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Abstract:
Low-frequency variability in large scale North Atlantic/Arctic properties
like Meridional Overturning Circulation, heat transport, deep water formation,
overflows, sea ice volume, thickness and extent, as well as the Arctic
fresh water budget are studied by means of ensemble simulations with the
global coupled ocean/sea ice model MPI-OM forced by realistic daily atmospheric
forcing data from the NCEP/NCAR Reanalysis for the period 1948-
2001. Major findings are that wintertime deep convection in the Labrador
Sea is dominated by atmospheric forcing, in particular by the North Atlantic
Oscillation. Intensified Labrador Sea convection induces substantial changes
in the Labrador Sea Water (LSW) properties, in particular colder, fresher
and denser LSW. The simulation links these changes to an increase in the
Atlantic Meridional Overturning Circulation (MOC) strength. However,
Labrador Sea deep convection is also strongly influenced by the presence of
surface salinity anomalies, which originate from anomalous Fram Strait sea
ice export events. These export events are shown to be mainly wind driven
and are the most probable cause of the observed Great Salinity Anomalies
of the 70th, 80th and 90th. In contrast to the Labrador Sea deep convection,
the Greenland-Island-Norwegian (GIN) Sea deep convection shows
a less clear imprint of the North Atlantic Oscillation variability. In the
simulation, inter-annual to decadal variability in the Atlantic MOC circulation
has its origin in the Labrador Sea, while longer term multi-decadal
trends in the MOC are governed by the properties of the overflow waters
from the GIN Sea. During the simulation period the strength of both over-
flows decreased, while the overflow water density increased. On one hand
low-frequency variability of the Arctic sea ice volume is related to sea ice
thickness changes, driven in equal parts through variability of atmospheric
thermal and fresh water fluxes, and on the other hand through variability
of the wind field. While there is a clear decrease of Arctic sea ice volume
during the 1990s, there is no such trend present over the full simulation period.
Arctic fresh water budget variability in the simulation is dominated by
exports of sea ice via Fram Strait, while the sea ice exports are governed by
variability of zonal planetary waves. Generally large parts of the observed
low frequency variability in the North Atlantic/Arctic can be understood
as a passive response of the ocean/sea ice system to variability of the large
scale atmospheric forcing.
