z- and ρ-AMOC under pre-industrial, historical and abrupt4xCO2 climates
in AWI-ESM2.1
Abstract
The Atlantic Meridional Overturning Circulation (AMOC) is one of the
most essential mechanisms influencing our climate system. By comparing
constant depth (z-AMOC) and density (ρ-AMOC) frameworks under
pre-industrial, historical and abrupt 4xCO2 scenarios we
analyze how the circulation mean state and variability differ amongst
them. Water mass transformations are also assessed as a matter of
analyzing surface-induced and interior-mixing-induced transformations.
As expected, both location and strength of AMOC maxima are deeply
affected by the framework choice, with the AMOC reaching a maximum
transport of 21 Sv at around 35°N under constant depth coordinates, as
opposed to ∼25 Sv at 55°N when diagnosed from density surfaces for both
pre-industrial and historical climate. When quadrupling the
CO2, both frameworks exhibit an abrupt AMOC weakening
followed by a steady recovery to maximum values of 10-15 Sv. The z-AMOC
maxima timeseries correlates more with those at 26°N (r ∼0.7) than with
the ρ-AMOC maxima (r ∼-0.3), due to the flatter isopycnals in the z
framework even in the subpolar North Atlantic, where isopycnals are, in
fact, steeper. Based on this discrepancy, we argue that the density
framework is more coherent to the physics of this circulation by
directly incorporating water mass transformations and their density
structure. We suggest that more analysis across timescales and under
different conditions must be performed with density surface outputs
being provided by as many models as possible, to enable a more
comprehensive analysis of these two frameworks and their applications.