Simulation of low-frequency climate variability in the North Atlantic Ocean and the Arctic

5 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 19482001. 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 overflows 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.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 19482001. 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 overflows 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.