer. Therefore, coupled climate models play an important role in the making of end of century climate projections on the AMOC strength. A known issue with the current generation of global coupled climate models is that the grid resolution is generally too coarse to resolve smaller scale processes such as mesoscale eddies. Observations and modelling studies suggest that mesoscale eddies play an important role in the exchange of water between convection regions and downwelling regions. When such processes are absent or parametrized incorrectly, it can have an influence on the climate projections based on these model simulations. This study analysed the AMOC characteristics in two different simulations of the Community Earth System Model; a reference simulation (referred to as piControl) and a simulation in which the atmospheric CO2 concentrations have been increased to four times the initial concentration (referred to as 1pctCO2). First, the AMOC characteristics in the piControl simulation are analysed using both a Eulerian and a Lagrangian approach. The Eulerian analysis shows that deep mixed layers, an indicator for convection, are present in the subpolar North Atlantic. Compared to observations and higher-resolution ocean-only models are these located closer to the West-Greenland coast. Strong vertical velocities are found over the continental slopes, especially over the steep continental slopes around Greenland. Second, the Lagrangian analysis showed the consequences of the coarse grid in the model. Only a single pathway around the subpolar gyre was observed. This implies that particles will experience convection while crossing the interior of the Labrador Sea, but only will experience downwelling when their individual pathway comes close enough to the continental slopes around Greenland. Furthermore, there is only limited exchange of particles with the regions north and south of the subpolar gyre. In the export of deep waters to the subtropical gyre, does it seem that the particles are being blocked by the North Atlantic Current. Third, the changes in the AMOC characteristics in the 1pctCO2 simulation are compared to the piControl simulation. In the 1pctCO2 simulation have deep mixed layers disappeared from the subpolar North Atlantic and convection in this region has shut down. A new fresh(er) surface layer in the Labrador Sea has intensified the stratification and prohibits the formation of deepmixed layers. Instead of the deep mixed layers in the subpolar North Atlantic have new deep mixed layers emerged between 30N and 40N. In general are velocities reduced in magnitude in the 1pctCO2 simulation, but the stronger vertical velocities can still be found over the continental slopes. In conclusion, the results show that the CESM model can reproduce the two components of the AMOC reasonably well, but the connection in the formof mesoscale eddies is missing. This illustrates that a overturning streamfunction simplifies the complexity of the overturning process. The overturning streamfunction is a measure for the overturning strength, but it does not take into account how and if the convection process and the exchange between convection and downwelling are represented.

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