Master´s thesis defense by Thomas Søndergaard Eriksen – Niels Bohr Institute - University of Copenhagen

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Master´s thesis defense by Thomas Søndergaard Eriksen

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

This study contains analysis of data obtained from the Climate Community Sys-
tem Model (CCSM) 3.5 with the objective of determining the driver of the Atlantic
Meridional Overturning Circulation (AMOC). The model was run with initial condi-
tions resembling a Pliocene climate. The main hypothesized drivers were the meridional
density di fference, 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 diff erent 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 trans-
porting 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 con-
vective feedback; when convection takes place the vertical density di fference is being
lowered due to mixing. This process decreases the energy input required for deep con-
vection, 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 takes 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.