Determining the driving processes of the Atlantic Meridional Overturning Circulation from CCSM data

This study contains analysis of data obtained from the Climate Community System Model (CCSM) 3.5 with the objective of determining the driver of the Atlantic Meridional Overturning Circulation (AMOC). The model was run with initial conditions resembling a Pliocene climate. The main hypothesized drivers were the meridional density difference, the strength of the winds over the Southern Ocean, the deep water formation in the north Atlantic, and the transport of the subpolar gyre. The analysis consisted of a quantitative and a qualitative part. In the quantitative part the focus was on finding the correlation coefficient between the AMOC and the hypothesized drives, and in the qualitative the purpose was to find a causal relation. Furthermore, the quantitative analysis provided a basis on which to perform the qualitative part. Because of distinctly different behavior of the AMOC in the early and latter part of the simulation both analyses were performed in two seperate temporal regimes. In both regimes the overall analysis of the data supported the claim that it is the deep water formation in the Denmark Strait and the Labrador Sea which should be considered the most important driver, although with a large influence of the subpolar gyre in transporting high salinity waters to these regions. Two feedback mechanisms in the behavior of the AMOC, deep water formation and subpolar gyre transport must be highlighted in this regard. First, the socalled salt-advection feedback; an increase in upper ocean northward salinity flux towards the subpolar Atlantic increases the density here. This enables deep water formation which again increases the overturning. Second, the convective feedback; when convection takes place the vertical density difference is being lowered due to mixing. This process decreases the energy input required for deep convection, which is thereby sustained. Since the deep water formation occurs in a region with a fresh water flux from the atmosphere to the ocean (since precipitation is higher than evaporation) a continuous freshening of the upper ocean take place. If convection does not occur for consecutive years, the fresh water accumulates and therefore lightens the upper ocean further inhibiting deep water formation. The analyzed data support the conclusion that both these feedback processes can have a very large influence on the deep water formation and therefore on the AMOC strength.