A model study of the interannual to decadal scale variability in the North Atlantic Ocean
Résumé
A forced experiment with the global version ORCA2LIM of the LODYC ocean-ice model has been carried out to simulate regional to basin scale variability in the ocean currents and the thermodynamic fields for the period 1948 to 1998. This study is part of the PREDICATE Work Package 2 (WP2) project, which purpose is to determine mechanisms of decadal fluctuations in the North Atlantic ocean. The realistic forcing fields are derived from the NCEP reanalysis using an iterative scheme (derived from the TOGA COARE scheme) for the bulk formula calculation. The thermohaline circulation (following the maximum of the meridional stream function above 20N) presents a strong jump (~5 Sv) in the early 70s, in accordance with an enhancement of the North Atlantic Oscillation some years earlier. The relationship between the THC and the atmospheric forcing (heat fluxes, wind stresses) is done through statistical methods (EOF-based methods, regression methods), in order to understand the respective roles of the atmospheric fields versus oceanic ones, like the salinity, the ice formation/dilution, and the convection. Strong variations in the thermohaline circulation are related to the mechanisms of deep-water formation in the Labrador Sea, region where the increasing salinity leads to the sinking of cold waters, the enhancement of the convection and of the thermohaline circulation. The understanding of the variability of these atmospheric and oceanic fields helps us to set up the characteristics of the simulated variability modes.
