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

• Main Author: Kusters, Niek (author)
• Other Authors: Katsman, C.A. (mentor), Pietrzak, J.D. (graduation committee), Petrini, M. (graduation committee), Vizcaino, M. (graduation committee), Delft University of Technology (degree granting institution)
• Format: Master Thesis
• Language: English
• Published: 2020
• Subjects: AMOC; Climate change; CESM; Lagrangian analysis; climate projections; Coupled models; Mixed Layer Depth; Greenland; Labrador Sea; North Atlantic
• Online Access: http://resolver.tudelft.nl/uuid:d9e1e567-ae94-4e55-8ae5-d16238ca4aa5

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, ...