Abstract
Recent studies, using data from the OSNAP observational campaign and
from numerical ocean models, suggest that surface buoyancy losses over
the Iceland Basin and the Irminger Sea may, in contradiction to the
established consensus, be more significant than those over the Labrador
Sea, and that these former regions are in fact the dominant sites for
formation of upper North Atlantic Deep Water), with the Labrador Sea
acting mainly as a region of further densification as the dense waters
flow around the gyre. Here we present a set of hindcast integrations of
a global 1/4° NEMO ocean configuration from 1958 until nearly the
present day, forced with three standard surface forcing datasets. We use
the surface-forced streamfunction, estimated from surface buoyancy
fluxes, along with the overturning streamfunction, similarly defined in
potential density space, to investigate the causal link between surface
forcing and decadal variability in the strength of the Atlantic
meridional overturning circulation (AMOC). A scalar metric based on the
surface forced streamfunction, evaluated in critical density and
latitude classes, and accumulated in time, is found to be a good
predictor of changes in the overturning strength, and the surface heat
loss from the Irminger Sea is confirmed to be the dominant mechanism for
decadal AMOC variability. We use the streamfunctions to demonstrate that
the watermasses in the simulations are transformed to higher densities
as they propagate around the subpolar gyre from their formation
locations in the north-east Atlantic and the Irminger Sea, consistent
with the picture emerging from observations.